In this study we analyzed the numbers and types of cultivable tylosin-resistant bacteria from six agricultural soils that differed with respect to their association with subtherapeutic antibiotic use. The proportion of tylosin-resistant bacteria to the total number of bacteria cultivated was significantly higher (7.2-16.5%) at three sites affected by subtherapeutic antibiotic use compared to three sites unaffected by subtherapeutic antibiotic use (0.7-2.5%). We also detected differences in the types of cultivable tylosin-resistant bacteria. At a site affected by subtherapeutic antibiotic use, we detected tylosin-resistant bacteria from the α- and β-subdivisions of Proteobacteria. In contrast, at a site unaffected by subtherapeutic use, we detected only Streptomyces-like (high-G+C Gram-positive) tylosin-resistant bacteria. Our results suggest a link between subtherapeutic use of antibiotics and the numbers and types of antibiotic-resistant bacteria in nearby soil. However, other factors, such as soil type and temporal variation, may have also contributed to the differences observed.
Bibliographical noteFunding Information:
We thank the numerous anonymous farmers who graciously allowed us to collect samples and Sara Firl for technical assistance. The University of Minnesota financially supported this research.
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