Superiority of D-zone Testing Method over Standard Method to detect Rnducible Resistance in Gram Positive Bacteria: a Prospective Surveillance from a Teaching Hospital in Saudi Arabia

Objective: In this prospective study, we determined phenotypic resistance to erythromycin among gram positive bacteria. Methods: Bacterial isolates were identified by conventional methods and by the MicroScan: D-test zone was performed according to the Clinical and Laboratory Standards institutes (CLSI) recommendations to determine inducible resistance to clindamycin on gram positive bacteria isolated from different clinical specimens .Bacterial isolates included : group A streptococci (GAS), group B streptococci (GBS), viridans streptococci, S.pneumoniae, Staphylococcus aureus (S.aureus) ( both methicillin susceptible (MSSA) and methicillin resistant (MRSA) . Results: A total of 1072 gram positive bacterial isolates were tested .The majority was from swabs collected from outpatient clinics. Erythromycin resistance was 8/23 (35%) for S. pneumoniae, 12/91(13%) for GAS and 17/300(5.7%) for GBS. All GAS and viridans streptococci possessed the efflux phenotype only, 8(8.8% and 1(20%), respectively. For GBS, cMLSB was 11(3.7%), 3 (1%) iMLSB and 2(0.33%) were of efflux phenotype. All S.pneumoniae strains possessed cMLSB phenotype. Seventy five isolates (16.3%) of MSSA were resistant to erythromycin compared to 160(83%) of MRSA. The majority of MSSA, 31/460 (6.7%) had an efflux phenotype while 26/460(5.6%) were of cMLSB and 19/460(4%) iMLSB phenotypes. Constitutive MLSB was the most predominant resistant phenotype, 152/193(78.8%) among MRSA. Conclusion: D-test zone should be considered for routine testing to detect inducible clindamycin resistance among significant gram positive bacteria.