Bdellovibrio bacteriovorus is a predatory bacterium which attacks a wide range of gram negative bacterial pathogens and is proposed to be a potential probiotic or living antibiotic in the future1 2 . In the current study, we are evaluating the effect of indole (a potential bacterial quorum sensing molecule) on the predatory ability of B. bacteriovorus HD100. Indole at concentration of 2 mM inhibited the predation on E. coli MG1655 and S. enterica KACC 10595 completely over 48 hours while at lower concentrations down to 0.25 mm delayed it significantly. This range includes physiological concentrations found within the human gut, making them relevant to applications of this bacterium. Microscopic analysis revealed that the indole inhibited the predator from attacking the prey. Furthermore, we found that indole didn’t kill the predator but slowed down its motility. Microarray and RT-PCR analyses revealed that the most down-regulated gene groups in presence of 1 and 2 mM indole are related with the flagellar, chemotaxis and motility functions. In addition, several other gene groups were also found to be significantly affected. Furthermore, addition of indole to already established intraperiplasmic culture, i.e., after the entrance of B. bacteriovorus into the prey periplasm, seemed to delay the bdelloblast lysis and release of the attack phase progeny. In conclusion, this study found indole has systemic effects on B. bacteriovorus and negatively impacts its ability to predate.