294 



YEASTS AND YEAST-LIKE FUNGI 



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In many species of Candida, especially those of medical impor- 

 tance, the yeast cell is the predominant growth form, mycelium being 

 produced in some species only under particular conditions. If it is 



grown on glucose agar slants, 

 for instance, there occurs a 

 creamy white growth of pasty 

 consistency and yeast-like odor. 

 Microscopic examination of such 

 a culture shows only round or 

 oval budding cells which cannot 

 be distinguished from true yeasts. 

 In old agar slant cultures, how- 

 ever, one may find the growth 

 dipping down into the agar in the 

 form of narrow bands or fine 

 filaments; and microscopic ex- 

 amination of this part of the cul- 

 ture will reveal, in addition to the 

 budding yeast cells, some fila- 

 ments of mycelium. The nature 

 of the mycelium depends in part 

 upon the culture medium. In 



VJ 



Fig. 115. Diagram illustrating the 

 characteristics of the pathogenic Can- 

 didas. In a gelatin stab culture, only 

 yeast-like cells are found at the sur- 

 face, but filaments of mycelium radiate 

 from the depths of the stab; these give 

 rise to yeast-like cells by budding. 



corn meal agar a true mycelium is formed and in several species of Can- 

 dida it is very extensive. A pseudomycelium may also be formed and 

 in some media with some species it is only mycelium that is formed. 

 One may demonstrate a mycelium by making a stab in beef pep- 

 tone gelatin. At the point of puncture there appears a heaped-up 

 colony of buttery consistency, composed entirely of yeast cells. But 

 after a few days there develop along the course of the stab many 

 fine tufts of mycelium that radiate into the gelatin at right angles 

 to the line of the stab, giving the growth a characteristic fir-tree 

 appearance. If the tube is cut and microscopic preparations are 

 made from these lateral projections, they will be found to be com- 

 posed of mycelium, from which yeast cells are budded off in clusters 

 along the sides and at the tips (Fig. 115). Such a stab culture may 

 be used to differentiate these "medical" species of Candida from true 

 yeasts. Corn meal agar is more convenient (see page 53) and on 

 this medium mycelium growth may usually be demonstrated from 

 the surface growth penetrating into the substrate. In glucose pour- 

 plate cultures the surface colonies appear exactly like yeast colonies, 

 and contain in many species only single budding cells. The deep 

 .colonies are usually at first lens-shaped and contain only yeast cells, 



