MUT056399 (Fab-001) is a highly potent new inhibitor of the FabI enzyme of both Staphylococcus aureus and Escherichia coli. MUT056399 is very active against S. aureus strains, including methicillin-susceptible S. aureus (MSSA), methicillin-resistant S. aureus (MRSA), linezolid-resistant, and multidrug-resistant strains, with MIC90s between 0.03 and 0.12 μg/ml. MUT056399 is also active against coagulase-negative staphylococci, with MIC90s between 0.12 and 4 μg/ml. MUT056399 is very active against the 118 S. aureus strains tested, including MSSA and MRSA isolates and linezolid-resistant and multidrug-resistant strains, with MIC90s between ≤0.03 and 0.12 μg/ml.