Some factors which affect the initiation of growth of Cryptococcus neoformans.

Howard, Dexter H. (University of California, Los Angeles). Some factors which affect the initiation of growth of Cryptococcus neoformans. J. Bacteriol. 82:430-435. 1961.-The observation that a strain of Cryptococcus neoformans failed to grow in a medium containing normal human serum led to an investigation of some of the physiological factors controlling initiation of growth by this fungus. The data presented show that the growth of C. neoformans is markedly inhibited in media with initial pH values slightly above neutrality. The growth of four strains of the fungus was completely suppressed in peptone broth at pH values above 8. The growth of two strains was partially inhibited at a pH of 7.3. Two other strains grew well at 7.3, but were partially inhibited at pH 7.5. The growth in media at pH values which partially inhibited multiplication was granular in appearance. Microscopically the granules were composed of clumps of yeast cells. The possibility is suggested that pH may exert a portion of its effect on the growth of C. neoformans by influencing those factors which affect the stability of the cells in suspension. At a temperature of 25 C, growth of the fungus was initiated in media with a pH which suppressed growth at a temperature of 37 C. Under the experimental conditions employed in these studies, proteose peptone no. 3 (Difco) serves as both a carbon and a nitrogen source. However, glucose, which is assimilated by C. neoformans in contrast to lactose, permitted the growth of the fungus at pH values which suppressed growth in the absence of the sugar. Since weakly buffered media, containing filtered, normal human serum, have an alkaline reaction higher than that shown to suppress the growth of C. neoformans, it was concluded that this factor was primarily responsible for the failure of the fungus to reproduce in tissue culture media. However, it is also possible to show that even under optimal pH conditions normal human serum exerts an anticryptococcal effect. This activity of human serum was relatively heat-stable and was expressed only in high concentrations of serum. Serum adsorbed with viable cells of C. neoformans also inhibited the growth of the fungus, but a partially purified globulin fraction of serum was not inhibitory. The data are discussed in relation to the work of other investigators.