Kinetics and Mechanisms of Drug Action on Microorganisms XVII: Bactericidal Effects of Penicillin, Kanamycin, and Rifampin with and without Organism Pretreatment with Bacteriostatic Chloramphenicol, Tetracycline, and Novobiocin

Escherichia coli (ATCC 12407), generating in the logarithmic growth phase, was killed by penicillin, kanamycin, and rifampin. The logarithm of number of viables of the drug-treated culture decreased linearly with time after a certain lag period and above a minimum drug concentration. The slopes of these plots were characteristic of kill rate constants and were linearly dependent on drug concentrations. The microorganisms developed resistance against the cidal action of the antibiotics. The lower the penicillin concentration and the rate of kill, the greater is the number of resistant organisms, most probably formed by generation in the presence of penicillin. Penicillin was inactivated with time in the culture of organisms, and the resistant individuals eventually grew and were shown to be insensitive to penicillin. When organisms were exposed to the cidal action of kanamycin or rifampin, the appearance of resistant individuals was ascertained. Penicillin yielded debris and ghost cells of E. coli. No such apparent lysis was observed with kanamycin or rifampin. The activity of kanamycin increased with the pH of the medium, so activity can be assigned to the uncharged or lesser charged fraction of the drug concentration. The addition of penicillin or kanamycin to the organisms treated with tetracycline, chloramphenicol, or novobiocin did not show any significant difference in killing rate from organisms not previously treated with such bacteriostatic agents. However, the combination of a bactericidal and a bacteriostatic antibiotic depressed the resistant mutant formation of resistant individuals over that of the bactericidal drug alone. When rifampin was added to organisms pretreated with tetracycline or chloramphenicol, the cidal action of rifampin was significantly reduced.