experiments with staphylococcus aureus and streptococcus pyogenes have shown that when cells are exposed to bactericidal concentration of benzylpenicillin or ampicillin and the antibiotic then inactivated with beta-lactamase the count of the surviving viable cells ( colony-forming units ) remains essentially constant for a period of approximately 1.5 -- 3 h before increasing at a normal rate .
C0030827
benzylpenicillin
C0002680
ampicillin