Conjugation is necessary for a bacterial plasmid to survive under protozoan predation

Abstract

Horizontal gene transfer by conjugative plasmids plays a critical role in the evolution of antibiotic resistance. Interactions between bacteria and other organisms can affect the persistence and spread of conjugative plasmids. Here we show that protozoan predation increased the persistence and spread of the antibiotic resistance plasmid RP4 in populations of the opportunist bacterial pathogen Serratia marcescens. A conjugation-defective mutant plasmid was unable to survive under predation, suggesting that conjugative transfer is required for plasmid persistence under the realistic condition of predation. These results indicate that multi-trophic interactions can affect the maintenance of conjugative plasmids with implications for bacterial evolution and the spread of antibiotic resistance genes.

Black Queen Evolution and Trophic Interactions Determine Plasmid Survival after the Disruption of the Conjugation Network

“…The addition of the protozoan predator T. thermophila to the system had a major impact on the frequency of plasmids maintaining a conjugative phenotype in both the absence and presence of antibiotics. This is in line with previous experiments denoting the importance of predation for conjugative plasmid persistence ( 24 ). Since a bacterial population reaches lower density under conditions of predation ( Fig.…”

“…Even more intriguing is the notion that predation can effectively counter the loss of resistance plasmids after the disruption of the conjugation network. Since predation selects for conjugative plasmids over nonconjugative ones ( 24 ), and given that conjugation may be lost by several types of mutations, of which only some are readily reversible (see Fig. S1 in the supplemental material), under oscillating selection pressures (for and against conjugation), mutations in these reversible sites could be considered contingency loci improving the survival of resistance plasmids.…”

“…Therefore, they can modulate the ecological landscape of (potential) plasmid hosts in many different types of environments. Recently, it was shown that, in contrast to a wild-type conjugative plasmid, a conjugation-deficient mutant was unable to persist in a bacterial population under conditions of predation ( 24 ). In contrast, in the absence of predation, the conjugation-defective mutant had higher persistence than the conjugative plasmid, suggesting that the relevance of conjugation to plasmid persistence may become evident only in a more realistic trophic setup.…”

Predator coevolution and prey trait variability determine species coexistence

“…Cultures were kept at 288C (+0.18C) with shaking at 50 r.p.m. Every four transfers (28 days), bacterial and predator densities were estimated using optical density as a proxy for bacterial biomass and direct ciliate counts as described previously [38], and samples were freeze-stored with glycerol at -208C for later analysis. This experiment had been running for 20 months when we isolated the populations for the current experiment.…”

Evolution in interacting species alters predator life history traits, behavior and morphology in experimental microbial communities

“…Every four transfers (28 days), bacterial and predator densities were estimated using optical density (1 mL sample at 600 nm wavelength) as a proxy for bacterial biomass and direct ciliate counts (5 × 0.5 µL droplets using light microscopy) as used in this context and described previously [30][31][32], and samples were freeze-stored with glycerol at -20 °C for later analysis. Since predators do not survive freeze-storage in these conditions, at time points 52 and 89 weeks, predator cultures were made axenic by transferring 400 µL into 100 mL of PPY medium containing an antibiotic cocktail (42, 50, 50 and 33 µg mL -1 of kanamycin, rifampicin, streptomycin and tetracycline, respectively) and stored in liquid nitrogen.…”