Koch’s postulates seem to be an attempt to formalise to some degree the scientific investigation of the relationship between microbes and the effect they may or may not have on living organisms. There are many pitfalls in attempting to formalise the scientific process and these postulates serve as a good example of a few of them.
The postulates themselves were formulated with respect to the scientific knowledge of the time. The term ‘microorganism’ obviously means ‘bacterium’ and the term ‘isolation’ refers to established methods of optical microscopy. The postulates are intended to establish the pattern or otherwise of bacterial infection, multiplication and transmission.
Attempts to extend this methodology to purported ‘viral’ particles fails for lack of well-defined isolation procedures.
Claims that the postulates can determine causality are deeply flawed.
Establishment of ‘causation’
The postulates are said to be useful in establishing a ‘causal relationship’ between a suspected ‘pathogen’ and a ‘disease state’, but ‘causation’ itself is a term with no precise meaning. Wikipedia falls into this trap:
“Koch’s postulates are four criteria designed to establish a causal relationship between a microbe and a disease.” – Wikipedia
“Causality is an abstraction that indicates how the world progresses As such a basic concept, it is more apt as an explanation of other concepts of progression than as something to be explained by others more basic. The concept is like those of agency and efficacy For this reason, a leap of intuition may be needed to grasp it” – Wikipedia
So causation’ is a matter of intuition, with no precise definition and yet Wikipedia is claiming it as the raison d’être of the postulates; they exist in this form only to justify something less well-defined than themselves.
Thought-crime
This is obviously a major thought-crime and ought not be allowed in scientific circles.
The idea of causation here is completely superfluous. It would be simple and informative to say something like “Our experiment passed all of Koch’s postulates” or “One to five of the Bradford-Hill criteria were satisfied“, but there is no need to bring in the additional concept, from philosophy, of ‘causation’.
‘Causation’ was not observed, was not measured and was never defined in scientific terms – so why mention it? Is life not difficult enough as it is? No scientific training is needed to work this out.
What is happening is that scientists themselves still think in primitive and imprecise layman terms and despite all their terminology and discoveries, nevertheless want to reduce their scientific findings to something that is regarded as more ‘fundamental’ and ‘basic’.
Ideas such as causation, existence, reality, proof, these ideas are not fundamental, are not basic and do not belong here; they are an old coat that needs discarding. Assessment according to the postulates should be enough by itself or what is the point of them?
Adding unnecessary and ill defined concepts into a scientific framework do not an can not improve it in any way.
The postulates as a definition of causality
Better, then, would be to try and use the postulates as an actual definition of biological causality in the specific case of bacteria. At least then we know what words actually mean!
The current situation is that the well defined postulates of Koch which have been devised for a specific usage are being used to support a far more general and woolly concept that even philosophers can’t define properly. Why?
Better to refer to ‘Koch causation’ or ‘Bradford Hill causation’ as at least then the terms used have well-defined meanings and their inadequacies are clearly on display for all to see. The word ‘causality’ should not be used without some sort of qualification.
Policy makers and snake-oil salesmen will no doubt want to simplify the language to ‘this causes that’ or whatever, but that doesn’t mean that scientists are obliged to play the same game.
Koch’s postulates
- The microorganism must be found in abundance in all organisms suffering from the disease but should not be found in healthy organisms.
- The microorganism must be isolated from a diseased organism and grown in pure culture.
- The cultured microorganism should cause disease when introduced into a healthy organism.
- The microorganism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.
Causation .. again!
The postulates are to determine causation according to Wikipedia but clause 3 above says that microorganism can only be said to cause disease if .. the microorganism causes disease!
The definition is circular and therefore the postulates are not fit for purpose..
What is meant by “The cultured microorganism should cause disease when introduced into a healthy organism.”? How long do you have to wait for the disease to appear? How much of the microorganism is required and how is it introduced? What is meant by ‘healthy’? At what point has ‘causation’ failed?
Bacteria. Koch repeatedly refers to ‘finding’, ‘isolating’ and ‘introducing’ the organism and so it is clear that he is taking it for granted that these things are feasible and he is therefore referring to bacteria when he says ‘microorganisms’.
The postulates then are not a general description of how to determine whether or not some invisible agent is causing disease, but are a description of typical behaviour we might expect from bacteria if they were in fact capable of causing disease. We don’t see this behaviour and so the theory can be scrapped.
This is important as some are trying to apply Koch’s postulates to so called ‘viruses’ .They are finding that isolation in the normal sense of the word is not possible and are saying that viruses do not exist for this reason.
Virus ‘isolation’ techniques however are long and tortuous procedures involving purported genome sequencing. Koch clearly did not intend this meaning and so his postulates do not apply. Just because the same word, ‘isolation’ is used it doesn’t mean that the same reasoning applies.
Key assumptions:
- That the microorganism can exist in an isolated state
- That the microorganism in vivo is identical to that in vitro
- That the microorganism can be unambiguously characterised
- That the microorganism can be reliably cultured
- That the state of the host is irrelevant – that there is no ‘immunity’ or adaptation
- That after infection a disease will manifest in a predictable amount of time
- That diseases themselves are unambiguously characterised and identified
- That experiments are repeatable in a meaningful way
- That the microorganism will not mutate significantly during the experiment
- That the transmissible agent is the same as the disease-causing agent
There is a great deal of doubt concerning all of these statements
The ViroLIEgy website comments “As for the third postulate, insisting that the same exact disease is established when a healthy host is subjected to a pure source of the presumed pathogen is entirely rational and should be the expected outcome. If no disease is produced or different symptoms are observed, then the presumed pathogen can hardly be the cause of the specific disease being investigated. “
The author has had to use the phrases “disease is established” and “disease is produced” to avoid using the word “cause”. The postulates themselves use the phrase “should cause disease”.
World Health Organisation
“The 13 laboratories have been working on meeting Koch’s postulates, necessary to prove disease causation. These postulates stipulate that to be the causal agent, a pathogen must meet four conditions: it must be found in all cases of the disease, it must be isolated from the host and grown in pure culture, it must reproduce the original disease when introduced into a susceptible host, and it must be found in the experimental host so infected.”
The word ‘reproduce‘ here means ‘causes‘ – what else can it mean?
The point is that if you are reproducing (causing) the disease already then why did you need the other postulates? What is their meaning?
So what do the postulates mean?
They are, by their own description, intended to test for a specific pattern of association between bacteria and disease where the transmissible agent is easily identifiable outside of the host, is identified as the disease causing agent and will unfailingly cause disease in any organism to which it is introduced and, moreover, will multiply within that host.
The idea of ’cause’ is taken as a given within the postulates and therefore cannot be determined by them. The idea of isolation is also taken as a given and must be properly defined before assessment by the same postulates. If no sensible isolation is possible then Koch’s postulates cannot be applied.
To clarify, maybe, consider the ‘virion’ theory of Hoyle et al:
Viruses from space and other matters – Hoyle et al [here]
Hoyle, Wickramasinghe and Watkins conceived the idea that some particles they called ‘virions’ came from space at certain times of the year and were inhaled by the population. These particles would promote the production of the flu virus within the body and disease would follow.
The person to person transmission of influenza as a disease was ruled out by their data and so they did not consider that the virus could be passed between people but remained within the host organism.
If we suppose all this is true then we have causation by a microorganism but the whole thing fails Koch’s postulates whatever we try.
The primary cause of disease is the introduction of a ‘virion’ but we don’t know what they look like, they do not replicate within the body and cannot be isolated from a sick person. The virions will not be found in abundance and cannot be cultured; they only come from space. Maybe it is possible to isolate an actual virus but the re-introduction into a new host will not produce disease because in this scenario, the production of virus particles is triggered by the virions and not other viral particles.
Conclusions
Koch’s test fails with the original intended microorganisms and is unsuitable for what is claimed to be a virus because of the undefined isolation procedure.
The postulates are too restrictive for general investigative work since they necessarily assume a particular mode of infection and therefore rule out other patterns a priori. To somehow claim that they can be used to determine ‘causation’ when clause 3 specifically uses the word ‘causation’ is about as confused as it gets.
These postulates should be consigned to history.