Koch's postulates nowadays
Since their introduction, in the late 19th century, Koch's postulates have "evolved". Are they still valid today?
Last week in Science, Julie Segre and Allyson Byrd wrote a nice Perspective on how to adapt Koch's postulates in light of our increased understanding about what causes disease (the piece can be found here). Briefly, they discuss why the "1 pathogen = 1 disease" label may not always apply, particularly in light of the existence of colonization resistance factors or protective microbial communities, which may prevent or delay disease onset.
I really like this "microbial community" view of pathogenesis, and would even dare to add some other examples where community interactions are essential for disease, which would fall under a different "1 pathogen + 1 community = 1 disease" scenario. Several of these interactions are discussed in one of our recent reviews, where Marvin Whiteley and colleagues discuss "The biogeography of polymicrobial infection". Just to quote a few of these examples:
"For example, some infections require colonization with multiple interacting microorganisms (for example, Porphyromonas gingivalis and commensal oral microorganisms in periodontal disease), whereas other infections are modulated in severity by the presence of co-infecting species (for example, Pseudomonas aeruginosa and Staphylococcus aureus in chronic wound infections)."
I think we'll see many more of these "group effort" examples pop up in the near future, especially given the current focus on the microbiome and the increased interest in studying microbial interactions. I also think this doesn't eliminate the merits of Koch's postulates, although it means that they'll need to be constantly reassessed to keep up with scientific advances. So I couldn't agree more with what Stanley Falkow wrote more than a decade ago in our journal (here):
"The power of Koch’s postulates comes not from their rigid application, but from the spirit of scientific rigour that they foster. The proof of disease causation rests on the concordance of scientific evidence, and Koch’s postulates serve as guidelines for collecting this evidence”.
"The molecular Koch’s postulates were not intended to be anything more than a means to provide a basis of dialogue among interested investigators. (This dialogue) now takes on less of a phenotypic description based on only a few, often observational, criteria. (It) now centres increasingly on better defined biochemical mechanisms that are less equivocal".