Detection of Inducible Clindamycin Resistance in Staphylococci by Broth Microdilution Using Erythromycin/Clindamycin Combination Wells

Antimicrobial Resistance Team, Division of Healthcare Quality Promotion, Centers for Disease Control and Prevention, Atlanta, GA 30333; BD Diagnostic Systems, Sparks, MD 21152; Massachusetts General Hospital, Boston, MA 02114; University of Rochester Medical Center Hospital, Rochester, NY 14642; Trek Diagnostic Systems, Cleveland, OH 44131; Duke University Hospital, Durham, NC, 27710; JMI Laboratories, North Liberty, IA, 52317; BioMérieux, Inc., Hazelwood, MO, 63042; Statens Serum Institut, Copenhagen, Denmark, 2300; Robert Wood Johnson Medical School, New Brunswick, NJ,08903; Dade Behring MicroScan, Inc., West Sacramento, CA, 95691

	Abstract

In an 11-laboratory study, we investigated the ability of 4 combinations of erythromycin (ERY) and clindamycin (CC) (ERY/CC in µg/ml: 4/0.5, 6/1, 8/1.5, and 0.5/2) in a single well of a broth microdilution panel to predict the presence of inducible CC resistance. Each laboratory tested approximately 30 S. aureus and 20 coagulase-negative staphylococci (CoNS) in a panel using cation-adjusted Mueller-Hinton broth from 3 different manufacturers. Only strains resistant to ERY and susceptible or intermediate to CC were included in the analysis (S. aureus, n=333, and CoNS, n=97). Results of the D-zone test were used as the gold standard. After 18h incubation the combination 4/0.5 performed the best, with 98-100% sensitivity and 100% specificity for both organism groups. After 24h incubation, the ERY/CC combinations of 4/0.5, 6/1, and 8/1.5 correlated well with the D-zone test.