Escherichia coli and Salmonella Typhimurium isolates from the pre-antibiotic era differ from recent isolates in their ability to take up foreign DNA

Since the discovery of penicillin by Alexander Fleming in 1928, the consumption of antibiotics in human and veterinary medicine has increased continuously. This has resulted in strong selection pressure on bacteria and the spread of multi-resistant microbes. Horizontal gene transfer (HGT) is an important process triggering bacterial evolution. Conjugation is the most important form of HGT, resulting in the rapid spread of plasmids among bacteria. We hypothesize that the increased use of antibiotics since the discovery of penicillin is a selective force that affects HGT rates in bacteria. The transfer frequency of a plasmid is known to be affected by many biotic and abiotic factors, such as growth phase, donor:recipient ratio, and environmental conditions (temperature, pH, mating time, etc.). However, factors affecting the capability of the recipient bacterial cell to take up foreign DNA are less well understood. Since the inhibition of conjugation could be a useful strategy to reduce resistance gene spread among bacteria, a better understanding of the molecular mechanisms that control plasmid spread is essential.

Here, we compared the DNA uptake efficiency of E. coli and Salmonella isolates collected in the early twentieth century, which means before the broad introduction of antibiotics into human medicine, with that of recent clinical isolates. The conjugation frequency, transformation efficiency, growth kinetics as well as the plasmid content of the selected strains was analyzed. In conjugation experiments, we examined the transfer of two different conjugative plasmids, the highly transmissible resistance plasmid RP4 and the ESBL plasmid pO104_90. E. coli MG1655 and S. Typhimurium LT2 served as donor strains. Interestingly, recent isolates often showed better recipient properties than historical isolates. These results should serve as a basis for us to learn more about the mechanisms and possible selection pressures involved in bacterial adaptation to increasing exposure to antibiotics and the emergence of multidrug-resistant strains.