Inhibits bacterial cell wall synthesis by binding to one or more of the penicillin-binding proteins. Usually bactericidal. Active in vitro and in clinical infections against Gram-positive aerobic bacteria, such as Staphylococcus aureus and S. epidermidis (including penicillinase-producing strains), Streptococcus pyogenes (group A β-hemolytic streptococci), and S. pneumoniae. Oxacillin-resistant (methicillin-resistant) staphylococci and most enterococci are resistant. Active in vitro and in clinical infections against Gram-negative aerobic bacteria such as Haemophilus influenzae, Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis. Inactive against anaerobic bacteria, fungi, and viruses.