Omadacycline in Vitro Activity Against a Molecularly Characterized Collection of Clinical Isolates with Known Tetracycline Resistance Mechanisms

This study evaluated the in vitro activity of omadacycline against a broad collection of clinical isolates with molecularly characterized tetracycline resistance mechanisms.1 A total of 167 gram-positive and gram-negative clinical isolates from the worldwide SENTRY Antimicrobial Surveillance Program, the majority (79%) of which were from the 2016 sampling year, were included.

Against tetracycline-resistant gram-positive isolates, omadacycline (minimal inhibitory concentration [MIC]_50/90, 0.12/0.25 μg/mL) and tigecycline (MIC_50/90, 0.06/0.25 μg/mL) showed similar results tested against Staphylococcus aureus carrying tet(K). Omadacycline (MIC₉₀, 0.25–2 μg/mL) and tigecycline (MIC₉₀, 0.12–1 μg/mL) showed potent MIC results against gram-positive isolates carrying tet(L) and/or tet(M). Tetracycline and doxycycline had MIC₉₀ values of ≥8 μg/mL against isolates carrying tet genes, except for doxycycline (MIC_50/90, 0.5/0.5 μg/mL) that was active against those carrying tet(K) genes. Against tetracycline-resistant gram-negative isolates, omadacycline (MIC_50/90, 1/4 μg/mL) and tigecycline (MIC_50/90, 0.25/0.5 μg/mL) had the lowest MIC results against gram-negative Enterobacteriaceae isolates carrying tet(B). Omadacycline showed MIC₅₀ results of 2, 4, and 4 μg/mL, respectively, against isolates carrying tet(D), tet(A), and tet(A) + tet(D). Among tetracyclines, omadacycline (MIC_50/90, 2/8 μg/mL) and tigecycline (MIC_50/90, 0.5/2 μg/mL) demonstrated the lowest MIC results when tested against gram-negative isolates harboring a combination of other tet genes. Tetracycline and doxycycline were not active in vitro against gram-negative Enterobacteriaceae isolates carrying tet genes.

These results indicate that omadacycline is not adversely affected against molecularly characterized gram-positive clinical isolates carrying acquired tet genes.