Hormetic dose-dependent response about typical antibiotics and their mixtures on plasmid conjugative transfer of Escherichia coli and its relationship with toxic effects on growth.

Bacterial resistance caused by the abuse of antibiotics has attracted worldwide attention. However, there are few studies exploring bacterial resistance under the environmental exposure condition of antibiotics that is featured by low-dose and mixture. In this study, sulfonamides (SAs), sulfonamide potentiators (SAPs) and tetracyclines (TCs) were used to determine the effects of antibiotics on plasmid RP4 conjugative transfer of Escherichia coli (E. coli) under single or combined exposure, and the relationship between the effects of antibiotics on conjugative transfer and growth was investigated. The results show that the effects of single or binary antibiotics on plasmid RP4 conjugative transfer all exhibit a hormetic phenomenon. The linear regression reveals that the concentrations of the three antibiotics promoting conjugative transfer are correlated with the concentrations promoting growth and the physicochemical properties of the compounds. The combined effects of SAs-SAPs and SAs-TCs on plasmid conjugative transfer are mainly synergistic and antagonistic. While SAPs provide more effective concentrations for the promotion of conjugative transfer in SAs-SAPs mixtures, SAs play a more important role in promoting conjugative transfer in SAs-TCs mixtures. Mechanism explanation shows that SAs, SAPs and TCs inhibit bacterial growth by acting on their target proteins DHPS, DHFR and 30S ribosomal subunit, respectively. This study indicates that toxic stress stimulates the occurrence of conjugative transfer and promotes the development of bacterial resistance, which will provide a reference for resistance risk assessment of antibiotic exposure.