Prevalence and Molecular Characterization of Tetracycline Resistance in Enterococcus Isolates from Food

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Abstract
In the present study, a collection of 187 Enterococcus food isolates mainly originating from European cheeses were studied for the phenotypic and genotypic assessment of tetracycline (TC) resistance. A total of 45 isolates (24%) encompassing the species Enterococcus faecalis ( n = 33), E. durans ( n = 7), E. faecium ( n = 3), E. casseliflavus ( n = 1), and E. gallinarum ( n = 1) displayed phenotypic resistance to TC with MIC ranges of 16 to 256 μg/ml. Eight of these strains exhibited multiresistance to TC, erythromycin, and chloramphenicol. By PCR detection, TC resistance could be linked to the presence of the tet (M) ( n = 43), tet (L) ( n = 16), and tet (S) ( n = 1) genes. In 15 isolates, including all of those for which the MIC was 256 μg/ml, both tet (M) and tet (L) were found. Furthermore, all tet (M)-containing enterococci also harbored a member of the Tn 916 -Tn 1545 conjugative transposon family, of which 12 erythromycin-resistant isolates also contained the erm (B) gene. Filter mating experiments revealed that 10 E. faecalis isolates, 3 E. durans isolates, and 1 E. faecium isolate could transfer either tet (M), tet (L), or both of these genes to E. faecalis recipient strain JH2-2. In most cases in which only tet (M) was transferred, no detectable plasmids were acquired by JH2-2 but instead all transconjugants contained a member of the Tn 916 -Tn 1545 family. Sequencing analysis of PCR amplicons and evolutionary modeling showed that a subset of the transferable tet (M) genes belonged to four sequence homology groups (SHGs) showing an internal homology of ≥99.6%. Two of these SHGs contained tet (M) mosaic structures previously found in Tn 916 elements and on Lactobacillus and Neisseria plasmids, respectively, whereas the other two SHGs probably represent new phylogenetic lineages of this gene.