A semisynthetic β-lactam antibiotic; inhibits the growth of 30 isolates of P. mirabilis and 89% of 30 E. coli strains at ≥5 and 10 μg/ml, respectively; susceptible to bacterial β-lactamases, which leads to resistance; active against β-lactamase-producing bacteria when used in combination with β-lactamase antibiotics such as clavulanic acid; inhibits growth of β-lactamase-producing E. coli when used in combination with clavulanic acid (MICs = 4,096 and 16 μg/ml without or with clavulanic acid, respectively)