Impact of mesophilic anaerobic digestion and post-treatment of digestates on the transfer of conjugative antimicrobial resistance plasmids

Manure is a major source of antimicrobial-resistant bacteria and resistance genes carried by mobile genetic el-ements such as plasmids. In France, the number of on-farm biogas plants has increased significantly in recent years. Our study investigated the impact of mesophilic anaerobic digestion (AD) and the post-treatment of digestates on the fate of conjugative plasmids, along with their potential transfer of antimicrobial resistance. Samples of raw manure, digestates and post-treated digestates were collected from three on-farm biogas plants. Conjugative plasmids were captured using the Escherichia coli CV601 recipient strain and media supplemented with rifampicin and kanamycin - to which the recipient strain is resistant - and tetracycline, sulfamethoxazole, gentamicin, trimethoprim, amoxicillin, cefotaxime, ciprofloxacin or colistin. Putative transconjugants were identified and characterised by disc diffusion and whole genome sequencing. The results showed that the antimicrobial resistance genes transferred from the different matrices conferred resistance to tetracyclines, sulphonamides, trimethoprim, and/or streptomycin. Transconjugants were obtained from raw manure samples but not from digestates or post-digestates, suggesting that mesophilic AD processes may produce fewer con-jugative plasmids potentially able to be transferred to Enterobacterales.

Automated summary

This study investigated the impact of mesophilic anaerobic digestion and post-treatment of digestates on the fate of conjugative plasmids and potential transfer of antimicrobial resistance. The results showed transfer of antimicrobial resistance genes in raw manure samples but not in digestates or post-digestates, indicating that mesophilic AD processes may reduce the production of potentially transferable conjugative plasmids.