Detection of mecA-mediated methicillin resistance and evaluation of disk-diffusion antimicrobial susceptibility characteristics of Staphylococcus saprophyticus isolates from geographically diverse locations

Staphylococcus saprophyticus, a common uropathogen, is usually susceptible to urine-concentrating antimicrobials, so routine AST is not recommended by CLSI. Our study evaluated the antimicrobial resistance profiles of 277 S. saprophyticus isolates from North America and a globally diverse cohort. Notably, 24% (67/277) of our isolates come from non-urinary sources. AST was performed against 12 antimicrobials using standard disk diffusion, PCR for mecA and mecC, PBP2a production assays, and cefinase. 5% (13/277) of isolates were mecA positive and cefinase positive, 63% (176/277) were mecA negative but cefinase positive, 4% (11/277) were mecA positive but cefinase negative, and 28% (77/277) were mecA and cefinase negative. All (277/277) isolates were susceptible to delafloxacin, ciprofloxacin, rifampin, linezolid, and nitrofurantoin and 95% (262/277) were susceptible to trimethoprim-sulfamethoxazole. Our results showed that regardless of using CLSI or EUCAST breakpoints oxacillin had low categorical agreement for mecA presence, making it unsuitable for surrogate testing, while cefoxitin disk diffusion had high very major error rate. If possible, PBP2a or mecA testing is recommended for guiding therapy for non-urinary infections. Our work supports CLSI guidelines on routine susceptibility to urinary tract antibiotics.