[In vitro studies on the activities of cephalothin against selected bacteria (author's transl)].

: Cephalothin antimicrobial activities were studied against selected bacteria with special reference to the time of exposure of microbes to the drug assessed in broth medium. The minimal concentration of this drug producing 10(0)-10(1) reduction of colony forming unit (CFU) per ml after given exposure times -an approximation to the theoretical bacteriostatic concentration inhibiting increase or decrease of CFU- was designated as minimally reducing concentration (MRC). The concentration was determined after 6 h, 18 h and 42 h of incubation and designated as 6-h, 18-h and 42-h MRC, respectively. Conventional minimal inhibitory concentration (MIC) with 18 h exposure time was also studied. The minimal concentration of the drug yielding 99.9% reduction of CFU after an exposure to the drug was determined and designated as minimal lethal concentration (MLC). The determinations were made at 6 h, 18 h and 42 h of incubation and designated as 6-h, 18-h and 42-h MLC, respectively. The data indicated that the bacteriostatic concentration for Escherichia coli and Klebsiella sp. with brief exposure time (6-h MRC) was 2-4 times lower than the MIC. If the exposure time was prolonged (18-42 h), the bacteriostatic concentration increased and almost agreed with the MIC. A significant effect of inoculum size was observed on the MRC values for the Gram-negatives when the values were determined with the inoculum concentrations at 10(6)-10(7) CFU/ml and 10(3)-10(4) CFU/ml. In contrast to these Gram-negatives, the 6-h MRC for most strains of Staphylococcus aureus and Enterococcus roughly agreed with the MIC. If the ratio of bacteriostatic to bactericidal concentrations was used as the criterion, the mode of action of cephalothin appeared to be bactericidal for most of the Gram-negatives. This drug was bacteriostatic for a number of the Gram-positive strains, in particular to those of Enterococcus, especially when microbes were exposed to the drug over a brief period of time.