Evolutionary rescue of bacteria via horizontal gene transfer under a lethal β-lactam concentration.

β-Lactams are a commonly used class of bactericidal antibiotics. The number of β-lactam-resistant pathogens is constantly increasing in hospitals around the world. Interestingly, most of the β-lactam-resistant bacteria carry mobile genetic elements, such as conjugative plasmids, that render the pathogen resistant. These elements mediate their own transfer from one bacterium to another, producing new resistant strains via horizontal gene transfer. Here we investigated whether it is possible that transfer of the resistance element from another bacterium may evolutionarily rescue a susceptible bacterium exposed to a lethal concentration of the β-lactam ampicillin. Indeed, the rescuing occurs even at very high, clinically significant antibiotic levels, suggesting that pathogens may acquire the resistance 'on the fly' from commensal bacteria during treatment.