Article

Phenotypic and Molecular Characterization of Tetracycline- and Erythromycin-Resistant Strains of Streptococcus pneumoniae

Department of Microbiology and Biomedical Sciences, University of Ancona Medical School, 60131 Ancona, Italy.
Antimicrobial Agents and Chemotherapy (Impact Factor: 4.45). 07/2003; 47(7):2236-41. DOI: 10.1128/AAC.47.7.2236-2241.2003
Source: PubMed

ABSTRACT Sixty-five clinical isolates of Streptococcus pneumoniae, all collected in Italy between 1999 and 2002 and resistant to both tetracycline (MIC, ≥8 μg/ml) and erythromycin (MIC, ≥1 μg/ml), were investigated. Of these strains, 11% were penicillin resistant and 23% were penicillin intermediate. With the use of the erythromycin-clindamycin-rokitamycin triple-disk test, 14 strains were assigned to the constitutive (cMLS) phenotype of macrolide resistance, 44 were assigned to the partially inducible (iMcLS) phenotype, 1 was assigned to the inducible (iMLS) phenotype, and 6 were assigned to the efflux-mediated (M) phenotype. In PCR assays, 64 of the 65 strains were positive for the tetracycline resistance gene tet(M), the exception being the one M isolate susceptible to kanamycin, whereas tet(K), tet(L), and tet(O) were never found. All cMLS, iMcLS, and iMLS isolates had the erythromycin resistance gene erm(B), and all M phenotype isolates had the mef(A) or mef(E) gene. No isolate had the erm(A) gene. The int-Tn gene, encoding the integrase of the Tn916-Tn1545 family of conjugative transposons, was detected in 62 of the 65 test strains. Typing assays showed the strains to be to a great extent unrelated. Of 16 different serotypes detected, the most numerous were 23F (n = 13), 19A (n = 10), 19F (n = 9), 6B (n = 8), and 14 (n = 6). Of 49 different pulsed-field gel electrophoresis types identified, the majority (n = 39) were represented by a single isolate, while the most numerous type included five isolates. By high-resolution restriction analysis of PCR amplicons with four endonucleases, the tet(M) loci from the 64 tet(M)-positive pneumococci were classified into seven distinct restriction types. Overall, a Tn1545-like transposon could reasonably account for tetracycline and erythromycin resistance in the vast majority of the pneumococci of cMLS, iMcLS, and iMLS phenotypes, whereas a Tn916-like transposon could account for tetracycline resistance in most M phenotype strains.

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    • "Early work aimed at detecting Tn1545-like elements by identification of the various resistance genes present in isolates by dot blot (Seral et al., 2001) showed a range of combinations of the genes present on Tn1545. Additionally, 63 out of 65 S. pneumoniae strains showed the presence of intTn demonstrating, that elements from the Tn916/Tn1545 group were likely to be common in this organism (Montanari et al., 2003). It is likely that this early work was in fact detecting some of the more recently characterized conjugative transposons such as Tn6002 and Tn6003. "
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