Detection of clindamycin susceptibility in macrolide resistant phenotypes of Staphylococcus aureus

The present study aimed at in vitro detection of macrolide resistant phenotypes of methicillin resistant Staphylococcus aureus (MRSA) and interpretation of susceptibility tests to guide therapy. The study included 25 MRSA strains that were resistant to erythromycin and clindamycin, 25 MRSA strains that were sensitive to both erythromycin and clindamycin and 100 MRSA isolates which displayed erythromycin resistant but clindamycin susceptible phenotype. Erythromycin and clindamycin double disc susceptibility testing was done to detect inducible clindamycin resistance. Dilution susceptibility testing for clindamycin and erythromycin alone and in combination was performed for all 150 strains. Seventy-six strains showed blunting around clindamycin disc (inducible resistance). After induction with erythromycin, minimum inhibitory concentration (MIC) of clindamycin was noticed to rise from at least 16 to 256 g/mL in iMLSB phenotypes indicating inducible resistance. The detailed result analysis suggests the possible role of clindamycin in treatment of some of the erythromycin resistant isolates (non inducible), as there are multiplicity of resistance mechanisms and diversity of phenotypic expressions.