Antimicrobial resistance (AMR) poses a significant threat to public health by compromising the effectiveness of infection treatments. Consuming food or water contaminated with AMR bacteria can lead to the transmission of resistant strains within the gut microbiota. However, the impact of intermittent clinical doses and prolonged sub-clinical exposure to antimicrobials on the persistence and transfer of AMR genes remains unclear.

To investigate this, five groups of mice were treated with ampicillin (Amp) administered in four intermittent clinical doses, with intervals of 20, 48, and 27 days between sequential treatments. During the intervals, the mice received additional tetracycline treatment in drinking water at concentrations ranging from 0 to 100 mg/L. The mice were orally inoculated with beta-lactam-resistant Salmonella Heidelberg carrying a mobile AMR plasmid (donor) following the first Amp treatment, E. coli labeled with green fluorescent protein (EC-GFP) as a recipient after the second Amp treatment, and beta-lactam-susceptible S. Heidelberg as another recipient after the third Amp treatment. Fecal samples were collected to enumerate donor, recipient, and transconjugant bacteria on selective agar media. Transconjugants were confirmed using qPCR targeting AMR genes.

The donor S. Heidelberg persisted throughout the experiment in all mice. Following each Amp treatment, the donor population reached a high density, but gradually declined until it became undetectable. The AMR plasmid was successfully transferred to commensal E. coli, EC-GFP, and the recipient S. Heidelberg after the first, second, and third Amp treatment, respectively. Tetracycline exposure enhanced both the persistence of the donor and the transfer of the AMR plasmid.

These findings underscore the significant impact of intermittent selective pressure on the persistence and transfer of AMR genes within the gut microbiota, reinforcing the need for more judicious antibiotic use to mitigate the spread of resistance.