Cefoxitin and Oxacillin Resistance Conundrums

Recently, mecA-negative Staphylococcus aureus strains with decreased susceptibility to oxacillin and cefoxitin have been sporadically reported worldwide. They are called as borderline oxacillin resistant Staphylococcus aureus (BORSA). Almost, more than 30% of such strains are often misinterpreted and reported as MRSA (methicillin resistant Staphylococcus aureus) due to the hyperproduction of beta-lactamase enzyme and other reasons which results in invitro reduced susceptibility to oxacillin as well as cefoxitin. Similar phenomenon is quite common in other staphylococci, micrococci and macrococci. In order to identify beta-lactamase hyperproducing staphylococci and other gram-positive cocci and to avoid false reporting as methicillin resistant, a one-year prospective study was conducted in Deccan college of medical sciences, Hyderabad, Telangana state, India in 2023. During one year duration, 5630 inpatient (IP) and 823 outpatient (OPD) samples accounting for total 6453 cultures were received in the microbiology laboratory. Out of this,100 cultures of different samples showing pure growth of gram-positive cocci suggestive of staphylococci, micrococci and other Gram-positive cocci were included in the present study and tested against different antibiotics panel as per CLSI guidelines (Clinical and Laboratory Standards Institute.)  Inclusion of amoxiclav disc to the antibiotics test panel as per recommendation by the special phenotypic methods for the detection of antibacterial resistance in the manual of clinical microbiology was helpful to detect the beta-lactamase hyperproducers (BHP) and helped to report correct identification of the organisms and curtailed the mismanagement of more than 95% of the patients. Therefore, I do recommend to differentiate borderline from true methicillin resistant strains to streamline the antibiotic therapy and hence further avoidance of selection of the resistant strains.