A study was conducted to determine the mode of action of antifungal agents in respect to the relationships between the antifungal activities and morphological effects, drug resistance and their effects on the metabolism of pathogenic fungi. The results obtained were as follows :
The antifungal activities and morphological effects of 36 compounds were tested against 8 different strains of pathogenic fungi, such as Trichophyton, Aspergillus, Candida, etc. by the agar plate-streaking method. These compounds can be classified into 6 groups in terms of the relationships between the antifungal activities and morphological effects. Several compounds which showed strong antifungal activity exhibited a griseofulvin-like action on the fungi, a morphologic change. In view of such results obtained, the examination of an in vitro antifungal activity of drugs should be tested in addition to observing any morphologic changes which occur.
The newly isolated strains of fungi were first examined for their susceptibility to nystatin, amphotericin B, trichomycin, variotin and merzonin; Trichophyton mentagrophytes, T. rubrum and Aspergillus fumigatus showed, more or less, differences in susceptibilities to these drugs. The differences in susceptibilities observed may have depended upon the strains employed; A. fumigatus was especially conspicuous in this sense. All of the test drugs employed did not produce a significant degree of increase to resistance in all strains of A. fumigatus tested. The population of the resistant cells of A. fumigatus was determined by the replica plating method. Cells which were highly resistant to amphotericin B or trichomycin were much more numerous than those resistant to nystatin, variotin or merzonin, and the drug resistance of these isolated cells were stable as demonstrated by ten successive transfers on drug-free agar media. Thus, it might be concluded that the pathogenic fungi have a difficulty in acquiring drug resistance to antifungal agents; however, the existence of highly resistant strains and cells may demonstrate the acquisition of an increase in drug resistance, similar to bacteria and other microorganisms, if such drugs were to be used extensively.
The effects of antifungal agents, especially polyene antibiotics, such as nystatin, trichomycin and amphotericin B, and, also, for comparison, pentachlorophenol, a synthetic drug, on the metabolism of Candida albicans or Candida pseudotropicalis were investigated in preliminary experiments in order to determine the primary action of these antifungal agents. The endogenous respiration, the O2 uptake by the utilization of glucose, the 32P-phosphate incorporation into the cells and into several kinds of metabolites, and the incorporation of 14C-amino acids mixture into the cells were examined in the presence of antifungal agents of concentration which inhibited the growth of 50% or more cells. Neither stimulation nor inhibition was observed in the endogenous respiration and the O2 uptake by the oxidation of glucose in the presence of the employed drugs. The incorporation of 32P-phosphate into the cells was markedly suppressed by these drugs, especially by the polyene antibiotics. Marked suppression of the incorporation of 32P-phosphate into the TCA-soluble fraction and a slight inhibition of it into the phospholipid and RNA fractions were demonstrated. The incorporation of 14C-amino acids into the cells was also inhibited.
