Objectives: To report the linezolid in vitro activity obtained during the 2015 ZAAPS Program.
Methods: In total, 7587 organisms causing documented infections were consecutively collected in 65 centres in 32 ex-USA countries. Broth microdilution susceptibility testing was performed. Isolates displaying linezolid MIC results of ≥ 4 mg/L were molecularly characterized.
Results: Linezolid inhibited >99.9% of Staphylococcus aureus at ≤ 2 mg/L, with MIC50 results of 1 mg/L, regardless of methicillin resistance. A similar linezolid MIC50 result (0.5 mg/L) was observed for CoNS, with the vast majority of isolates (99.7%) also inhibited at ≤ 2 mg/L. Three CoNS (linezolid MIC, 16-64 mg/L) from Italy were found to contain alterations in the 23S rRNA and/or L3/L4 ribosomal proteins. One isolate also harboured cfr. Linezolid exhibited consistent modal MIC and MIC50 results (1 mg/L) for enterococci regardless of species or vancomycin resistance. One Enterococcus faecalis (linezolid MIC, 8 mg/L) from Galway, Ireland, carried optrA. One Enterococcus faecium (linezolid MIC, 16 mg/L) from Italy contained a G2576T mutation in the 23S rRNA. All Streptococcus pneumoniae, viridans group streptococci and β-haemolytic streptococci were inhibited by linezolid at ≤ 2, ≤ 2 and ≤ 1 mg/L, respectively, with equivalent MIC90 results (1 mg/L for all groups).
Conclusions: These results document the continued long-term and stable in vitro potency of linezolid and a limited number of isolates with decreased susceptibility to linezolid (MIC, ≥4 mg/L). The latter isolates showed primarily mutations in the 23S rRNA gene and/or L3/L4 proteins, with plasmid-mediated resistance (cfr and optrA) also present, albeit at a low prevalence.
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