DISTRIBUTION OF THE STREPTOMYCIN-RESISTANCE TRANSPOSON TN5393 AMONG PHYLLOPLANE AND SOIL BACTERIA FROM MANAGED AGRICULTURAL HABITATS

The distribution of the strA-strB streptomycin-resistance (Sm-r) genes associated with Tn5393 was examined in bacteria isolated from the phy lloplane and soil of ornamental pear and tomato. Two ornamental pear n urseries received previous foliar applications of streptomycin, wherea s the tomato fields had no prior exposure to streptomycin bactericides . Although the recovery of culturable Sm-r bacteria was generally high er from soil, the highest occurrence of Sm-r was observed in phyllopla ne bacteria of an ornamental pear nursery that received 15 annual appl ications of streptomycin during the previous 2 years. Twenty-two and 1 2% of 143 Gram-negative phylloplane and 163 Gram-negative soil isolate s, respectively, contained sequences that hybridized to probes specifi c for the strA-strB Sm-r genes and for the transposase and resolvase g enes of Tn5393. These sequences were located on large plasmids (>60 kb ) in 74% of the isolates. The 77 Sm-r Gram-positive bacteria isolated in the present study showed no homology to the Tn5393-derived probes. Although the repeated use of a single antibiotic in clinical situation s is known to favor the development of strains with resistance to othe r antibiotics, we found no evidence that intensive streptomycin usage in agricultural habitats favors the development of resistance to tetra cycline, an antibiotic also registered for disease control on plants. The detection of Tn5393 in bacteria with no prior exposure to streptom ycin suggests that this transposon is indigenous to both phylloplane a nd soil microbial communities.