High prevalence of inducible erythromycin resistance among Streptococcus bovis isolates in Taiwan

Susceptibilities to 13 antimicrobial agents were determined by measurement of MICs for 60 isolates of Streptococcus bovis from blood cultures. Thirty-eight isolates (63.3%) had high-level resistance to erythromycin (MICs, >or=128 microg/ml). Among the 38 erythromycin-resistant strains, 21 isolates (55%) had inducible resistance to macrolides-lincosamides-streptogramin B (iMLS isolates) and 17 (45%) had constitutive resistance to macrolides-lincosamides-streptogramin B (cMLS isolates). Tetracycline resistance was also found among all of the erythromycin-resistant strains. None of the strains displayed resistance to penicillin, chloramphenicol, or vancomycin. Detection of erythromycin resistance genes by PCR and sequencing indicated that all 17 cMLS isolates were positive for the ermB gene and that 7 of 21 iMLS isolates carried the ermB gene and the remaining 14 iMLS isolates carried the ermT gene. Sequence analysis of amplified partial ermB fragments (594 bp) from S. bovis isolates revealed a 99.8% nucleotide identity and a 100% amino acid homology compared with the sequences from gene banks. The sequences of amplified fragments with primers targeted for ermC were shown to be very similar to that of ermGT (ermT) from Lactobacillus reuteri (98.5% nucleotide identity). This is the first report to describe the detection of the ermT class of erythromycin resistance determinants in S. bovis. The high rate of inducible erythromycin resistance among S. bovis isolates in Taiwan was not reported before. The iMLS S. bovis isolates were shown to be heterogeneous by randomly amplified polymorphic DNA analysis. These results indicate that the prevalence of inducible erythromycin resistance in S. bovis in Taiwan is very high and that most of the resistant strains carry the ermT or the ermB gene.