Drug resistance in a mutant strain of Aspergillus nidulans

Parsons, Sheila G. (1962) Drug resistance in a mutant strain of Aspergillus nidulans. Masters thesis, Memorial University of Newfoundland.

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Abstract

The effects of the inhibitors proflavine and phenylmercuric acetate upon germination of conidia of a mutant of Aspergillus nidulans were investigated. Conidia were observed microscopically in the germination process, on media containing various concentrations of the inhibitors. Resistance was induced to gradually increasing concentrations of the inhibitors by continual subculture of the organism in their presence. The effect of this training process upon resistance to the inhibitor, during the various phases of germination and other stages of the asexual life cycle of the organism, were studied at length. -- The untrained strain of the mutant proved relatively resistant to proflavine, and a small inoculum could grow on petri plates containing concentrations of the inhibitor up to 400 ppm. This strain could be trained to resist concentrations of proflavine up to 475 ppm on petri plates. The principal effects of proflavine on germination were shown to be inhibition of spore swelling, prevention of the production of a germ tube, and decreased production of mycelium. In growth chambers, strains either trained or untrained to proflavine, germinated almost equally well on concentrations up to 800 ppm, but on 2000 ppm a greater proportion of spores of the resistant strain succeeded in germinating than did those of the untrained strain. -- Nearness of spores was found to increase the initial germination rate of both strains. The size of conidia of trained and untrained strains just prior to germination was found to be between 7.1 and 8.2 μ. The majority of those spores which failed to germinate on 2000 ppm proflavine retained a diameter of less than 4.4 μ. -- The mutant had been trained to grow on 0.50 ppm phenylmercuric acetate in petri plates. In growth chambers (where the inoculum used was much heavier than that added to petri plates) growth of both trained and untrained strains was obtained on 0.50 ppm. The closeness of spores increased the germination rate. The inhibitor at concentrations higher than 0.50 ppm completely inhibited the swelling of the spores and other phases of germination. Heavy deposits were seen in the hyphae of both trained and untrained strains growing in the presence of phenylmercuric acetate. These deposits were observed to appear much earlier in the hyphae of the resistant strain than in those of the untrained strain. Hyphal growth was retarded in both strains, and sporulation did not occur in the sensitive strain in the presence of the drug. Conidiophores produced by the trained strain growing in the presence of phenylmercuric acetate were considerably shorter than normal, though the conidial heads and conidia appeared normal. The trained strain was found able to overcome the effects of the inhibitor on all phases of the asexual life cycle, after continual subculture in its presence, at concentrations up to 1.0 ppm. -- These findings are discussed in the light of the available literature. Two major theories which might explain the attainment of resistance by training, namely enzyme adaptation and gene mutation, are considered. The possibility that phenylmercuric acetate may be absorbed in a non-toxic form by the hyphae of resistant strains is discussed.

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