Protozoal food vacuoles enhance transformation in Vibrio cholerae through SOS-regulated DNA integration

Abstract

Vibrio cholerae, the bacterial pathogen responsible for the diarrheal disease cholera, resides in the aquatic environment between outbreaks. For bacteria, genetic variation by lateral gene transfer (LGT) is important for survival and adaptation. In the aquatic environment, V. cholerae is predominantly found in biofilms associated with chitinous organisms or with chitin “rain”. Chitin induces competency in V. cholerae, which can lead to LGT. In the environment, V. cholerae is also subjected to predation pressure by protist. Here we investigated whether protozoal predation affected LGT using the integron as a model. Integrons facilitate the integration of mobile DNA (gene cassettes) into the bacterial chromosome. We report that protozoal predation enhances transformation of a gene cassette by as much as 405-fold. We show that oxidative radicals produced in the protozoal phagosome induces the universal SOS response, which in turn upregulates the integron-integrase, the recombinase that facilitates cassette integration. Additionally, we show that during predation, V. cholerae requires the type VI secretion system to acquire the gene cassette from Escherichia coli. These results show that protozoal predation enhances LGT thus producing genetic variants that may have increased capacity to survive grazing. Additionally, the conditions in the food vacuole may make it a “hot spot” for LGT by accumulating diverse bacteria and inducing the SOS response helping drive genetic diversification and evolution.

Protozoan predation enhances stress resistance and antibiotic tolerance in Burkholderia cenocepacia by triggering the SOS response

“…Importantly, because the SOS response is a conserved mechanism, our study suggests that protozoan food vacuoles likely constitute an important reservoir that promotes the switch to persister in bacteria that survive intracellular digestion. Furthermore, it has been recently shown that oxidative radicals produced within protozoan food vacuoles facilitate genetic diversification in V. cholerae through SOS-induced DNA integration [ 67 ]. Exchange of antibiotic resistance genes has been shown to be favored in protozoan food vacuoles, where different bacterial species accumulate under conditions that favor major mechanisms for horizontal gene transfer [ 67–72 ].…”

“…Furthermore, it has been recently shown that oxidative radicals produced within protozoan food vacuoles facilitate genetic diversification in V. cholerae through SOS-induced DNA integration [ 67 ]. Exchange of antibiotic resistance genes has been shown to be favored in protozoan food vacuoles, where different bacterial species accumulate under conditions that favor major mechanisms for horizontal gene transfer [ 67–72 ]. Therefore, over the last years evidence is accumulating about the role of protozoan food vacuoles as privileged niches that foster genetic diversification and adaptation in bacteria.…”

Protistan predation selects for antibiotic resistance in soil bacterial communities

“…Our work has revealed a strong increase in ARGs encoding multidrug and tetracycline efflux pumps under the increasing pressure of soil protists. Another potential reason for the increase in ARGs in soils is lateral gene transfer among undigested bacteria inside food vacuoles of protists under SOS response [56].…”

Phagotrophic protists preserve antibiotic-resistant opportunistic human pathogens in the vegetable phyllosphere

“…For example, the conjugation frequency of E. coli strains engulfed by the ciliate increased 2000- to 4000-fold after a full digestion cycle [ 108 ]. Similarly, protozoal predation enhances the transformation of a gene cassette in Vibrio cholerae by as much as 405-fold through SOS-regulated DNA integration [ 109 ]. These indicated that the HGT of ARGs, VFGs, and MRGs among endosymbiont OHPs may be further expedited under the predation pressure of protists [ 110 ], and thus contribute to the survival of more endosymbionts [ 111 ] and the emergence of resistant OHPs.…”