The Prevalence of Inducible and Constitutive Macrolide-lincosamide-streptogramin B-Resistant (iMLSB and cMLSB) Phenotypes among Clinical Isolates of Staphylococcus aureus at a Tertiary Care Hospital in South Mumbai

Abstract

Staphylococcus aureus is one of the most common causative organisms of health care as well as community-acquired infections in every region of the world. Moreover, increase in the prevalence of methicillin resistance among Staphylococci is a matter of concern.[1] Due to this, there has been a renewed interest in the usage of macrolide-lincosamide-streptogramin B (MLSB) antibiotics for the treatment of S. aureus infections. Moreover, clindamycin is the preferred agent because of its exceptional pharmacokinetic properties.[2] Clindamycin can be used as an alternative antibiotic in penicillin-allergic patients for the treatment of skin and soft-tissue infections caused by S. aureus. It achieves high intracellular levels in phagocytic cells, high levels in bone, and appears to be able to reduce toxin production in toxin-producing strains of staphylococci. Except central nervous system, it has very good tissue penetration.[3] It is a good option for outpatient prescription or as a follow-up drug after intravenous therapy because of its good oral absorption.[4] However, a possibility of inducible clindamycin resistance among Staphylococcal isolates is a major concern in use of clindamycin.[5] There are three mechanisms of resistance to the MLSB class of antibiotics: Modification of target site, enzymatic inactivation, and impermeability or macrolide efflux pumps.[6] Resistance due to modification of ribosomal target is mediated by erythromycin ribosomal methylases encoded by ermA/ ermC genes and it affects the activities of macrolides as well as clindamycin. This type of resistance may be inducible or constitutive.[7,8] The constitutive resistance MLSB (cMLSB) strains can easily be detected by standard susceptibility testing methods because they are resistant to both macrolides and lincosamides alike. However, the inducible resistant MLSB (iMLSB) strains appear erythromycin resistant and clindamycin sensitive in routine laboratory tests, unless the tests include measures that result in induction of clindamycin resistance.[4] In such cases, therapy with clindamycin may ABSTRACT