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.

Keywords: RP4 plasmid; Serratia marcescens; Tetrahymena thermophila; conjugation; evolution of antibiotic resistance; protozoan predation.

Figure 1.

Prevalence of conjugative and conjugation-defective RP4 plasmid and evolution in prey defence trait, D, over time (mean ± s.e.). (a–d) The conjugative plasmid in the 0 µg ml^–1 kanamycin environment without (a) or with (b) predators, and in the 1.15 µg ml^–1 kanamycin environment without (c) or with (d) predators. (e–h) The conjugation-defective plasmid in the 0 µg ml^–1 kanamycin environment without (e) or with (f) predators, and in the 1.15 µg ml^–1 kanamycin environment without (g) or with (h) predators.

Figure 2.

Effect of predation on change in RP4 plasmid frequency [(start frequency/end frequency) − 1] when low numbers of plasmid-carrying bacteria were introduced to a plasmid-free population (mean ± s.e.). Colours indicate presence (grey boxes) or absence (white boxes) of predators.