In vitro activity of linezolid, tigecycline, and daptomycin on methicillin-resistant Staphylococcus aureus blood isolates from adult patients, 2006-2008: stratified analysis by vancomycin MIC.

BACKGROUND: The recent molecular epidemiological studies concerning epidemiological studies concerning methicillin-resistant Staphylococcus aureus (MRSA) blood isolates from adult patients and susceptibilities of MRSA isolates with high vancomycin minimum inhibitory concentrations (MICs) (≥2 mg/L) to linezolid, tigecycline, and daptomycin in Taiwan remain limited. The objectives of the study were (1) to better understand the change of molecular epidemiology of MRSA blood isolates and (2) to evaluate the in vitro activity of new anti-Gram-positive agents, including linezolid, tigecycline, and daptomycin.

METHODS: A total of 470 nonduplicate MRSA blood isolates from adult patients (older than 18 years) were collected from January 2006 to December 2008. The MICs of these isolates to various antibiotics were determined. Multilocus sequence typing was also performed in all isolates.

RESULTS: Three sequence types (STs) constitute most (92.1%) of these 470 MRSA isolates: ST239 (53.2%), ST59 (23.2%), and ST5 (15.7%). Throughout the 3-year study, the ST239 strain remained predominant but with a significant trend of declining annually (p=0.03). In contrast, the proportion of isolates of ST59 increased, although the increment was insignificant (p=0.14). The proportion of MRSA isolates with a vancomycin MIC of 2 mg/L was 17.2%. All of these isolates with a vancomycin MIC of 2 mg/L were susceptible to linezolid and tigecycline, whereas most of them (98.8%) were susceptible to daptomycin.

CONCLUSIONS: ST239 remained predominant during the 3-year period but with a significant trend of declining. Moreover, linezolid, tigecycline, and daptomycin remained highly active against MRSA blood isolates, even with a vancomycin MIC of 2 mg/L.