Sulopenem is a thiolanylthiopenem derivative patented by American multinational pharmaceutical corporation Pfizer Inc as an antibiotic with broad-spectrum antibacterial activity against most gram-positive and gram-negative bacteria. Sulopenem showed concentration-dependent bactericidal activities against Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, and Acinetobacter calcoaceticus. Morphological observation using a phase-contrast microscope revealed that sulopenem induced spherical cell formation with E. coli and K. pneumoniae at lower concentrations and bacteriolysis at higher concentrations. Therapeutic efficacies of sulopenem against systemic infections in mice were almost equal to those of imipenem against Streptococcus pneumoniae.