The Effect of Indole Production on the Growth of Escherichia coli when Co-Cultured with Enterococcus faecalis
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In nature, bacteria live in dynamic communities surrounded by a vast number of other bacterial species. Recent studies indicate that one mechanism by which Escherichia coli thrives within such a multitude is via production of the molecule indole. Evidence indicates that indole thwarts the quorum sensing system of acyl-homoserine lactone (AHL) producing bacteria such as Pseudomonas aeruginosa, Chromobacterium violaceum, and Pseudomonas aureofaciens. Impeding the signaling system of these bacteria ultimately leads to a decrease in toxic secretions such as pyocyanin and proteases. The aim of this research was to determine if the production of indole by E. coli is a general mechanism by which it competes in mixed culture. To do this, the effect of indole on the growth of E. coli in mixed culture with Enterococcus faecalis was studied. E. faecalis is a Gram-positive, non-AHL producing bacteria found alongside E. coli as normal flora in the human intestine. E. faecalis has increasingly become a concern as it is now a leading cause of hospital-acquired infection and has developed resistance to “last- line” antibiotics such as vancomycin. Colony counts and turbidity of ΔtnaA (the E. coli mutant incapable of degrading tryptophan and thus deficient in indole production) were measured in mixed culture with E. faecalis. Indole was then reintroduced at physiologically relevant concentrations and its effect was measured. Contrary to previous research, in competition with E. faecalis the population size of E. coli is inhibited and indole has a further inhibitory effect.