Distribution and Characterization of Antibiotic Resistant Bacteria in the Sediment of Southern Basin of Lake Biwa

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The distribution of antibiotic-resistant bacteria in Lake Biwa sediment with depth from a relatively shallow (0−1 cm) to a relatively deep (9−10 cm) layer was investigated. More colonies formed on 100-fold diluted nutrient broth (DNB) agar medium than nutrient broth (NB) medium. The colony number and its difference between the two media decreased with depth. The resistance to antibiotics, ampicillin, chloramphenicol, erythromycin, rifampicin, streptomycin, sulfamethoxazole and tetracycline, was examined. The ratio of antibiotic-resistant strains increased with the depth of sediment. The isolates obtained from the deepest layer using antibiotic-free DNB medium included significantly more strains which were resistant to higher concentrations and many more kinds of antibiotics. In contrast, the isolates from the antibiotic-containing medium showed no such features. According to the 16S rDNA sequence, the isolates from the shallowest layer belonged to the Actinobacteria phylum and the Alpha-, Beta- and Gamma-proteobacteria. The isolates from the deepest layer, however, belonged to the Firmicutes phylum and the Alphaproteobacteria with some exceptions. Many of the multi-resistant isolates were similar to the genus Ralstonia, Afipia, or Bacillus cereus, known for the biodegradation of xenobiotics or pathogenicities mediated via plasmids.