Subminimal inhibitory concentrations of antibiotics and anaerobic conditions promote Escherichia coli cell-to-cell plasmid transformation in biofilms

In recent years, subminimal inhibitory concentrations (sub-MIC) of antibiotics have been found to exert unexpected physiological effects on bacterial cells, beyond their common growth-inhibition properties. Our previous research demonstrated that sub-MIC ampicillin, combined with mild mechanical stimulation using glass balls, significantly promotes intercellular plasmid transformation (cell-to-cell plasmid transformation) in Escherichia coli within air-solid biofilms. In this study, we investigated whether other antibiotics with diverse mechanisms of action similarly enhance plasmid transformation. Our findings revealed that various antibiotics indeed promote cell-to-cell plasmid transformation, and this effect was observed under both aerobic and anaerobic conditions. Interestingly, anaerobic conditions resulted in higher frequencies of plasmid transformation compared to aerobic conditions. Supporting these results, we found that several single-gene knockouts of aerobic respiratory chain components under aerobic conditions also enhanced plasmid transformation. This suggests that the unavailability of aerobic respiration may favor the process of intercellular plasmid transfer. Collectively, our results indicate that a wide range of sub-MIC antibiotics can stimulate horizontal plasmid transfer and that anaerobic conditions are particularly conducive to this process. Based on these findings, we hypothesize that the anaerobic gut environment of antibiotic-treated animals or humans, characterized by biofilm-like high cell densities of antibiotic-exposed bacteria and regular peristaltic and segmental movements, could serve as a favorable niche for horizontal gene transfer via intercellular plasmid transformation.