50 STUDYING MOLDS, YEASTS, AND ACTINOMYCETES 



the dilution plate method in soil or other habitats with a large fungus 

 flora. Such a medium has been devised by Waksman *^ especially for 

 soil work, but will prove of value for isolations from other material. 

 It consists of 



Just before use, i.e., after sterilizing and while melted, sulphuric acid 

 is added until the reaction is pH 3.8 to 4.0. About 0.5 to 0.6 ml. of 

 normal acid is sufficient for 100 ml. This medium has less sugar than 

 the preceding. The use of a mineral acid eliminates the danger of 

 the acid being destroyed by the growth of the mold, as might occur 

 with tartaric acid, thus eventually allowing the bacteria to develop. 



Hydrogen-ion Concentration. Although these acid media serve 

 well for the isolation of most of the common molds and yeasts, they 

 will not permit a growth of all fungi. Many of the pathogenic species 

 in particular may fail to grow. The pH limits of growth have been 

 determined for only a few species of molds and yeasts. Talice *^ 

 published data for a number of fungi pathogenic for animals and for 

 some common saprophytes, when grown on three different agar nu- 

 trient media. Very little difference was found with the different 

 media. A few examples will show the ranges. The organism of North 

 American blastomycosis grew between pH 5 and pH 8, the optimum 

 at pH 7; Sprotrichum Beurmdnni from pH 3.0 to pH 9.6, the optimum 

 at pH 5.0; Microsporum Audouini from pH 5 to pH 7, the optimum 

 at pH 6. Saprophytic species showed a wider range. Rhizopus nigri- 

 cans, for example, grew from pH 2.2 to pH 9.6; Penicillium citrinum 

 showed a similar range, as did also Oospora verticilloides. In many 

 cases fungi which tolerated a wide range of hydrogen-ion concentra- 

 tions failed to show a distinct optimum, the top of the curve form- 

 ing a broad plateau rather than a peak. The parasitic species, 

 Candida albicans, resembled the saprophytic species, growing at all 

 levels from pH 2.2 to pH 9.6, the optimum being pH 7. 



Some molds may grow in media much more acid than pH 2.2. 

 Starkey and Waksman ^'^ have grown certain molds in poorly buffered 

 media of a normality of as much as 2 and 2.5! These observations 

 have been confirmed. Mr. Owen Sletten (unpublished data) has 

 isolated several molds growing in 2A^ sulphuric acid reagent, pre- 

 sumably deriving their energy from traces of organic materials. 



