Wednesday, June 6, 2018


Post # 20
Donald A. Windsor

An interaction between a parasite and a host is like a chemical reaction.

Parasite + Host ==> ParasiteHost complex P + H ==> PH

A biobroker acts as a catalyst. P + H == biobroker ==> PH

This analogy helps me to understand my basic amazement at the complexities of biology. Nature seems to be a giant nexus of multiple species so tightly bound together that it resembles sticky cotton candy on a hot humid day.

Consequently, I visualize interactions between two species as involving multiple species. Species interactions are not between species, but among species. Many of these interactions have participants that operate behind the scenes.

Some of the behind the scenes mechanisms of biobrokers are especially interesting. A recent article reports that Bacteroides fragilis attaches itself to the gut epithelium of mice by adhering to the mouse’s immunoglobulin A. The bacterium then rules by excluding invasive pathogenic microbes (1). It acts as a gatekeeper, or in my view, a biobroker. The presence or absence of this bacterial species determines whether or not a pathogenic bacterial species (a parasite) can or cannot infect this mouse and use it as a host. The host species has retained Bacteroides fragilis to exclude parasites and to allow commensals.

                                       / ==> P + H Interaction prohibited; parasite excluded.
C + P + H == biobroker
                                       \ ==> CH Interaction accepted; commensal allowed.

To infect this host a parasite would have to overcome the biobroker.

P + H === biobroker ===> PH Interaction overcame biobroker.

In metazoan parasites and hosts this biobroker role could be played by competitive parasites.

References cited:

1. Donaldson, G.P. ; et al Gut microbiota utilize immunoglobulin A for mucosal colonization. Science 2018 May 18; 360(6390): 795-800.


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