AbstractBackgroundOmadacycline (ZL-2401) is a semi-synthetic derivative of minocycline. It has a broadspectrum activity against Gram-positive and Gram-negative bacteria, and atypical pathogens. The objective of this study was to evaluate the antibacterial activity of omadacycline against recently collected bacterial isolates from Chinese patients.ResultsOmadacycline showed potent activity against all Gram-positive pathogens:S. aureusMICs were low regardless of susceptibility to methicillin (methicillin-resistantStaphylococcus aureus, MRSA:N = 97, MIC_50/900.12/0.25 mg/L, 98.5% susceptible; methicillin-sensitiveStaphylococcus aureus, MSSA:N = 100, MIC_50/900.12/0.12 mg/L, 100.0% susceptible). Omadacycline was also very effective against β-haemolytic streptococci (MIC_50/90, 0.06/0.12 mg/L), viridans group streptococci (MIC_50/90,<0.03/0. 06 mg/L), and enterococci (MIC_50/90, 0.03/0.12 mg/L). AgainstS. pneumoniae, omadacycline was highly active regardless of penicillin-resistance (MIC₉₀0.06 mg/L) and despite the fact that less than 10.0% of these strains were susceptible to tetracycline. Omadacycline exhibited good in vitro activity againstEnterobacteralesisolates (MIC_50/90, 2/8 mg/L), inhibiting 81.7% of the isolates at ≤4 mg/L.M. catarrhalisisolates (MIC_50/90, 0.12/0.25 mg/L) were fully susceptible to omadacycline at ≤0.5 mg/L.ConclusionsOmadacycline showed potent in vitro activity against most common bacterial pathogens, and even against highly resistant problem pathogens, such as MRSA, penicillin-R and tetracycline-RS. pneumoniaeand enterococci. The susceptibility rate of Chinese isolates was similar to those reported in other countries, but the decreased activity againstK. pneumoniaeisolates in the present study should be noted.