Impact of Exposure to Enrofloxacin on Dynamics of Plasmid-Mediated Quinolone Resistance Genes and Enrofloxacin-Resistant Bacteria Rates on Lettuces at Harvest

Multiple drug resistance of animal pathogens poses a serious threat to global animal and human health. The overuse of antibiotics in livestock makes manure a primary source for the spread of drug resistance. Approximately 30 to 90% of antibiotics used for livestock and poultry are excreted and enter into the environment. Antibiotic resistance genes may enter plants through endophytic bacteria and plant roots play an important role in microbial gene exchange. There is a potential threat to human health caused by human consumption of infectant vegetables (especially some raw vegetables, such as lettuce). Therefore, it is significantly important to study the feasibility and migration of ARGs from vegetables to humans. We simulated manure-amended field environments using enrofloxacin with chicken and pig manure in test soils for lettuce cultivation. We found different rates of enterofloxicin degradation depending upon the initial amount of drug added. The resistance rates of endophytic bacteria in lettuce tissues were slightly higher than those of the soil bacteria. We screened for the presence of the plasmid-mediated quinolone resistance (PMQR) genes qnrA, qnrB, qnrC, qnrS, qepA, aac(6′)-Ib, oqxB and qnrS, aac (6′)-Ib, and oqxB and qepA were the most abundant in all samples. The total PMQR abundance in soil samples was significantly higher than that in lettuce tissues (P ≤ 0.05) and the chicken manure treatment group had significantly higher levels than the pig manure treatment group (P ≤ 0.05). Enrofloxacin was also degraded most slowly in the lettuce rhizosphere and the relative abundance was also greater that of the bulk soil. The genes qnrS, aac(6′)-Ib, qepA, and oqxB were detected in both soil and lettuce tissues even at 60 d. Overall, exposure to enrofloxacin and manure induced resistance in the indigenous bacterial community and resistance genes entered plants more frequently under these conditions. The accumulation of antibiotics in vegetable tissues is a risk to human health and we suggest a delay of at least one season between applications of manure-based fertilizer and harvest of fresh produce.