Background: Macrolide (MLS
B ) resistance is the most widespread and clinically important mechanism of resistance encountered with Gram-positive organisms. Resistance may be constitutive (cMLS
B phenotype) or inducible (iMLS
B phenotype). The iMLS
B phenotypes are not differentiated by using standard susceptibility test methods, but can be distinguished by erythromycin-clindamycin disk approximation test (D-test) and demonstration of resistance genes by molecular methods.
Aims: To demonstrate in vitro inducible clindamycin resistance (iMLS
B ) in erythromycin-resistant (ER) and clindamycin-susceptible (CLI-S) clinical isolates of
Staphylococci
spp., and interpretation of susceptibility tests to guide therapy.
Materials and Methods: Eight hundred and fifty-one isolates of
Staphylococci spp. were recovered from various clinical specimens. All the
Staphylococcal spp. were identified by conventional microbiological methods including colony morphology, Gram stain, catalase, slide coagulase and tube coagulase. Antibiotic susceptibility testing was performed by Kirby Bauer disc diffusion method. Erythromycin-resistant isolates were examined for inducible clindamycin resistance (iMLS
B ) by using double disk approximation test (D-test) at 15 mm disk separation.
Results: The
Staphylococci spp. isolated were 379 S. aureus [31.60% methicillin-resistant
S. aureus (MRSA), 12.92% methicillin-sensitive
S. aureus (MSSA)] and 472 coagulase-negative
Staphylococci (CNS) [37.60% methicillin-resistant coagulase-negative
Staphylococci (MRCNS), 17.86% methicillin-sensitive coagulase-negative
Staphylococci (MSCNS)]. Four hundred and thirty (50.52%) Staphylococcal spp. isolates showed erythromycin resistance. Constitutive resistance was demonstrated in 202 (46.97%), inducible clindamycin resistance (iMLS
B ) in 101 (23.48%), and non-inducible (MS) in 127 (29.53%). Two distinct induction phenotypes, D (18.13%) and D
+ (5.34%) were observed. All iMLS
B isolates were susceptible to linezolid and vancomycin while 78.78% to ciprofloxacin.
Conclusions: Fifty percent of
Staphylococcal spp. were ER among which 23.48% were iMLS
B phenotypes. Eighty-seven per cent of iMLS
B phenotypes were observed to be methicillin-resistant. The high frequency of methicillin resistance isolates (87.12%) with
in vitro inducible clindamycin resistance at our institute raises concern of clindamycin treatment failures with methicillin-resistant infections. So we recommend that microbiology laboratories should include the D-test for inducible resistance to clindamycin in the routine antibiotic susceptibility testing.
