Indole Inhibits IncP-1 Conjugation System Mainly Through Promoting and Expression.

Indole works as an interspecies signal molecule to regulate multiple physiological activities, like antibiotic resistance, acid resistance, and virulence. However, the effect of indole on conjugation is unknown. Here, with λπ as a donor strain that carries a chromosomally integrated conjugative RP4 plasmid, we explored the effect of indole on conjugation of a mobilizable pUCP24T plasmid imparting gentamycin resistance. The results showed that exogenous indole treatment inhibited conjugative transfer of pUCP24T from λπ to recipient strains, and . Furthermore, raising endogenous indole production through overexpression of TnaA, a tryptophanase, in λπ significantly inhibited both λπ- and λπ- conjugation, whereas deficiency of reversed the phenotype. Subsequent mechanistic studies revealed that exogenous indole significantly inhibited the expression of mating pair formation gene () and the DNA transfer and replication gene (), mainly due to the promotion of regulatory genes ( and ), and the result was confirmed in knockout and overexpression strains. Additionally, we found that both extracellular indole production and expression of λπ were downregulated by ciprofloxacin (CIP). Intriguingly, one-eighth minimum inhibitory concentration of CIP treatment clearly facilitated both λπ- and λπ- conjugation, and indole inhibited CIP-induced conjugation frequency. These data suggest that indole may play a negative role in the process of CIP-induced conjugation. This is the first study to reveal the biological function of indole-inhibiting conjugation and its role in CIP-induced conjugation, which may be developed into a new way of controlling the spread of antibiotic resistance.