Antibiotic Resistance in Airborne Bacteria Near Conventional and Organic Beef Cattle Farms in California, USA

摘要

Levels of antibiotic resistance genes (ARGs) and the fractions of antibiotic resistant bacteria (ARB) among culturable heterotrophic bacteria were compared in outdoor air near conventional (n = 3) and organic (n = 3) cattle rearing facilities. DNA extracts from filters taken from 18 locations were analyzed by quantitative polymerase chain reaction (qPCR) for five ARGs. At the reference (non-agricultural) site, all genes were below detection. ARGs sul1, bla(SHV), erm(B), and bla(TEM) were more frequently detected and at higher levels (up to 870 copies m(-3) for bla(SHV) and 750 copies m(-3) for sul1) near conventional farms compared to organic locations while the opposite was observed for erm(F) (up to 210 copies m(-3)). Isolates of airborne heterotrophic bacteria (n = 1295) collected from the sites were tested for growth in the presence of six antibiotics. By disk diffusion on a subset of isolates, the fractions of ARB were higher for conventional sites compared to organic farms for penicillin (0.9 versus 0.63), cloxacillin (0.74 versus 0.23), cefoperazone (0.58 versus 0.14), and sulfamethazine (0.50 versus 0.33) for isolates on nutrient agar. All isolates' Delta A600(pres)/Delta A600(abs) were measured for each of the six tested antibiotics; isolates from farms downwind of organic sites had a lower average Delta A600(pres)/Delta A600(abs) for most antibiotics. In general, all three analyses used to indicate microbial resistance to to 210 copies m(-3)). Isolates of airborne heterotrophic bacteria (n = 1295) collected from the sites were tested for growth in the presence of six antibiotics. By disk diffusion on a subset of isolates, the fractions of ARB were higher for conventional sites compared to organic farms for penicillin (0.9 versus 0.63), cloxacillin (0.74 versus 0.23), cefoperazone (0.58 versus 0.14), and sulfamethazine (0.50 versus 0.33) for isolates on nutrient agar. All isolates' Delta 600(pres)/Delta A600(abs) were measured for each of the six tested antibiotics; isolates from farms downwind of organic sites had a lower average Delta 600(pres)/Delta A600(abs) for most antibiotics. In general, all three analyses used to indicate microbial resistance to antibiotics showed increases in air samples nearby conventional versus organic cattle rearing facilities. Regular surveillance of airborne ARB and ARGs near conventional and organic beef cattle farms is suggested.