Phenotypic and molecular characterization of macrolide and streptogramin resistance in Streptococcus mitis from neutropenic patients

Objectives: 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.