Independent antibiotic actions of hetacillin and ampicillin revealed by fast methods.

Fast methods for measuring inhibition of growth of Escherichia coli or Staphylococcus aureus by hetacillin or ampicillin showed that the rate of inhibition was a function of the antibiotic concentration. When ampicillin concentrations were measured rapidly, hetacillin was shown to have a half-life of 20 min at 37 C and pH 6.7. Hetacillin was much less susceptible to penicillinase (beta-lactamase) than was ampicillin. An inhibitory effect of hetacillin was shown in the presence of beta-lactamase, which destroyed ampicillin activity very rapidly.