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Objectives:  The aim of the study was to characterize phenotypically and genotypically an uncommon mechanism of resistance to macrolides, lincosamides, and streptogramins (MLS) in a Streptococcus milleri group clinical isolate.  Materials and Methods:  The isolate UCN96 was recovered from an osteoradionecrosis wound, and was identified using the matrix assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry and the partial sequencing of the sod A gene. Antimicrobial susceptibility testing were carried out by the disk diffusion method and minimal inhibitory concentrations (MICs) were determined by the broth microdilution technique. PCR screening was performed for MLS resistance genes described in Gram-positive bacteria. Specific mutations in the ribosomal proteins L3-, L4-, and L22-encoding genes were also screened and those in domain V of the 23S rRNA gene ( rrl ). The number of mutated copies of the rrl gene was determined using amplification-refractory mutation system quantitative-polymerase chain reaction (qPCR) analysis.  Results:  The clinical isolate UCN96 was unambiguously identified as Streptococcus constellatus . It was susceptible to all macrolides and lincosamides (ML) antibiotics except spiramycin (MIC &gt;256 mg/L) while it was also resistant to streptogramins. Screening for all acquired resistance genes was negative and no mutation was found in genes coding for L3, L4, and L22 ribosomal proteins. Of interest, a single mutation, A2062C (according to Escherichia coli numbering), was detected in the domain V of 23S rRNA.  Conclusion:  Mutations at the position 2062 of 23S rRNA have been detected once in Streptococcus pneumoniae , and not yet in other Streptococcus spp. This mechanism is very likely uncommon in Gram-positive bacteria because different copies of 23S rRNA operons should be mutated for development of such a resistance pattern.

Paradoxical High-Level Spiramycin Resistance and Erythromycin Susceptibility due to 23S rRNA Mutation in Streptococcus constellatus