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 (HgR ) plasmid pQBR57. HGT expanded the selective conditions allowing the spread of HgR : Chromosomal HgR only increased in frequency under positive selection, whereas plasmid-encoded HgR reached fixation with or without positive selection. Tracking plasmid dynamics over time revealed that the mode of HgR inheritance varied across mercury environments. Under mercury selection, the spread of HgR was driven primarily by clonal expansion while in the absence of mercury HgR dynamics were dominated by infectious transfer. Thus, HGT is most likely to drive the spread of resistance genes in environments where resistance is useless.