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 (including β-lactamase-producing strains), Streptococcus pneumoniae, and S. pyogenes (group A β-hemolytic streptococci). Also active in vitro against Enterococcus durans, E. faecalis, Listeria monocytogenes, Staphylococcus epidermidis, S. saprophyticus, S. warneri, Streptococcus agalactiae (group B streptococci), and groups C, D, F, and G streptococci. Active in vitro and in clinical infections against Gram-negative aerobic bacteria such as Haemophilus influenzae and Moraxella catarrhalis (including β-lactamase-producing strains). Also active in vitro against Citrobacter diversus, Escherichia coli, Klebsiella pneumonia, Neisseria gonorrhoeae, (including β-lactamase-producing strains), Proteus mirabilis, Salmonella, Shigella, and Vibrio. Inactive against most strains of Acinetobacter, Enterobacter, Morganella morganii, P. vulgaris, Providencia, Pseudomonas, and Serratia. Active in vitro against anaerobes such as Prevotella melaninogenicus, Clostridium difficile, C. perfringens, Fusobacterium, Peptostreptococcus, and Propionibacterium acnes. Most strains of Bacteroides fragilis are resistant. Inactive against fungi and viruses.