Distribution of tetracycline resistance gene, tet(M), in Gram-positive and Gram-negative bacteria isolated from sediment and seawater at a coastal aquaculture site in Japan

We found increased numbers of oxytetracycline (OTC)-resistant bacteria in sediment and seawater around a marine aquaculture site after OTC therapy. Samples were collected at an aquaculture site along the coast of the Seto Inland Sea, Japan in 2004. In April, the percentage of bacteria resistant to 60 mu g mL(-1) OTC in the surface sediment was 6.8%-20.0%. The percentages increased during OTC therapy in the summer reaching 53.3%-60.7% in September. Ninety-two days after drug cessation, the percentages decreased to below 22.9%. Tet(M)-positive bacteria were detected in the sediment and seawater samples. Tet(M) was evident in both Gram-positive and Gram-negative bacteria from various genera, and was newly identified in Paenibacillus, Sporosarcina, Shewanella, and Pseudoalteromonas. The dominant tet(M)-positive isolates were strains of Vibrio suggesting that this genus is an important reservoir for tet(M) in the marine environment. Two different alleles were found, tet(M)-A and tet(M)-B, each in isolates from five genera. The data suggests drug therapy used in the aquaculture acted as a selective pressure promoting increased numbers of resistant bacteria.