Sunday, January 17, 2016

Interaction among microorganisms controls virulence

Adapting Koch’s postulates Criteria for disease causation must take microbial interactions into account

Koch proposed three postulates regarding disease causation by an infecting organism. First, the microorganism occurs in every case of the disease; second, it is not found in healthy organisms; and third, after the microorganism has been isolated from a diseased organism and propagated in pure culture, the proposed pathogen can induce disease anew. The often-cited fourth postulate (not from Koch himself) is that the microorganism must then be re-isolated from the experimentally infected host.

These authors point out that these postulates cannot be applied in their strictest form because of the need to include the role of microbial communities in disease pathogenesis. For example in the gut, C. scindens modifies endogenous bile acids so that they inhibit C. difficile growth and certain strains of E. coli can exert a protective effect against Salmonella by outcompeting pathogens for iron acquisition.

Comment: An interesting thought is that because of these interactions, culturing techniques may not detect all the organisms present. It has been observed that certain ‘obviously infected’ cases are culture negative. When sequencing is used along with culturing, all organisms present in a sample can be observed without the constraints imposed by culture requirements alone.

In the world of failed shoulder arthroplasty, we are still working to understand the interactions between two commonly cultured species: Propionibacterium and coagulase negative Staphylococcus that (as is the case for the Jekyll and Hyde duality) can be benign commensal organisms or pathogens. One approach is to seek ‘virulence factors’ that may distinguish the ‘good bugs’ from the ‘bad bugs’. However, it may be at least equally important to use sequencing data from harvested specimens to explore the possibility that virulence may relate to the nature of the microbial community into which the potential pathogen is introduced.

Accepting the abundance of data showing that Propionibacterium are commonly introduced to the surgical wound at the time of shoulder arthroplasty, perhaps we should seek to understand why the rate of periprosthetic infections with this organism is as low as it appears to be. Might certain commensal organisms have a probiotic effect? Might it be possible to use probiotic organisms to repopulate the skin of individuals determined to be at high risk for Propionibacterium infection? Additionally intriguing is the possibility that the fungal, yeast (vis the use of Saccharomyces boulardii in gut probiotics) and viral (phage) components of the microbial communities may play an important role. Stay tuned.

This article was called to my attention by
Frederick "Erick" Matsen, Associate Member
Fred Hutchinson Cancer Research Center

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