To determine the prevalence of macrolide and streptogramin resistance in Streptococcus mitis isolates from neutropenic patients and to identify mechanisms of macrolide and streptogramin resistance in resistant isolates.Methods
MICs of erythromycin, spiramycin, lincomycin and pristinamycin were determined for S. mitis isolates. Macrolide-resistance genes were characterized by PCR and ribosomal mutations by sequencing.Results
A total of 169 S. mitis isolates were recovered from 66 patients at the Tunisian Bone Marrow Transplant Centre. Of these, 120 (70%) were non-susceptible to erythromycin and one was resistant to pristinamycin; 48.5% of isolates had an MLSB phenotype with cross-resistance between erythromycin, spiramycin and lincomycin, 4% had a dissociated MLSB phenotype with resistance to erythromycin and spiramycin but apparent susceptibility to lincomycin and 47.5% displayed the M phenotype. Resistance determinants were characterized in 33 isolates. Ten of 14 isolates with the cross MLSB resistance contained an erm(B)-like gene and four a combination of erm(B)- and mef(A)-like genes. Four of the five isolates with a dissociated MLSB phenotype contained erm(B)-like and one a combination of erm(B)- and mef(A)-like genes. All the 14 isolates with an M phenotype contained mef(A)-like genes. The pristinamycin-resistant strain had G105 and A108 substitutions in the conserved C terminus of the L22 ribosomal protein.Conclusions
The prevalence of macrolide resistance is high in S. mitis from neutropenic patients and is due to the spread of erm(B)- or mef(A)-like genes alone or combined. Resistance to streptogramins is rare and in this case associated with ribosomal mutation.