In Vitro Activity of Fidaxomicin (OPT-80) Tested against Contemporary Clinical Isolates of Staphylococcus spp. and Enterococcus spp.

tigational, nonabsorbed oral agent being developed for the treatment of Clostridium difficile infection (CDI), which is not uncommon among hospitalized patients, resulting in longer hospital stays and increased medical care costs and mortality (2, 6, 7, 9–13, 15, 16). Current CDI therapies are compromised by high recurrence rates and risk of selection for vancomycin-resistant enterococci (VRE) and methicillin-resistant Staphy-lococcus aureus (MRSA) colonization (1, 14). Fidaxomicin, similar to other RNA polymerase inhibitors (8), is active against gram-positive organisms while exhibiting little activity against gram-negative species (7). Furthermore, fidaxomicin clinical trials (phase 2 dose-ranging and phase 3 versus vanco-mycin for the treatment of CDI) have demonstrated a safety profile (15) similar to that of vancomycin but with a statistically and clinically significant reduction in relapse rate (P 0.004) and improved clinical “global cure ” (cure with no relapse; P 0.006) rates compared to vancomycin (2, 9, 12, 13). While several in vitro studies have noted fidaxomicin to be highly active against gram-positive anaerobes, including C. difficile (2, 6, 7), data on its activity against gram-positive aerobic patho-gens that can colonize the intestinal tract remain limited. In this study, we evaluated the potency and activity spectrum of fidaxomicin tested against an international collection of com-mon gram-positive isolates recovered from hospitalized pa-tients, including VRE and MRSA. All organisms tested (215 strains) originated from clinical sources (Tables 1 and 2), primarily skin and skin structure infections and bacteremia, and were collected from 2004 t