Phenotypic detection of nosocomial mecA-positive coagulase-negative staphylococci from neonates

Over a 3-year period, we screened antimicrobial resistance genotype (mecA p ositive or -negative) in clinically significant coagulase-negative staphylo cocci isolated from patients residing in our neonatal intensive care unit. For the 152 study strains, the accuracy of standard methods (agar dilution MIG, disc diffusion and agar screen tests) in detecting oxacillin resistanc e during 48 h of incubation was evaluated. Using mecA gene PCR and Southern blot hybridization as the gold standard, the differential in MICs of addit ional antibiotics selected for their relevant clinical use in our setting w as also compared with mecA status of the isolates. The frequency of mecA wa s 48.6% among study strains. When applying the previous (1998) and most cur rent (1999) NCCLS interpretive criteria, the specificities of oxacillin aga r dilution MICs in detecting the 78 mecA-negative isolates were 100 and 89. 7%, respectively, at 24 h, and 100 and 80.7%, respectively, at 48 h. In thi s respect, the sensitivities of oxacillin agar dilution MICs in detecting t he 74 mecA-positive strains were 75.6 and 97.2%, respectively, at 24 h, and 86.4 and 100%, respectively, at 48 h. When applying the previous and most current NCCLS zone size interpretive criteria, oxacillin zone diameters wer e in false-susceptible error for 13.5 and 8.1%, respectively, of the 74 mec A-positive strains tested at 24 h, and for 6.7 and 2.7%, respectively, at 4 8 h. Accordingly, when the 78 mecA-negative strains were considered, oxacil lin zone diameters were in false-resistant error for 2.5 and 8.9%, respecti vely, at 24 h, and for 8.9 and 15.3%, respectively, at 48 h. The oxacillin salt agar screen assay accurately identified all mecA-negative strains at b oth 24 and 48 h. However, 26 (35.1%) and 7 (9.4%) of the mecA-positive stra ins were misinterpreted as susceptible by the agar screen test at 24 and 48 h, respectively. Using the presence of mecA as the reference standard for interpreting oxacillin susceptibility results, strains lacking mecA were mo re likely to be susceptible to ampicillin, ceftazidime, gentamicin, netilmi cin and rifampicin than were mecA-positive strains. Vancomycin was the only antibiotic tested for which all strains, regardless of mecA status, remain ed susceptible.