Fecal microbiota transplant (FMT), also known as a stool transplant,
is the process of transferring fecal bacteria and other microbes from a
healthy individual into another individual. FMT is an effective
treatment for Clostridioides difficile infection (CDI). For recurrent CDI, FMT is more effective than vancomycin alone, and may improve the outcome after the first index infection.
Scanning electron micrograph of Clostridioides difficile bacteria from a stool sample
Fecal microbiota transplant is approximately 85–90% effective in
people with CDI for whom antibiotics have not worked or in whom the
disease recurs following antibiotics. Most people with CDI recover with one FMT treatment.
A 2009 study found that fecal microbiota transplant was an
effective and simple procedure that was more cost-effective than
continued antibiotic administration and reduced the incidence of antibiotic resistance.
Once considered to be a "last resort therapy" by some medical
professionals, due to its unusual nature and invasiveness compared with
antibiotics, perceived potential risk of infection transmission, and
lack of Medicare coverage for donor stool, position statements by specialists in infectious diseases and other societies have been moving toward acceptance of FMT as a standard therapy for relapsing CDI and also Medicare coverage in the United States.
It has been recommended that endoscopic FMT be elevated to first-line treatment for people with deterioration and severe relapsing C. difficile infection.
In November 2022, faecal microbiota transplant (Biomictra) was approved for medical use in Australia,and fecal microbiota, live (Rebyota) was approved for medical use in the United States.
Fecal microbiota spores, live (Vowst) was approved for medical use in the United States in April 2023. It is the first fecal microbiota product that is taken by mouth.
Other conditions
Ulcerative colitis
In May 1988, Australian professor Thomas Borody treated the first ulcerative colitis patient using FMT, which led to longstanding symptom resolution.
Following on from that, Justin D. Bennet published the first case
report documenting reversal of Bennet's own colitis using FMT. While C. difficile
is easily eradicated with a single FMT infusion, this generally appears
to not be the case with ulcerative colitis. Published experience of
ulcerative colitis treatment with FMT largely shows that multiple and
recurrent infusions are required to achieve prolonged remission or cure.
Cancer
Clinical trials are underway to evaluate if FMT from anti-PD-1 immunotherapy donors can promote a therapeutic response in immunotherapy-refractory patients.
Autism
Once linked with naturopathy, there have been serious studies into treating Autism Spectrum Disorder with fecal microbiota transplants. One such study was conducted in Shanghai, China, and an earlier study led by Arizona State University. The Arizona treatment has received a United States Patent (#11,202,808) and the researchers hope for FDA approval.
Adverse effects
Adverse effects were poorly understood as of 2016. They have included bacterial blood infections, fever, SIRS-like syndrome, exacerbation of inflammatory bowel disease
in people who also had that condition, and mild GI distress which
generally resolve themselves soon after the procedure, including
flatulence, diarrhea, irregular bowel movements, abdominal
distension/bloating, abdominal pain/tenderness, constipation, cramping,
and nausea. There are also concerns that it may spread COVID-19.
A person died in the United States in 2019, after receiving an
FMT that contained drug-resistant bacteria, and another person who
received the same transplant was also infected. The US Food and Drug Administration
(FDA) issued a warning against potentially life-threatening
consequences of transplanting material from improperly screened donors.
Technique
There
are evidence-based consensus guidelines for the optimal administration
of FMT. Such documents outline the FMT procedure, including preparation
of material, donor selection and screening, and FMT administration.
The gut microbiota comprises all microorganisms that reside along
the gastrointestinal tract, including commensal, symbiotic and
pathogenic organisms. FMT is the transfer of fecal material containing
bacteria and natural antibacterials from a healthy individual into a
diseased recipient.
Donor selection
Preparing
for the procedure requires careful selection and screening of the
potential donor. Close relatives are often chosen on account of ease of
screening; however, in the case of treatment of active C. diff., family members and intimate contacts may be more prone to be carriers themselves. This screening involves medical history questionnaires, screening for various chronic medical diseases (e.g. irritable bowel diseases, Crohn's disease, gastrointestinal cancer, etc.), and laboratory testing for pathogenic gastrointestinal infections (e.g. CMV, C. diff., salmonella, Giardia, GI parasites, etc.).
Specimen preparation
No laboratory standards have been agreed upon,
so recommendations vary for size of sample to be prepared, ranging from
30 to 100 grams (1.1 to 3.5 ounces) of fecal material for effective
treatment. Fresh stool is used to increase viability of bacteria within the stool and samples are prepared within 6–8 hours. The sample is then diluted with 2.5–5 times the volume of the sample with either normal saline, sterile water, or 4% milk. Some locations mix the sample and the solvent with a mortar and pestle, and others use a blender. There is concern with blender use on account of the introduction of air which may decrease efficacy as well as aerosolization of the feces contaminating the preparation area. The suspension is then strained through a filter and transferred to an administration container. If the suspension is to be used later, it can be frozen after being diluted with 10% glycerol, and used without loss of efficacy compared to the fresh sample. The fecal transplant material is then prepared and administered in a clinical environment to ensure that precautions are taken.
Administration
After being made into suspensions, the fecal material can be given through nasogastric and nasoduodenal tubes, or through a colonoscope or as a retention enema.
Mechanism of action
One
hypothesis behind fecal microbiota transplant rests on the concept of
bacterial interference, i.e., using harmless bacteria to displace pathogenic organisms, such as by competitive niche exclusion. In the case of CDI, the C. difficile pathogen is identifiable.
Recently, in a pilot study of five patients, sterile fecal filtrate was
demonstrated to be of comparable efficacy to conventional FMT in the
treatment of recurrent CDI. The conclusion from this study was that soluble filtrate components (such as bacteriophages, metabolites, and/or bacterial components, such as enzymes) may be the key mediators of FMT's efficacy, rather than intact bacteria. It has now been demonstrated that the short-chain fatty acidvalerate
is restored in human fecal samples from CDI patients and a bioreactor
model of recurrent CDI by FMT, but not by antibiotic cessation alone;
as such, this may be a key mediator of FMT's efficacy. Other studies
have identified rapid-onset but well-maintained changes in the gut bacteriophage profile after successful FMT (with colonisation of the recipient with donor bacteriophages), and this is therefore another key area of interest.
In contrast, in the case of other conditions such as ulcerative colitis, no single culprit has yet been identified.
However, analysis of gut microbiome and metabolome changes after FMT as
treatment for ulcerative colitis has identified some possible
candidates of interest.
History
The
first use of donor feces as a therapeutic agent for food poisoning and
diarrhea was recorded in the Handbook of Emergency Medicine by a Chinese
man, Hong Ge, in the 4th century. Twelve hundred years later Ming
dynasty physician Li Shizhen used "yellow soup" (aka "golden syrup") which contained fresh, dry or fermented stool to treat abdominal diseases. "Yellow soup" was made of fecal matter and water, which was drunk by the person.
The consumption of "fresh, warm camel feces" has also been recommended by Bedouins as a remedy for bacterial dysentery; its efficacy, probably attributable to the antimicrobial subtilisin produced by Bacillus subtilis, was anecdotally confirmed by German soldiers of the Afrika Korps during World War II. However, this story is likely a myth; independent research was not able to verify any of these claims.
The first use of FMT in western medicine was published in 1958 by
Ben Eiseman and colleagues, a team of surgeons from Colorado, who
treated four critically ill people with fulminant pseudomembranous
colitis (before C. difficile was the known cause) using fecal enemas, which resulted in a rapid return to health. For over two decades, FMT has been provided as a treatment option at the Centre for Digestive Diseases in Five Dock, by Thomas Borody, the modern-day proponent of FMT. In May 1988 their group treated the first ulcerative colitis patient using FMT, which resulted in complete resolution of all signs and symptoms long term. In 1989 they treated a total of 55 patients with constipation, diarrhea, abdominal pain, ulcerative colitis, and Crohn's disease with FMT. After FMT, 20 patients were considered "cured" and a further 9 patients had a reduction in symptoms. Stool transplants are considered about 90 percent effective in those with severe cases of C. difficile colonization, in whom antibiotics have not worked.
The first randomized controlled trial in C. difficile infection was published in January 2013.
The study was stopped early due to the effectiveness of FMT, with 81%
of patients achieving cure after a single infusion and over 90%
achieving a cure after a second infusion.
Since that time various institutions have offered FMT as a therapeutic option for a variety of conditions.
Society and culture
Regulation
Interest in FMT grew in 2012 and 2013, as measured by the number of clinical trials and scientific publications.
In July 2013, the FDA issued an enforcement policy ("guidance") regarding the IND requirement for using FMT to treat C. difficile infection unresponsive to standard therapies (78 FR42965, July 18, 2013).
In March 2014, the FDA issued a proposed update (called "draft
guidance") that, when finalized, is intended to supersede the July 2013
enforcement policy for FMT to treat C. difficile infections
unresponsive to standard therapies. It proposed an interim discretionary
enforcement period, if 1) informed consent is used, mentioning
investigational aspect and risks, 2) stool donor is known to either the
person with the condition or physician, and 3) stool donor and stool are
screened and tested under the direction of the physician (79 FR10814, February 26, 2014).
Some doctors and people who want to use FMT have been worried that the
proposal, if finalized, would shutter the handful of stool banks which
have sprung up, using anonymous donors and ship to providers hundreds of
miles away.
As of 2015, FMT for recurrent C. difficile infections can
be done without mandatory donor and stool screening, whereas FMT for
other indications cannot be performed without an IND.
The FDA has issued three safety alerts regarding the transmission
of pathogens. The first safety alert, issued in June 2019, described
the transmission of a multidrug resistant organism from a donor stool
that resulted in the death of one person.
The second safety alert, issued in March 2020, was regarding FMT
produced from improperly tested donor stools from a stool bank which
resulted in several hospitalizations and two deaths. A safety alert in late March 2020, was due to concerns of transmission of COVID-19 in donor stool.
In November 2022, the AU Therapeutic Goods Administration approved faecal microbiota under the brand name Biomictra, and the US FDA approved a specific C. difficile fecal microbiota treatment under the brand name Rebyota, administered rectally. In April 2023, the FDA approved a live spore capsule that can be taken by mouth, under the brand name Vowst.
Across Europe, numerous stool banks have emerged to serve the increasing demand. While consensus exists, standard operation procedures still differ. Institutions in the Netherlands have published their protocols for managing FMT, and in Denmark institutions manages FMT according to the European Tissue and Cell directive.
Names
Previous terms for the procedure include fecal bacteriotherapy, fecal transfusion, fecal transplant, stool transplant, fecal enema, and human probiotic infusion (HPI).
Because the procedure involves the complete restoration of the entire
fecal microbiota, not just a single agent or combination of agents,
these terms have been replaced by the term fecal microbiota transplantation.
Research
Cultured intestinal bacteria are being studied as an alternative to fecal microbiota transplant.
One example is the rectal bacteriotherapy (RBT), developed by Tvede and
Helms, containing 12 individually cultured strains of anaerobic and
aerobic bacteria originating from healthy human faeces.
Research has also been done to identify the most relevant microbes
within fecal transplants, which could then be isolated and manufactured
via industrial fermentation;
such standardized products would be more scalable, would reduce the
risk of infections from unwanted microbes, and would improve the
scientific study of the approach, since the same substance would be
administered each time.
Veterinary use
Elephants, hippos, koalas, and pandas
are born with sterile intestines, and to digest vegetation need
bacteria which they obtain by eating their mothers' feces, a practice
termed coprophagia. Other animals eat dung.
In veterinary medicine
fecal microbiota transplant has been known as "transfaunation" and is
used to treat ruminating animals, like cows and sheep, by feeding rumen
contents of a healthy animal to another individual of the same species
in order to colonize its gastrointestinal tract with normal bacteria.
Research concerning the relationship between the thermodynamic quantity entropy and both the origin and evolution of life began around the turn of the 20th century. In 1910 American historian Henry Adams printed and distributed to university libraries and history professors the small volume A Letter to American Teachers of History proposing a theory of history based on the second law of thermodynamics and on the principle of entropy.
The 1944 book What is Life? by Nobel-laureate physicist Erwin Schrödinger stimulated further research in the field. In his book, Schrödinger originally stated that life feeds on negative entropy, or negentropy
as it is sometimes called, but in a later edition corrected himself in
response to complaints and stated that the true source is free energy. More recent work has restricted the discussion to Gibbs free energy
because biological processes on Earth normally occur at a constant
temperature and pressure, such as in the atmosphere or at the bottom of
the ocean, but not across both over short periods of time for individual
organisms. The quantitative application of entropy balances and Gibbs
energy considerations to individual cells is one of the underlying
principles of growth and metabolism.
Ideas about the relationship between entropy and living organisms
have inspired hypotheses and speculations in many contexts, including
psychology, information theory, the origin of life, and the possibility of extraterrestrial life.
Early views
In 1863 Rudolf Clausius published his noted memoir On the Concentration of Rays of Heat and Light, and on the Limits of Its Action, wherein he outlined a preliminary relationship, based on his own work and that of William Thomson (Lord Kelvin), between living processes and his newly developed concept of entropy. Building on this, one of the first to speculate on a possible thermodynamic perspective of organic evolution was the Austrian physicist Ludwig Boltzmann. In 1875, building on the works of Clausius and Kelvin, Boltzmann reasoned:
The general struggle for existence
of animate beings is not a struggle for raw materials – these, for
organisms, are air, water and soil, all abundantly available – nor for
energy which exists in plenty in any body in the form of heat, but a struggle for [negative] entropy, which becomes available through the transition of energy from the hot sun to the cold earth.
In 1876 American civil engineer Richard Sears McCulloh, in his Treatise on the Mechanical Theory of Heat and its Application to the Steam-Engine,
which was an early thermodynamics textbook, states, after speaking
about the laws of the physical world, that "there are none that are
established on a firmer basis than the two general propositions of Joule and Carnot;
which constitute the fundamental laws of our subject." McCulloh then
goes on to show that these two laws may be combined in a single
expression as follows:
When we reflect how generally
physical phenomena are connected with thermal changes and relations, it
at once becomes obvious that there are few, if any, branches of natural science
which are not more or less dependent upon the great truths under
consideration. Nor should it, therefore, be a matter of surprise that
already, in the short space of time, not yet one generation, elapsed
since the mechanical theory of heat has been freely adopted, whole branches of physical science have been revolutionized by it.
McCulloh gives a few of what he calls the "more interesting examples"
of the application of these laws in extent and utility. His first
example is physiology, wherein he states that "the body of an animal, not less than a steamer, or a locomotive, is truly a heat engine, and the consumption of food in the one is precisely analogous to the burning of fuel in the other; in both, the chemical process is the same: that called combustion." He then incorporates a discussion of Antoine Lavoisier's
theory of respiration with cycles of digestion, excretion, and
perspiration, but then contradicts Lavoisier with recent findings, such
as internal heat generated by friction, according to the new theory of heat,
which, according to McCulloh, states that the "heat of the body
generally and uniformly is diffused instead of being concentrated in the
chest". McCulloh then gives an example of the second law, where he
states that friction, especially in the smaller blood vessels, must
develop heat. Undoubtedly, some fraction of the heat generated by
animals is produced in this way. He then asks: "but whence the
expenditure of energy causing that friction, and which must be itself
accounted for?"
To answer this question he turns to the mechanical theory of heat
and goes on to loosely outline how the heart is what he calls a
"force-pump", which receives blood and sends it to every part of the
body, as discovered by William Harvey,
and which "acts like the piston of an engine and is dependent upon and
consequently due to the cycle of nutrition and excretion which sustains
physical or organic life". It is likely that McCulloh modeled parts of
this argument on that of the famous Carnot cycle. In conclusion, he summarizes his first and second law argument as such:
Everything physical being subject to the law of conservation of energy,
it follows that no physiological action can take place except with
expenditure of energy derived from food; also, that an animal performing
mechanical work must from the same quantity of food generate less heat than one abstaining from exertion, the difference being precisely the heat equivalent of that of work.
Negative entropy
In the 1944 book What is Life?, Austrian physicist Erwin Schrödinger, who in 1933 had won the Nobel Prize in Physics, theorized that life – contrary to the general tendency dictated by the second law of thermodynamics,
which states that the entropy of an isolated system tends to increase –
decreases or keeps constant its entropy by feeding on negative entropy. The problem of organization in living systems increasing despite the second law is known as the Schrödinger paradox. In his note to Chapter 6 of What is Life?, however, Schrödinger remarks on his usage of the term negative entropy:
Let me say first, that if I had been catering for them [physicists] alone I should have let the discussion turn on free energy instead. It is the more familiar notion in this context. But this highly technical term seemed linguistically too near to energy for making the average reader alive to the contrast between the two things.
This, Schrödinger argues, is what differentiates life from other forms of the organization of matter.
In this direction, although life's dynamics may be argued to go against
the tendency of the second law, life does not in any way conflict with
or invalidate this law, because the principle that entropy can only
increase or remain constant applies only to a closed system
which is adiabatically isolated, meaning no heat can enter or leave,
and the physical and chemical processes which make life possible do not
occur in adiabatic isolation, i.e. living systems are open systems.
Whenever a system can exchange either heat or matter with its
environment, an entropy decrease of that system is entirely compatible
with the second law.
Schrödinger asked the question: "How does the living organism
avoid decay?" The obvious answer is: "By eating, drinking, breathing and
(in the case of plants) assimilating." While energy from nutrients is
necessary to sustain an organism's order, Schrödinger also presciently
postulated the existence of other molecules equally necessary for
creating the order observed in living organisms: "An organism's
astonishing gift of concentrating a stream of order on itself and thus
escaping the decay into atomic chaos – of drinking orderliness from a
suitable environment – seems to be connected with the presence of the
aperiodic solids..." We now know that this "aperiodic" crystal is DNA,
and that its irregular arrangement is a form of information. "The DNA
in the cell nucleus contains the master copy of the software, in
duplicate. This software seems to control by specifying an algorithm, or
set of instructions, for creating and maintaining the entire organism
containing the cell."
DNA and other macromolecules
determine an organism's life cycle: birth, growth, maturity, decline,
and death. Nutrition is necessary but not sufficient to account for
growth in size, as genetics
is the governing factor. At some point, virtually all organisms
normally decline and die even while remaining in environments that
contain sufficient nutrients to sustain life. The controlling factor
must be internal and not nutrients or sunlight acting as causal
exogenous variables. Organisms inherit the ability to create unique and
complex biological structures; it is unlikely for those capabilities to
be reinvented or to be taught to each generation. Therefore, DNA must be
operative as the prime cause in this characteristic as well. Applying
Boltzmann's perspective of the second law, the change of state from a
more probable, less ordered, and higher entropy arrangement to one of
less probability, more order, and lower entropy (as is seen in
biological ordering) calls for a function like that known of DNA. DNA's
apparent information-processing function provides a resolution of the
Schrödinger paradox posed by life and the entropy requirement of the
second law.
Gibbs free energy and biological evolution
In recent years, the thermodynamic interpretation of evolution in relation to entropy has begun to use the concept of the Gibbs free energy, rather than entropy.
This is because biological processes on Earth take place at roughly
constant temperature and pressure, a situation in which the Gibbs free
energy is an especially useful way to express the second law of
thermodynamics. The Gibbs free energy is given by:
and exergy
and Gibbs free energy are equivalent if the environment and system
temperature are equivalent. Otherwise, Gibbs free energy will be less
than the exergy (for systems with temperatures above ambient). The
minimization of the Gibbs free energy is a form of the principle of minimum energy (minimum 'free' energy or exergy), which follows from the entropy maximization principle for closed systems. Moreover, the Gibbs free energy equation, in modified form, can be used for open systems, including situations where chemical potential terms are included in the energy balance equation. In a popular 1982 textbook, Principles of Biochemistry, noted American biochemistAlbert Lehninger
argued that the order produced within cells as they grow and divide is
more than compensated for by the disorder they create in their
surroundings in the course of growth and division. In short, according
to Lehninger, "Living organisms preserve their internal order by taking
from their surroundings free energy, in the form of nutrients or sunlight, and returning to their surroundings an equal amount of energy as heat and entropy."
Similarly, according to the chemist John Avery, from his 2003 book Information Theory and Evolution,
we find a presentation in which the phenomenon of life, including its
origin and evolution, as well as human cultural evolution, has its basis
in the background of thermodynamics, statistical mechanics, and information theory.
The (apparent) paradox between the second law of thermodynamics and the
high degree of order and complexity produced by living systems,
according to Avery, has its resolution "in the information content of
the Gibbs free energy that enters the biosphere from outside sources."
In a study titled "Natural selection for least action" published in the Proceedings of the Royal Society A., Ville Kaila and Arto Annila of the University of Helsinki describe how the process of natural selection
responsible for such local increase in order may be mathematically
derived directly from the expression of the second law equation for
connected non-equilibrium open systems. The second law of thermodynamics
can be written as an equation of motion to describe evolution, showing
how natural selection and the principle of least action can be connected
by expressing natural selection in terms of chemical thermodynamics. In
this view, evolution explores possible paths to level differences in
energy densities and so increase entropy most rapidly. Thus, an organism
serves as an energy transfer mechanism, and beneficial mutations allow
successive organisms to transfer more energy within their environment.
Counteracting the second law tendency
Second-law
analysis is valuable in scientific and engineering analysis in that it
provides a number of benefits over energy analysis alone, including the
basis for determining energy quality (or exergy content),
understanding fundamental physical phenomena, improving performance
evaluation and optimization, or in furthering our understanding of
living systems.
The second law describes a universal tendency towards disorder
and uniformity, or internal and external equilibrium. This means that
real, non-ideal processes cause entropy production. Entropy can also be
transferred to or from a system as well by the flow or transfer of
matter and energy. As a result, entropy production does not necessarily
cause the entropy of the system to increase. In fact the entropy or
disorder in a system can spontaneously decrease, such as an aircraft gas
turbine engine cooling down after shutdown, or like water in a cup left
outside in sub-freezing winter temperatures. In the latter, a
relatively unordered liquid cools and spontaneously freezes into a
crystalized structure of reduced disorder as the molecules ‘stick’
together. Although the entropy of the system decreases, the system
approaches uniformity with, or becomes more thermodynamically similar to
its surroundings.
This is a category III process, referring to the four combinations of
either entropy (S) up or down, and uniformity (Y) - between system and
its environment – either up or down.
Four categories of processes given entropy up or down and uniformity up or down
The second law can be conceptually stated
as follows: Matter and energy have the tendency to reach a state of
uniformity or internal and external equilibrium, a state of maximum
disorder (entropy). Real non-equilibrium processes always produce
entropy, causing increased disorder in the universe, while idealized
reversible processes produce no entropy and no process is known to exist
that destroys entropy. The tendency of a system to approach uniformity
may be counteracted, and the system may become more ordered or complex,
by the combination of two things, a work or exergy source and some form
of instruction or intelligence. Where ‘exergy’ is the thermal,
mechanical, electric or chemical work potential of an energy source or
flow, and ‘instruction or intelligence’, is understood in the context
of, or characterized by, the set of processes that are within category
IV.
Consider as an example of a category IV process, robotic
manufacturing and assembly of vehicles in a factory. The robotic
machinery requires electrical work input and instructions, but when
completed, the manufactured products have less uniformity with their
surroundings, or more complexity (higher order) relative to the raw
materials they were made from. Thus, system entropy or disorder
decreases while the tendency towards uniformity between the system and
its environment is counteracted. In this example, the instructions, as
well as the source of work may be internal or external to the system,
and they may or may not cross the system boundary. To illustrate, the
instructions may be pre-coded and the electrical work may be stored in
an energy storage system on-site. Alternatively, the control of the
machinery may be by remote operation over a communications network,
while the electric work is supplied to the factory from the local
electric grid. In addition, humans may directly play, in whole or in
part, the role that the robotic machinery plays in manufacturing. In
this case, instructions may be involved, but intelligence is either
directly responsible, or indirectly responsible, for the direction or
application of work in such a way as to counteract the tendency towards
disorder and uniformity.
As another example, consider the refrigeration of water in a warm
environment. Due to refrigeration, heat is extracted or forced to flow
from the water. As a result, the temperature and entropy of the water
decreases, and the system moves further away from uniformity with its
warm surroundings. The important point is that refrigeration not only
requires a source of work, it requires designed equipment, as well as
pre-coded or direct operational intelligence or instructions to achieve
the desired refrigeration effect.
Hummingbird in flight.
Observation is the basis for the understanding that category IV
processes require both a source of exergy as well as a source or form of
intelligence or instruction. With respect to living systems, sunlight
provides the source of exergy for virtually all life on Earth, i.e.
sunlight directly (for flora) or indirectly in food (for fauna). Note
that the work potential or exergy of sunlight, with a certain spectral
and directional distribution, will have a specific value
that can be expressed as a percentage of the energy flow or exergy
content. Like the Earth as a whole, living things use this energy,
converting the energy to other forms (the first law), while producing
entropy (the second law), and thereby degrading the exergy or quality of
the energy. Sustaining life, or the growth of a seed, for example,
requires continual arranging of atoms and molecules into elaborate
assemblies required to duplicate living cells. This assembly in living
organisms decreases uniformity and disorder, counteracting the universal
tendency towards disorder and uniformity described by the second law.
In addition to a high quality energy source, counteracting this tendency
requires a form of instruction or intelligence, which is contained
primarily in the DNA/RNA.
In the absence of instruction or intelligence, high quality
energy is not enough on its own to produce complex assemblies, such as a
house. As an example of category I in contrast to IV, although having a
lot of energy or exergy, a second tornado will never re-construct a
town destroyed by a previous tornado, instead it increases disorder and
uniformity (category I), the very tendency described by the second law. A
related line of reasoning is that, even though improbable, over
billions of years or trillions of chances, did life come about
undirected, from non-living matter in the absence of any intelligence?
Related questions someone can ask include; can humans with a supply of
food (exergy) live without DNA/RNA, or can a house supplied with
electricity be built in the forest without humans or a source of
instruction or programming, or can a fridge run with electricity but
without its functioning computer control boards?
The second law guarantees, that if we build a house it will, over
time, have the tendency to fall apart or tend towards a state of
disorder. On the other hand, if on walking through a forest we discover a
house, we likely conclude that somebody built it, rather than
concluding the order came about randomly. We know that living systems,
such as the structure and function of a living cell, or the process of
protein assembly/folding, are exceedingly complex. Could life have come
about without being directed by a source of intelligence – consequently,
over time, resulting in such things as the human brain and its
intelligence, computers, cities, the quality of love and the creation of
music or fine art? The second law tendency towards disorder and
uniformity, and the distinction of category IV processes as
counteracting this natural tendency, offers valuable insight for us to consider in our search to answer these questions.
Entropy of individual cells
Entropy balancing
An entropy balance for an open system, or the change in entropy over time for a system at steady state, can be written as:
Assuming a steady state system, roughly stable pressure-temperature conditions, and exchange through cell surfaces only, this expression can be rewritten to express entropy balance for an individual cell as:
Where
heat exchange with the environment
partial molar entropy of metabolite B
partial molar entropy of structures resulting from growth
rate of entropy production
and terms indicate rates of exchange with the environment.
This equation can be adapted to describe the entropy balance of a
cell, which is useful in reconciling the spontaneity of cell growth
with the intuition that the development of complex structures must
overall decrease entropy within the cell. From the second law, ; due to internal organization resulting from growth, will be small. Metabolic
processes force the sum of the remaining two terms to be less than zero
through either a large rate of heat transfer or the export of high
entropy waste products.
Both mechanisms prevent excess entropy from building up inside the
growing cell; the latter is what Schrödinger described as feeding on
negative entropy, or "negentropy".
Implications for metabolism
In fact it is possible for this "negentropy" contribution to be large enough that growth is fully endothermic,
or actually removes heat from the environment. This type of metabolism,
in which acetate, methanol, or a number of other hydrocarbon compounds
are converted to methane (a high entropy gas), is known as acetoclastic methanogenesis; one example is the metabolism of the anaerobic archaebacteria Methanosarcina barkeri. At the opposite extreme is the metabolism of anaerobicthermophile archaebacteria Methanobacterium thermoautotrophicum, for which the heat exported into the environment through fixation is high (~3730 kJ/C-mol).
Generally, in metabolic processes, spontaneous catabolic processes that break down biomolecules provide the energy to drive non-spontaneous anabolic reactions that build organized biomass from high entropy reactants.
Therefore, biomass yield is determined by the balance between coupled
catabolic and anabolic processes, where the relationship between these
processes can be described by:
where
total reaction driving force/ overall molar Gibbs energy
biomass produced
Gibbs energy of catabolic reactions (-)
Gibbs energy of anabolic reactions (+)
Organisms must maintain some optimal balance between and to both avoid thermodynamic equilibrium (),
at which biomass production would be theoretically maximized but
metabolism would proceed at an infinitely slow rate, and the opposite
limiting case at which growth is highly favorable (),
but biomass yields are prohibitively low. This relationship is best
described in general terms, and will vary widely from organism to
organism. Because the terms corresponding to catabolic and anabolic
contributions would be roughly balanced in the former scenario, this
case represents the maximum amount of organized matter that can be
produced in accordance with the 2nd law of thermodynamics for a very
generalized metabolic system.
Entropy and the origin of life
The second law of thermodynamics applied to the origin of life
is a far more complicated issue than the further development of life,
since there is no "standard model" of how the first biological lifeforms
emerged, only a number of competing hypotheses. The problem is
discussed within the context of abiogenesis, implying gradual pre-Darwinian chemical evolution.
Relationship to prebiotic chemistry
In 1924 Alexander Oparin suggested that sufficient energy for generating early life forms from non-living molecules was provided in a "primordial soup".
The laws of thermodynamics impose some constraints on the earliest
life-sustaining reactions that would have emerged and evolved from such a
mixture. Essentially, to remain consistent with the second law of
thermodynamics, self organizing systems that are characterized by lower
entropy values than equilibrium must dissipate energy so as to increase
entropy in the external environment.
One consequence of this is that low entropy or high chemical potential
chemical intermediates cannot build up to very high levels if the
reaction leading to their formation is not coupled to another chemical
reaction that releases energy. These reactions often take the form of redox couples, which must have been provided by the environment at the time of the origin of life. In today's biology, many of these reactions require catalysts (or enzymes)
to proceed, which frequently contain transition metals. This means
identifying both redox couples and metals that are readily available in a
given candidate environment for abiogenesis is an important aspect of
prebiotic chemistry.
The idea that processes that can occur naturally in the
environment and act to locally decrease entropy must be identified has
been applied in examinations of phosphate's role in the origin of life,
where the relevant setting for abiogenesis is an early Earth lake
environment. One such process is the ability of phosphate to concentrate
reactants selectively due to its localized negative charge.
In the context of the alkaline hydrothermal vent (AHV) hypothesis
for the origin of life, a framing of lifeforms as "entropy generators"
has been suggested in an attempt to develop a framework for abiogenesis
under alkaline deep sea conditions. Assuming life develops rapidly under
certain conditions, experiments may be able to recreate the first
metabolic pathway, as it would be the most energetically favorable and
therefore likely to occur. In this case, iron sulfide compounds may have
acted as the first catalysts.
Therefore, within the larger framing of life as free energy converters,
it would eventually be beneficial to characterize quantities such as
entropy production and proton gradient dissipation rates quantitatively for origin of life relevant systems (particularly AHVs).
Other theories
The
evolution of order, manifested as biological complexity, in living
systems and the generation of order in certain non-living systems was
proposed to obey a common fundamental principal called "the Darwinian
dynamic".
The Darwinian dynamic was formulated by first considering how
microscopic order is generated in relatively simple non-biological
systems that are far from thermodynamic equilibrium (e.g. tornadoes, hurricanes). Consideration was then extended to short, replicating RNA molecules assumed to be similar to the earliest forms of life in the RNA world.
It was shown that the underlying order-generating processes in the
non-biological systems and in replicating RNA are basically similar.
This approach helps clarify the relationship of thermodynamics to
evolution as well as the empirical content of Darwin's theory.
In 2009 physicist Karo Michaelian published a thermodynamic dissipation theory for the origin of life
in which the fundamental molecules of life; nucleic acids, amino acids,
carbohydrates (sugars), and lipids are considered to have been
originally produced as microscopic dissipative structures (through
Prigogine's dissipative structuring)
as pigments at the ocean surface to absorb and dissipate into heat the
UVC flux of solar light arriving at Earth's surface during the Archean,
just as do organic pigments in the visible region today. These UVC
pigments were formed through photochemical dissipative structuring from
more common and simpler precursor molecules like HCN and H2O under the UVC flux of solar light. The thermodynamic function of the original pigments (fundamental molecules of life) was to increase the entropy production
of the incipient biosphere under the solar photon flux and this, in
fact, remains as the most important thermodynamic function of the
biosphere today, but now mainly in the visible region where photon
intensities are higher and biosynthetic pathways are more complex,
allowing pigments to be synthesized from lower energy visible light
instead of UVC light which no longer reaches Earth's surface.
Jeremy England developed a hypothesis of the physics of the origins of life, that he calls 'dissipation-driven adaptation'.
The hypothesis holds that random groups of molecules can self-organize
to more efficiently absorb and dissipate heat from the environment. His
hypothesis states that such self-organizing systems are an inherent part
of the physical world.
Other types of entropy and their use in defining life
Like a thermodynamic system, an information system has an analogous concept to entropy called information entropy.
Here, entropy is a measure of the increase or decrease in the novelty
of information. Path flows of novel information show a familiar pattern.
They tend to increase or decrease the number of possible outcomes in
the same way that measures of thermodynamic entropy increase or decrease
the state space. Like thermodynamic entropy, information entropy uses a
logarithmic scale: –P(x) log P(x), where P is the probability of some
outcome x. Reductions in information entropy are associated with a smaller number of possible outcomes in the information system.
In 1984 Brooks and Wiley introduced the concept of species
entropy as a measure of the sum of entropy reduction within species
populations in relation to free energy in the environment.
Brooks-Wiley entropy looks at three categories of entropy changes:
information, cohesion and metabolism. Information entropy here measures
the efficiency of the genetic information in recording all the potential
combinations of heredity which are present. Cohesion entropy looks at
the sexual linkages within a population. Metabolic entropy is the
familiar chemical entropy used to compare the population to its
ecosystem. The sum of these three is a measure of nonequilibrium entropy
that drives evolution at the population level.
A 2022 article by Helman in Acta Biotheoretica suggests
identifying a divergence measure of these three types of entropies:
thermodynamic entropy, information entropy and species entropy. Where these three are overdetermined, there will be a formal freedom that arises similar to how chirality
arises from a minimum number of dimensions. Once there are at least
four points for atoms, for example, in a molecule that has a central
atom, left and right enantiomers are possible. By analogy, once a
threshold of overdetermination in entropy is reached in living systems,
there will be an internal state space that allows for ordering of
systems operations. That internal ordering process is a threshold for
distinguishing living from nonliving systems.
Entropy and the search for extraterrestrial life
In 1964 James Lovelock was among a group of scientists requested by NASA to make a theoretical life-detection system to look for life on Mars during the upcoming Viking missions.
A significant challenge was determining how to construct a test that
would reveal the presence of extraterrestrial life with significant
differences from biology as we know it. In considering this problem,
Lovelock asked two questions: "How can we be sure that the Martian way
of life, if any, will reveal itself to tests based on Earth's life
style?", as well as the more challenging underlying question: "What is
life, and how should it be recognized?"
Because these ideas conflicted with more traditional approaches
that assume biological signatures on other planets would look much like
they do on Earth, in discussing this issue with some of his colleagues
at the Jet Propulsion Laboratory,
he was asked what he would do to look for life on Mars instead. To
this, Lovelock replied "I'd look for an entropy reduction, since this
must be a general characteristic of life." This idea was perhaps better
phrased as a search for sustained chemical disequilibria associated with
low entropy states resulting from biological processes, and through
further collaboration developed into the hypothesis that biosignatures
would be detectable through examining atmospheric compositions. Lovelock
determined through studying the atmosphere of Earth that this metric
would indeed have the potential to reveal the presence of life. This had
the consequence of indicating that Mars was most likely lifeless, as
its atmosphere lacks any such anomalous signature.
This work has been extended recently as a basis for biosignature
detection in exoplanetary atmospheres. Essentially, the detection of
multiple gases that are not typically in stable equilibrium with one
another in a planetary atmosphere may indicate biotic production of one
or more of them, in a way that does not require assumptions about the
exact biochemical reactions extraterrestrial life might use or the
specific products that would result. A terrestrial example is the
coexistence of methane and oxygen, both of which would eventually
deplete if not for continuous biogenic production. The amount of
disequilibrium can be described by differencing observed and equilibrium
state Gibbs energies
for a given atmosphere composition; it can be shown that this quantity
has been directly affected by the presence of life throughout Earth's
history.
Imaging of exoplanets by future ground and space based telescopes will
provide observational constraints on exoplanet atmosphere compositions,
to which this approach could be applied.
But there is a caveat related to the potential for chemical
disequilibria to serve as an anti-biosignature depending on the context.
In fact, there was probably a strong chemical disequilibrium present on
early Earth before the origin of life due to a combination of the
products of sustained volcanic outgassing and oceanic water vapor. In
this case, the disequilibrium was the result of a lack of organisms
present to metabolize the resulting compounds. This imbalance would
actually be decreased by the presence of chemotrophic
life, which would remove these atmospheric gases and create more
thermodynamic equilibrium prior to the advent of photosynthetic
ecosystems.
In 2013 Azua-Bustos and Vega argued that, disregarding the types
of lifeforms that might be envisioned both on Earth and elsewhere in the
Universe, all should share in common the attribute of decreasing their
internal entropy at the expense of free energy obtained from their
surroundings. As entropy allows the quantification of the degree of
disorder in a system, any envisioned lifeform must have a higher degree
of order than its immediate supporting environment. These authors showed
that by using fractal mathematics analysis alone, they could readily
quantify the degree of structural complexity difference (and thus
entropy) of living processes as distinct entities separate from their
similar abiotic surroundings. This approach may allow the future
detection of unknown forms of life both in the Solar System and on
recently discovered exoplanets based on nothing more than entropy
differentials of complementary datasets (morphology, coloration,
temperature, pH, isotopic composition, etc.).
Entropy in psychology
The notion of entropy as disorder has been transferred from thermodynamics to psychology by Polish psychiatristAntoni Kępiński, who admitted being inspired by Erwin Schrödinger. In his theoretical framework devised to explain mental disorders (the information metabolism
theory), the difference between living organisms and other systems was
explained as the ability to maintain order. Contrary to inanimate
matter, organisms maintain the particular order of their bodily
structures and inner worlds which they impose onto their surroundings
and forward to new generations. The life of an organism or the species ceases as soon as it loses that ability.
Maintenance of that order requires continual exchange of information
between the organism and its surroundings. In higher organisms,
information is acquired mainly through sensory receptors and metabolised in the nervous system. The result is action – some form of motion, for example locomotion, speech, internal motion of organs, secretion of hormones, etc. The reactions of one organism become an informational signal to other organisms. Information metabolism,
which allows living systems to maintain the order, is possible only if a
hierarchy of value exists, as the signals coming to the organism must
be structured. In humans that hierarchy has three levels, i.e.
biological, emotional, and sociocultural.
Kępiński explained how various mental disorders are caused by
distortions of that hierarchy, and that the return to mental health is
possible through its restoration.
The idea was continued by Struzik, who proposed that Kępiński's information metabolism theory may be seen as an extension of Léon Brillouin's negentropy principle of information. In 2011, the notion of "psychological entropy" was reintroduced to psychologists by Hirsh et al. Similarly to Kępiński, these authors noted that uncertainty management is a critical ability for any organism. Uncertainty, arising due to the conflict between competing perceptual and behavioralaffordances, is experienced subjectively as anxiety. Hirsh and his collaborators proposed that both the perceptual and behavioral domains may be conceptualized as probability distributions and that the amount of uncertainty associated with a given perceptual or behavioral experience can be quantified in terms of Claude Shannon's entropy formula.
Objections
Entropy is well defined for equilibrium systems, so objections to the
extension of the second law and of entropy to biological systems,
especially as it pertains to its use to support or discredit the theory
of evolution, have been stated. Living systems and indeed many other systems and processes in the universe operate far from equilibrium.
However, entropy is well defined much more broadly based on the probabilities
of a system's states, whether or not the system is a dynamic one (for
which equilibrium could be relevant). Even in those physical systems
where equilibrium could be relevant, (1) living systems cannot persist
in isolation, and (2) the second principle of thermodynamics does not
require that free energy be transformed into entropy along the shortest
path: living organisms absorb energy from sunlight or from energy-rich
chemical compounds and finally return part of such energy to the
environment as entropy (generally in the form of heat and low
free-energy compounds such as water and carbon dioxide).
The Belgian scientist Ilya Prigogine
has, throughout all his research, contributed to this line of study and
attempted to solve those conceptual limits, winning the Nobel prize in
1977. One of his major contributions was the concept of the dissipative system, which describes the thermodynamics of open systems in non-equilibrium states.