High concentrations of polymyxin B inhibited the growth of Candida albicans and Saccharomyces cerevisiae. When these yeasts were incubated with concentrations of polymyxin B too low to affect growth, and were then exposed to tetracycline, protein synthesis was inhibited and at least 99% of the organisms were killed. Neither inhibition of protein synthesis nor cell death occurred in cultures treated with high concentrations of tetracycline alone. We conclude that polymyxin B at high concentrations affects the cell membrane of yeasts, which results in inhibition of growth. At low concentrations, it increases the permeability of the yeast cell membrane to tetracycline, which then inhibits protein synthesis and leads to cell death.Polymyxin B is a surface-active bactericidal antibiotic which alters the permeability of the bacterial cell envelope by binding to the negatively charged phospholipid component of the membrane and causing cell lysis (4). Because eukaryotic cells also contain phospholipids in their cell membranes (7), it was reasonable to expect that polymyxin B would disrupt permeability barriers in them as well. Two previous studies have shown this to be true in a protozoan (6) and in the yeast Candida tropicalis (5). The latter study, however, failed to demonstrate a polymyxin B effect in a variety of other yeasts.Several agents have been shown to be ineffective against yeasts because of failure to penetrate the cytoplasmic membrane and gain access to their site of action. For example, tetracycline has been shown to inhibit protein synthesis in extracts of yeasts (1) but was ineffective against whole organisms (C. N. Kwan et al., Bacteriol. Proc., p. 179, 1972). In this report, we have investigated the antifungal properties of polymyxin B against certain yeasts, and have exploited the changes in membrane permeability caused by this antibiotic to potentiate the antifungal effect of tetracycline. Synergism studies. The susceptibility of each organism to polymyxin B and tetracycline was determined by the broth-dilution method, as previously described (2). A wide range of antibiotic concentrations was examined for each organism, and cell viability was determined by colony counts after 1, 2, 3, and 7 days of incubation. The minimal inhibitory concentration (MIC) was defined as the lowest concentration of drug required to inhibit completely the growth of the organism over the designated time interval.Synergism was also determined by colony counts of each organism in the presence of the antibiotics used singly and in combination. Exposure to the two drugs was done either by 24-hr pretreatment of the yeasts with polymyxin B or by simultaneous addition of the polymyxin B and tetracycline. Our definition of antifungal drug synergy was a decrease of 100-fold or more in colony counts caused by the drugs in combination as compared with the counts when the drugs were used singly (3). The level of each antibiotic used in the combined drug studies was well below the respective MIC. In the MIC and synergism studies, a s...

No abstract

No abstract