Borderline oxacillin-resistant Staphylococcus aureus (BORSA) represents a quite 
poorly understood and inadequately defined phenotype of methicillin resistance. 
BORSA strains show low, borderline resistance to penicillinase-resistant 
penicillins (PRPs), with oxacillin MICs typically equal to 1-8 µg ml-1, and in 
contrast to methicillin-resistant S. aureus (MRSA), do not have an altered 
penicillin-binding protein, PBP2a, encoded by the mecA or mecC gene. Their 
resistance is typically associated with hyperproduction of beta-lactamases or, 
in some cases, point mutations in PBP genes. BORSA cannot be classified as 
either truly methicillin-resistant or truly methicillin-susceptible strains. 
However, they are frequently misidentified, which poses an obvious 
epidemiological and therapeutic threat. BORSA strains are commonly isolated from 
humans and animals, and are found both in hospitals and in a community setting. 
The epidemiology and clinical presentation of BORSA infections seem to be 
similar to those for MRSA; these infections are usually more severe than those 
caused by methicillin-sensitive S. aureus (MSSA). Treatment of severe infections 
caused by BORSA may be ineffective, even with larger doses of oxacillin. The 
available evidence suggests that BORSA represent a frequently neglected problem, 
and their emergence in new environments implies that they need to be monitored 
and accurately distinguished from MSSA and MRSA.