A large number of gram-positive and gramnegative bacteria (but not P-lactamase-producing strains) are sensitive to the semisynthetic broad-spectrum penicillins (ampicillin and amoxicillin). Susceptible genera include Staphylococcus, Streptococcus, Arcanobacterium, Clostridium, Escherichia, Klebsiella, Shigella, Salmonella, Proteus, and Pasteurella.