Detection of Oxacillin Resistance in Staphylococcus aureus Isolated from the Neonatal and Pediatric Units of a Brazilian Teaching Hospital

Abstract

Objective:To determine, by phenotypic and genotypic methods, oxacillin susceptibility in Staphylococcus aureus strains isolated from pediatric and neonatal intensive care unit patients seen at the University Hospital of the Botucatu School of Medicine.Methods:A total of 100 S. aureus strains isolated from the following materials were studied: 25 blood cultures, 21 secretions, 12 catheters, 3 cannulae and one chest drain from 62 patients in the neonatal unit, and 36 blood cultures, one pleural fluid sample and one peritoneal fluid sample from 38 patients in the pediatric unit. Resistance of the S. aureus isolates to oxacillin was evaluated by the disk diffusion method with oxacillin (1 μg) and cefoxitin (30 μg), agar screening test using Mueller-Hinton agar supplemented with 6 μg/ml oxacillin and 4% NaCl, and detection of the mecA gene by PCR. In addition, the isolates were tested for β-lactamase production using disks impregnated with Nitrocefin and hyperproduction of β-lactamase using amoxicillin (20 μg) and clavulanic acid (10 μg) disks.Results:Among the 100 S. aureus strains included in the study, 18.0% were resistant to oxacillin, with 16.1% MRSA being detected in the neonatal unit and 21.0% in the pediatric unit. The oxacillin (1 μg) and cefoxitin (30 μg) disk diffusion methods presented 94.4% and 100% sensitivity, respectively, and 98.8% specificity. The screening test showed 100% sensitivity and 98.8% specificity. All isolates produced β-lactamase and one of these strains was considered to be a hyperproducer.Conclusions:The 30 μg cefoxitin disk diffusion method presented the best result when compared to the 1 μg oxacillin disk. The sensitivity of the agar screening test was similar to that of the cefoxitin disk diffusion method and higher than that of the oxacillin disk diffusion method. We observed variations in the percentage of oxacillin-resistant isolates during the study period, with a decline over the last years which might be related to improved nosocomial infection control and the rational use of antibiotics.

Prevalence of Staphylococcus aureus and Methicillin Resistant Staphylococcus aureus from Milk and Milk Products in and around Srinagar Area of Kashmir, India

“…Biochemical characterization of the presumptive S. aureus isolates was performed as described by Bennett and Lancette (2001). Thus, confirmed S. aureus isolates were inoculated and cultured on Oxacillin resistance screening agar (ORSA) for detection of Methicillin Resistant S. aureus (Pereira et al, 2009). …”

Occurrence and genetic characterization of Staphylococcus aureus in milk samples of cattle with mastitis, and in the Veterinary Hospital personnel and dairy workers

“…Inactivation of oxacillin by β-lactamase hyperproduction and the production of modified intrinsic PBPs with affinity for altered oxacillin (MOD-SA) can be some of the causes for borderline resistance (PEREIRA et al, 2009).…”

Antibiotic Resistance of Community and Hospital Acquired Methicillin-Resistant <i>Staphylococcus aureus</i> Isolates from Clinical Specimens

“…PBP2a, as a transpeptidase, facilitates cell wall synthesis and bacterial growth at concentrations of βlactams inhibitory to native penicillin-binding proteins (Kim, 2009). PBP2a acts as a surrogate transpeptidase that takes over the biosynthetic functions of the normal PBPs when the bacteria encounter β-lactam antibiotics in the environment (Oliveira et al, 2002;Pereira et al, 2009). MRSA has persisted and implies cross-resistance to all available β-lactam antibiotics (Glikman et al, 2008).…”