Oxacillin is approved for the treatment of infections caused by penicillinase producing staphylococci which have demonstrated susceptibility to this antibacterial drug.

In 2017, the Clinical Laboratory and Standards Institute (CLSI) updated the oxacillin breakpoints for Staphylococcus spp. based on the ability to detect oxacillin (methicillin) resistance by phenotypic methods among Staphylococcus spp. other than S. aureus or S. lugdunensis. Since then, CLSI has revised the oxacillin breakpoints several times (2018 through 2021) primarily due to newer bacterial identification methods that can readily identify staphylococci to the species level. Historically, mecA gene mediated methicillin resistance had been uncommon in staphylococci other than S. aureus; however, several studies over the past decade have demonstrated that methicillin-resistance in other Staphylococcus spp. is increasingly common. Similar to methicillin-resistant S. aureus, these Staphylococcus spp. are often resistant to many antibacterial drug classes, posing a public health challenge. CLSI’s differentiation of Staphylococcus spp. has optimized the phenotypic methods for detecting mecA mediated oxacillin-resistance among the species.

There was no change in the oxacillin susceptibility test interpretive criteria for S. aureus or S. lugdunensis; disk diffusion correlates are not recommended for these species. The clinical microbiology analyses showed that adjusting the oxacillin MIC susceptible breakpoint to ≤ 0.5 mcg/mL and the resistant breakpoint to ≥ 1mcg/mL led to fewer major errors (0.3%) and very major errors (2.8%) for the most commonly isolated species of coagulase-negative Staphylococcus other than S. epidermidis (i.e., S. capitis, S. haemolyticus, S. hominis and S. warneri). When data for these four species were combined with data generated for S. epidermidis, S. pseudintermedius and S. schleiferi, the new oxacillin MIC breakpoints also resulted in fewer major errors (0.9%) and very major errors (1.6%). Analysis of the oxacillin disk diffusion correlates demonstrated there was a need to revise the susceptible and resistant breakpoints, ≥ 18 mm and ≤ 17 mm, respectively, for S. pseudintermedius, S. schleiferi and S. epidermidis. However, there was no change to the other Staphylococci spp. oxacillin disk diffusion correlates as shown in the following table.

Table. Recognized Susceptibility Interpretive Criteria for Oxacillin

Based on the available microbiology information, FDA concurs with the revised CLSI breakpoints for designated Staphylococcus species.