Gene mobility promotes the spread of resistance in bacterial populations

Theory predicts that horizontal gene transfer (HGT) expands the selective conditions under which genes spread in bacterial populations. Whereas vertically inherited genes can only spread by positively selected clonal expansion, mobile genetic elements can drive fixation of genes by infectious HGT. We tested this using populations of Pseudomonas fluorescens and the conjugative mercury resistance (Hg^R) plasmid pQBR57. HGT expanded the selective conditions allowing the spread of Hg^R: Chromosomal Hg^R only increased in frequency under positive selection, whereas plasmid-encoded Hg^R reached fixation with or without positive selection. Tracking plasmid dynamics over time revealed that the mode of Hg^R inheritance varied across mercury environments. Under mercury selection, the spread of Hg^R was driven primarily by clonal expansion while in the absence of mercury Hg^R dynamics were dominated by infectious transfer. Thus, HGT is most likely to drive the spread of resistance genes in environments where resistance is useless.