Growth of soil bacteria, on penicillin and neomycin, not previously exposed to these antibiotics

There is growing evidence that bacteria, in the natural environment (e.g. the soil), can exhibit naturally occurring resistance/degradation against synthetic antibiotics. Our aim was to assess whether soils, not previously exposed to synthetic antibiotics, contained bacterial strains that were not only antibiotic resistant, but could actually utilize the antibiotics for energy and nutrients. We isolated 19 bacteria from four diverse soils that had the capability of growing on penicillin and neomycin as sole carbon sources up to concentrations of 1000mgL−1. The 19 bacterial isolates represent a diverse set of species in the phyla Proteobacteria (84%) and Bacteroidetes (16%). Nine antibiotic resistant genes were detected in the four soils but some of these genes (i.e. tetM, ermB, and sulI) were not detected in the soil isolates indicating the presence of unculturable antibiotic resistant bacteria. Most isolates that could subsist on penicillin or neomycin as sole carbon sources were also resistant to the presence of these two antibiotics and six other antibiotics at concentrations of either 20 or 1000mgL−1. The potentially large and diverse pool of antibiotic resistant and degradation genes implies ecological and health impacts yet to be explored and fully understood. Three bacterial isolates (PenS4C4, PenS2D4 and NeoS2D1) were obtained from soil not previously treated with the antibiotics penicillin and neomycin. Not only were they resistant to the presence of these antibiotics, but could use the nutrients and energy contained in these antibiotic molecules (at 1000mgL−1) to support growth (see growth curves below). Soils contain a potentially large and diverse pool of antibiotic resistant and degradation genes. This suggests potential ecological and health impacts yet to be explored and fully understood. [Display omitted] •Nineteen bacterial isolates from four soils subsisted on penicillin or neomycin as sole C sources.•Most of the isolates were resistant to six other antibiotics at concentrations as high as 1000mgL−1.•The 19 isolates were assigned to diverse orders within the phyla Proteobacteria and Bacteroidetes.•Some antibiotic resistance genes were found in soil but not in the bacterial isolates.•Soils are an under-appreciated reservoir of antibiotic resistance/degradation machinery.