Inducible clindamycin resistance in Staphylococcus aureus isolates causing bacteremia at a university hospital in southern Taiwan

Novel Structure of Enterococcus faecium-Originated ermB-Positive Tn1546-Like Element in Staphylococcus aureus

We determined the resistance determinants in 274 erythromycin-resistant methicillin-susceptible Staphylococcus aureus (MSSA) isolates during a 13-year period, 2000 to 2012. The resistance phenotypes, inducible macrolide-lincosamide-streptogramin (iMLS), constitutive MLS (cMLS), and macrolide-streptogramin (MS) resistance phenotypes, were examined by a double-disk diffusion D test. The ermB gene was more frequent (35%; 97/274) than ermC (27%; 75/274) or ermA (21%; 58/274). All 97 ermB-positive isolates harbored Tn551 and IS1216V The majority (89/97) of ermB-positive isolates displayed the cMLS phenotype and carried mobile element structure (MES)-like structures, which has been previously reported in sequence type 59 (ST59) methicillin-resistant S. aureus (MRSA). The remaining 8 ermB-carrying isolates, belonging to ST7 (n = 4), ST5 (n = 3), and ST59 (n = 1), were sasK intact and did not carry MES-like structures. Unlike a MES-like structure that was located on the chromosome, the ermB elements on sasK-intact isolates were located on plasmids by S1 nuclease pulsed-field gel electrophoresis (PFGE) analysis and conjugation tests. Sequence data for the ermB-containing region (14,566 bp) from ST59 NTUH_3874 revealed that the best match was a Tn1546-like element in plasmid pMCCL2 DNA (GenBank accession number AP009486) of Macrococcus caseolyticus Tn1546 is recognized as an enterococcal transposon and was known from the vancomycin resistance gene cluster in vancomycin-resistant Enterococcus (VRE). So far, acquisitions of Tn1546 in S. aureus have occurred in clonal complex 5 (CC5) MRSA, but not in MSSA. This is the first report that MSSA harbors an Enterococcus faecium-originated ermB-positive Tn1546-like element located on a plasmid.

Presence of inducible clindamycin resistance phenotype and erm genes in foodborne Staphylococcus aureus isolates

The resistance of foodborne Staphylococcus aureus isolates to antimicrobial agents is an increasing problem. In this study, 154 S. aureus food isolates were examined on the basis of inducible clindamycin resistance and the presence of ermABC genes. Furthermore, the genetic relatedness of S. aureus strains having inducible and constitutive clindamycin resistance was investigated by pulsed-field gel electrophoresis. Inducible clindamycin resistance was determined in five strains using the D-test. Only S. aureus strain HE7A showed constitutive resistance. Polymerase chain reaction results demonstrated that all constitutive and inducible clindamycin resistant strains contained only the ermC gene. However, the presence of the ermC gene was also detected in three strains without inducible or constitutive resistance. Nucleotide variations on the sequences of ermC genes were also determined. Pulsed-field gel electrophoresis analysis of two groups of S. aureus strains (S4BY, PY62A, PY38BY/S and S35A, S35B) with inducible resistance demonstrated 100% homology, but the other susceptible and ermC gene carrying S. aureus strains showed different band patterns with 44-88% homology. This is the first study reporting inducible clindamycin resistance in foodborne S. aureus isolates in Turkey.