In vitro activity of beta-lactam antibiotics to community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA)

Community-associated (CA) MRSA often display low MIC values against oxacillin. The in vitro activity of various beta-lactam antibiotics against heterogeneous CA-MRSA (n = 98) isolated in a low endemic area was determined by Etest, and Mueller-Hinton agar (MUHAP) was compared with Mueller-Hinton agar supplemented with 2% NaCl (MUHSP). In general, the CA-MRSA isolates showed higher MIC values for the various beta-lactam antibiotics on MUHSP compared with MUHAP. MIC values for oxacillin ranged from 1 to > 256 mg/L on MUHSP. Cephalothin, representing the first generation of cephalosporins, showed MICs from 0.75 to 96 mg/L and the MIC50 and MIC90 for cefuroxime, cefotaxime and cefepime, representing the second, third and fourth generations, respectively, were rather high. However, the MIC50 and MIC90 for ceftobiprole (fifth generation) were 1.5 and 2 mg/L, respectively, on MUHSP. The MIC50 and MIC90 for imipenem were 0.75 and 2 mg/L, respectively, on MUHSP. Only 3/98 (3%) CA-MRSA isolates showed a MIC > 4 mg/L. Consequently, low MIC values for imipenem, lower than those of the newly developed fifth generation cephalosporins, were found among CA-MRSA. These findings may be considered for further studies including clinical trials in order to evaluate carbapenems as a potential treatment option for infections caused by CA-MRSA.