Epidemiology of borderline oxacillin-resistant Staphylococcus aureus in pediatric cystic fibrosis

A single-center retrospective study was undertaken in children with cystic fibrosis (CF) to evaluate (1) risk factors for acquisition; (2) molecular epidemiology; and (3) impact on disease progression of borderline oxacillin-resistant Staphylococcus aureus (BORSA) versus mecA-positive methicillin-resistant Staphylococcus aureus (MRSA). The study comprised of (1) identification of all children with at least one respiratory specimen positive for either BORSA or MRSA during the study period; (2) compilation of relevant clinical and epidemiological data from 12-month period leading up to first positive (index) culture; (3) microbiological and molecular characterization of index isolates and (4) measurement of subsequent clinical outcome. Thirty-eight children were identified with at least one positive isolate; 4 were excluded due to insufficient clinical or laboratory data. Eighteen children (53%) grew BORSA in their index culture. Children who acquired BORSA only (n = 16) were more likely to have had prior MSSA colonization (P < 0.0001). Usage of oral cephalexin (P < 0.01) and inhaled tobramycin (P < 0.03) prior to index culture was significantly and independently associated with acquisition of BORSA. The majority of BORSA isolates were hyper β-lactamase producers and susceptible to a greater range of antibiotics. Strain relatedness was not evident within the BORSA group. There was no difference in disease progression between the two groups. This is the first study to demonstrate that a significant proportion of S. aureus isolates with methicillin resistance in the CF population are BORSAs that lack mecA. Antibiotic pressure may lead to the development of BORSA in CF patients. Prospective studies are needed to assess its clinical impact.