Novel penem molecules with heterocycle substitutions at the 6 position via a methylidene linkage were investigated for their activities and efficacy as β-lactamase inhibitors. The concentrations of these molecules that resulted in 50% inhibition of enzyme activity were 0.4 to 3.1 nM for the TEM-1 enzyme, 7.8 to 72 nM for Imi-1, 1.5 to 4.8 nM for AmpC, and 14 to 260 nM for a CcrA metalloenzyme. All the inhibitors were more stable than imipenem against hydrolysis by hog and human dehydropeptidases. Piperacillin was combined with a constant 4-μg/ml concentration of each inhibitor for MIC determinations. The combinations reduced piperacillin MICs by 2- to 32-fold for extended-spectrum β-lactamase (ESBL)-producing Escherichia coli and Klebsiella pneumoniae strains. The MICs for piperacillin-resistant (MIC of piperacillin, >64 μg/ml) strains of Enterobacter spp., Citrobacter spp., and Serratia spp. were reduced to the level of susceptibility (MIC of piperacillin, ≤16 μg/ml) when the drug was combined with 4, 2, or 1 μg of these penem inhibitors/ml. Protection against acute lethal bacterial infections with class A and C β-lactamase- and ESBL-producing organisms in mice was also demonstrated with piperacillin plus inhibitor. Median effective doses were reduced by approximately two- to eightfold compared to those of piperacillin alone when the drug was combined with the various inhibitors at a 4:1 ratio. Pharmacokinetic analysis after intravenous administration of the various inhibitors showed mean residence times of 0.1 to 0.5 h, clearance rates of 15 to 81 ml/min/kg, and volumes of distribution between 0.4 and 2.5 liters/kg. The novel methylidene penem molecules inhibit both class A and class C enzymes and warrant further investigation for potential as therapeutic agents when used in combination with a β-lactam antibiotic.