Distribution of Tetracycline Resistance Genes and Transposons among Phylloplane Bacteria in Michigan Apple Orchards

Abstract

ABSTRACT The extent and nature of tetracycline resistance in bacterial populations of two apple orchards with no or a limited history of oxytetracycline usage were assessed. Tetracycline-resistant (Tc r ) bacteria were mostly gram negative and represented from 0 to 47% of the total bacterial population on blossoms and leaves (versus 26 to 84% for streptomycin-resistant bacteria). A total of 87 isolates were screened for the presence of specific Tc r determinants. Tc r was determined to be due to the presence of Tet B in Pantoea agglomerans and other members of the family Enterobacteriacae and Tet A, Tet C, or Tet G in most Pseudomonas isolates. The cause of Tc r was not identified in 16% of the isolates studied. The Tc r genes were almost always found on large plasmids which also carried the streptomycin resistance transposon Tn 5393 . Transposable elements with Tc r determinants were detected by entrapment following introduction into Escherichia coli . Tet B was found within Tn 10 . Two of eighteen Tet B-containing isolates had an insertion sequence within Tn 10 ; one had IS 911 located within IS 10 -R and one had Tn 1000 located upstream of Tet B. Tet A was found within a novel variant of Tn 1721 , named Tn 1720 , which lacks the left-end orfI of Tn 1721 . Tet C was located within a 19-kb transposon, Tn 1404 , with transposition genes similar to those of Tn 501 , streptomycin ( aadA2 ) and sulfonamide ( sulI ) resistance genes within an integron, Tet C flanked by direct repeats of IS 26 , and four open reading frames, one of which may encode a sulfate permease. Two variants of Tet G with 92% sequence identity were detected.