There are at least two lines of communication that occur in the host; 1) bacterial cell-cell communication, i.e., quorum sensing, and 2) cross-kingdom host: bacterial communication which is exemplified by host-derived chemicals, principally hormones, that can act as substitutes for bacterial molecular autoinducer signals. These forms of chemical communication can affect phenotypic expression of virulence factors and susceptibility to antimicrobial agents. There is currently an extensive effort to design analogs for the homoserine-lactones, which comprise some of the known quorum autoinducers for Gram negative bacteria, as well as the small peptides and macrocyclic peptides involved in Gram positive bacterial communication. However, it has not been recognized until fairly recently that bacteria as well as fungal and protozoan pathogens exhibit phenotype alterations in response to host-derived hormones and other molecules, e.g., cholesterol. This cross-kingdom communication presents a particularly intriguing and difficult line of communication to intercept due to the apparent phylogenetic microbial origins of what are viewed as hormones in the mammalian host.
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