Indole negatively impacts predation by Bdellovibrio bacteriovorus and its release from the bdelloplast

Bdellovibrio bacteriovorus is a predatory bacterium that attacks a wide range of Gram-negative bacterial pathogens and is proposed to be a potential living antibiotic. In this study, we evaluated the effects of indole, a bacterial signalling molecule commonly produced within the gut, on the predatory ability of B. bacteriovorus HD100. Indole significantly delayed predation on Escherichia coli MG1655 and Salmonella enterica KACC 11595 at physiological concentrations (0.25 to 1 mM) and completely inhibited predation when present at 2 mM. Microscopic analysis revealed that indole blocked the predator from attacking the prey. Furthermore, indole was not toxic to the predator but slowed down its motility. Microarray and reverse transcription quantitative polymerase chain reaction (RT-qPCR) analyses confirmed that as the gene group showing the greatest downregulation in the presence of indole was flagellar assembly genes. Indole also caused a wide spectrum changes in gene expression including general downregulation of genes involved in ribosome assembly. Furthermore, indole addition to the predatory culture after the entrance of B. bacteriovorus into the prey periplasm slowed down bdelloplast lysis. In conclusion, indole can have significant impacts on the predation efficiency, which should be taken into consideration especially if B. bacteriovorus is to be applied as a probiotic or living antibiotic.