In Vitro Study on Effects of Combined Use of Antibiotics

The following in vitro studies were carried out using 23 strains of Klebsiella pneumoniae isolated from clinical materials in recent years:1) The MIC of Cephalothin (CET) against K1. pneumoniae was determined by the agar dilution method. Then, the amount of 0.002ml of CET-inactivating substance, which is supposed to consist mainly of Cephalosporinase with the ability to inactivate 250, 000μg of CET/ml was added to each of surfaces of the plate which had previously been inoculated with test bacteria. After an overnighti ncubation at 37°C, the minimal concentration of CET at which no colony appeared was judged as “MBC”. As a result, it was found that the values of MIC and “MBC” were markedly affected by the number of inoculated bacteria. In the group with the undiluted culture of bacteria, the MIC was 25-50μg/ml in most of the strains and “MBC” was 400μg/ml or more in almost all of the strains; on the other hand, in the group with 10, 000 fold dilutions, MIC and “MBC” showed almost the same values, from 3.13 to 6.25μg/ ml in most of the strains.2) The effects of the combined use of Gentamicin (GM) and CET were examined using agar plate inoculated with the undiluted culture of bacteria by the two-dimensional, two-fold dilution procedure. The combined use of GM and CET did not show such a significant difference between MIC and “MBC” as seen in the single use of CET.3) The effects of the combined use of GM and CET on 20 strains of K 1. pneumoniae were examined by the agar dilution method. By the two-dimensional, two-fold dilution procedure of these drugs, the MIC of CET which was used in combination with a certain concentration of GM and the MIC of GM which was used in combination with a certain concentration of CET were examined. Then, the number of bacteria having these MICs was counted and shown in the chess-board.Our survey on the number of bacteria, which growth was inhibited by the various combinations of GM and CET, clearly demonstrated that the in vitro antibacterial effects of GM and CET upon K1. pneumoniae were synergistic. In addition, it was confirmed that the concentrations of both antibiotics needed to inhibit the growth of almost all of the strains correspond to the blood concentrations which are obtained by usual doses of these antibiotics.