Evaluation of the in vitro activity of tigecycline against multiresistant Gram-positive cocci containing tetracycline resistance determinants

This study was undertaken to test the in vitro activity of tigecycline against 117 clinically relevant Gram-positive pathogens and to correlatethis activity with their resistance gene content. Overall, tigecycline showed a minimal inhibitory concentration (MIC) range of 0.015–0.5 mg/L,able to inhibit potently all multiresistant streptococci, enterococci and MR staphylococci. Tigecycline was active against methicillin-resistantStaphylococcus aureus (MRSA) and enterococci, with MICs for 90% of the organisms (MIC90) of 0.25 mg/L and 0.12 mg/L, respectively, beingmore active than linezolid (MIC90 = 2 mg/L) and quinupristin/dalfopristin (MIC90 = 0.5 and 2–4 mg/L, respectively). Molecular characterisationof resistance determinants demonstrated the concomitant presence of different classes of genes: in particular, tet(M), erm(B) and erm(C) inStreptococcus agalactiae; tet(O), variably associated with different erm genes, in Streptococcus pyogenes; vanA, tet(M) and erm(B) inEnterococcus faecalis, and vanA and erm(B) in Enterococcus faecium. All the MRSA strains harboured SCCmec and erm genes and 50%possessed tet(K) with tet(M) genes. Staphylococcus epidermidis strains were only resistant to erythromycin. These results clearly demonstratethat tigecycline has a MIC90 range of 0.015–0.5 mg/L both against tetracycline-susceptible and -resistant isolates carrying other resistancedeterminants, suggesting that this drug could play an important role in the treatment of infections caused by these multiresistant Gram-positivepathogens.