Longitudinal Analysis of Tigecycline Activity against US Isolates of Staphylococcus aureus Based on Patient Location and Specimen Source

Background: Tigecycline (TIG) is a novel glycylcycline that was approved in 2005 in the US for treatment of complicated skin and skin structure infections and complicated intra-abdominal infections. S. aureus (SA), including methicillin-resistant S. aureus (MRSA), is a leading cause of skin and skin structure infections. This study examines the in vitro activity of TIG against SA, including MRSA, over the last five years. Additionally, data were stratified according to patient location (PL) and specimen source (SS) to determine if any variation in TIG activity against SA was apparent among these subpopulations. Methods: SA isolates were collected from multiple locations across all nine US Bureau of Census regions during ’03—’04 (N = 1,796), ’05 (N = 1,802), and ’06—’07 (N = 1,800) and centrally tested using broth microdilution according to current CLSI standards. TIG activity was analyzed by patient locations (PL; outpatient [OP], intensive-care unit [ICU], and inpatient non-ICU [IP]) and by specimen source (SS; blood [BL], respiratory [RP], and skin and skin structure [SST]). FDA breakpoints were used to interpret all TIG MIC results. Results: Against SA overall, TIG had an MIC50/90 of 0.12/0.12mg/L in ’03—’04 and an MIC50/90 of 0.12/0.25mg/L in both ’05 and ’06—’07. Throughout the study periods, percent susceptibility (%S) of TIG remained ≥99.5%. TIG activity by MIC90 was largely unaffected by methicillin susceptibility status of the isolate (MIC90: 0.12mg/L for MSSA and 0.25mg/L for MRSA in ’03—’04, 0.25mg/L for MSSA and MRSA in ’05, and ’06—’07). In each study period, the activity of TIG by MIC90 was identical among all PL (OP, ICU, and IP) and SS (BL, RP, SST) evaluated relative to TIG activity against SA overall. In the most recent study period (’06—’07), SA isolates were ≥99% S to TIG regardless of PL or SS. Conclusion: By MIC90, TIG in vitro activity was stable against SA over the past five years spanning both its development and introduction to use and the %S of SA isolates to TIG remained unaltered and high (>99.5%). TIG activity by MIC90 was not notably affected by the methicillin susceptibility status of the isolate and was consistent across all PL and SS evaluated.