Distribution of streptomycin resistance and biosynthesis genes in streptomycetes recovered from different soil sites.

Streptomycin resistant streptomycetes were isolated from four diverse sites. Two sites in Germany were sampled and included an agricultural experimental plot of apple trees which had a history of streptomycin application (AR) and a control site without antibiotic application (CR). Two sites in the UK were sampled; a pastureland site which had sewage injection (DW) and an agricultural site (CW). The actinomycete counts indicated a high proportion of streptomycin resistant isolates in the CW soil. Streptomycetes were identified by partial sequencing of the 16S rDNA. PCR product of the hypervariable gamma region of 16S rDNA allowed analysis by denaturing gradient gel electrophoresis to assess the diversity within the isolates. The streptomycin and sewage sludge treated sites showed decreased diversity within streptomycete populations. Isolates were screened for the streptomycin resistance gene, strA, and flanking biosynthesis gene, strB1. Distribution of these genes indicated the prevalence of str genes in the streptomycin treated soil. Evidence of horizontal gene transfer was recorded in isolates identified as Streptomyces platensis recovered from CR and AR sites which had acquired a streptomycin resistance gene homologous to that found in Streptomyces griseus. Members of the latter species were the most abundant streptomycin resistant streptomycetes isolated from all soils.