Incidence of Constitutive and Inducible Clindamycin Resistance in Clinical Isolates of Methicillin Resistant Staphylococcus aureus

Without the double-disk test, all the Staphylococcus aureus isolates with inducible clindamycin resistance would have been misclassified as clindamycin susceptible, resulting in an underestimated clindamycin resistance rate. Clindamycin resistance rates may vary by geographic region and methicillin susceptibility. Hence it should be determined in individual settings. The high frequency of methicillin resistant S. aureus (MRSA) and methicillin susceptible S. aureus (MSSA) isolates with in vitro inducible clindamycin resistance at hospitals raises concern that clindamycin treatment failures may occur with MSSA as well as with MRSA infections. Clinical laboratories should report in vitro inducible clindamycin resistance in S. aureus isolates and clinicians should be aware of the potential of clindamycin treatment failure in patients with infections caused by inducible resistant strains. In this study, the percentage of inducible clindamycin resistance at two hospitals (Maharaj Nakhon Si Thammarat Hospital, Nakhon Si Thammarat, Thailand and Chumphon Khet Udomsak Hospital, Chumphon, Thailand) were 50 (25/50) and 8.3 (1/12) for methicillin resistant Staphylococcus aureus, respectively. Given the data of inducible resistance to clindamycin found in the two hospitals, we conclude that susceptibility testing of staphylococci should include the disk diffusion induction test (D-test) for usefulness of therapeutic treatment of staphylococci infections.