Methicillin-resistant Staphylococcus aureus isolates with SCCmec type V and spa types t437 or t1081 associated to discordant susceptibility results between oxacillin and cefoxitin, Central Taiwan

- The prevalence rate of these OX(S)-FOX(R) MRSA isolates could be up to around 1% in all MRSA isolates and usually belong to SCCmec type V and spa type t437-ST59 or t1081-ST45 in Central Taiwan.
- Patients with OX(S)-FOX(R) MRSA infections and responses to treatment did not always receive effective antimicrobial agents according to the susceptibility results.
- Concomitant testing using both cefoxitin- and oxacillin-based methods, but not oxacillin alone, is a reasonable and practical strategy for OS-MRSA detection.

Staphylococcus aureus isolates with discordant susceptibility results between oxacillin and cefoxitin obtained using automated microbiology systems are infrequently observed. From April 2013 to December 2014, 1956 methicillin-resistant S. aureus (MRSA) and 1761 methicillin-susceptible S. aureus isolates were obtained from different patients. Forty isolates (1.1% and 2% in case of S. aureus and MRSA, respectively) with discordant susceptibility results (oxacillin susceptible and cefoxitin resistant) and carrying mecA gene were obtained. Except 2 SCCmec type IV isolates, 38 MRSA isolates were all SCCmec type V (VT or non-VT), which were further divided into VT (n = 22) and non-VT (n = 16). The most common spa type in VT and non-VT isolates were t437 (n = 19) and t1081 (n = 13), respectively. Only 55% of patients received effective antimicrobial agents; 2 mortalities were not attributable to MRSA infection. Using standard agar dilution, 17 MRSA isolates (0.46% and 0.87% in case of S. aureus and MRSA, respectively) had oxacillin MIC in the susceptible ranges (oxacillin-susceptible MRSA [OS-MRSA]); all carried SCCmec type V (VT, n = 8; non-VT, n = 9). The most common spa-MLST types of OS-MRSA in VT and non-VT were t437-ST59 (n = 4) and t1081-ST45 (n = 7), respectively. Concomitant testing by both cefoxitin- and oxacillin-based methods is a practical strategy for OS-MRSA detection in the clinical laboratories. Continuous monitoring of OS-MRSA isolates is necessary to elucidate their impact in clinical infectious diseases.