An international activity and spectrum analysis of linezolid: ZAAPS Program results for 2011.

Through a continuing resistance surveillance monitoring program, linezolid was shown to maintain its spectrum and potency against a collection of 8059 clinically relevant Gram-positive strains collected from patients at 79 medical centers in 33 countries and Hong Kong. Linezolid MIC90 values were 2 µg/mL for methicillin-resistant and -susceptible Staphylococcus aureus and enterococci, and the MIC90 value was 1 µg/mL for coagulase-negative staphylococci (CoNS), ß-hemolytic streptococci, Streptococcus pneumoniae, and viridans group streptococci. Reference broth microdilution susceptibility testing for linezolid demonstrated a 99.83% susceptibility rate for all organisms. All S. aureus were inhibited by ≤2 µg/mL. Three (0.3%) of 928 strains of CoNS had a linezolid MIC of 4 µg/mL and contained the cfr resistance gene; 1 also had a mutation in L3. There were 14 linezolid-resistant strains detected from 7 countries (Brazil [5], France [1], Germany [2] Greece [2], Italy [2], Ireland [1], and Spain [1]) representing 5 species (E. faecium, S. capitis, S. epidermidis, S. hominis, S. lugdenensis). A mobile cfr gene was noted in 2 species having elevated linezolid MIC values; one was a S. haemolyticus isolate with a MIC at 4 µg/mL. Resistance rates were as follows for the 6 groups of organisms sampled in the 2011 ZAAPS Program: CoNS, 1.2%; enterococci, 0.39%; among S aureus, S. pneumoniae, viridans group streptococci, and ß-hemolytic streptococci, no resistance was detected. As the activities of commonly used antimicrobials continue to be compromised by evolving resistance mechanisms in Gram-positive pathogens, linezolid-resistant strains remain uncommon and without increasing occurrence.