438 BACTERIA PATHOGENIC TO MAN 



present in the air and in cultures made from the throat. They consist 

 of round or oval cells, usually many times larger than the bacteria. 

 They usually reproduce themselves by budding, a portion of the proto- 

 plasm budding and finally being cut off to form a new individual. 



For many centuries blastomycetes, or yeasts, have proven themselves 

 to be of great benefit to man, untold millions of them being used daily 

 in breweries, distilleries, and other industries. Until a few years ago 

 this group of organisms stood alone among other allied forms of life 

 as being the only one in which pathogenic species were unknown. It 

 is not more than ten years since the discovery of the first of the disease- 

 producing yeasts. Since that time these organisms have been studied 

 not only because of their interesting biological and physiological 

 characteristics, but also from the point of view of the physician and 

 etiologist. Our present knowledge concerning the pathogenic yeasts 

 may be briefly summarized as follows: 



The position which the yeasts occupy in systematic biology (botany) 

 has not, thus far, been accurately determined. In fact, it is even doubted 

 whether they constitute independent fungi or are perhaps a particular 

 form of growth of more highly organized fungi, especially of the mould 

 fungi. This hypothesis was formulated by Brefeld about thirty years 

 ago, but has not thus far been proved. For the present it seems advis- 

 able to retain the yeasts in a group of fungi by themselves. 



The chief characteristic of the yeasts is their peculiar method of 

 reproduction which in most cases is by means of budding. For this 

 reason these organisms go by the name of blastomycetes in contrast to 

 the fission fungi, or schizomycetes, and the mould fungi, or hyphomycetes. 

 A transition between the blastomycetes and the hyphomycetes is 

 formed by the oidien, which at one time grow to long threads, at another 

 time (under certain conditions almost exclusively) multiply by budding. 

 But no hard-and-fast line exists between these classes, for the yeasts 

 can at times develop short hyphoe, at other times, in rare cases, form 

 new individuals by segmentation. 



The most important property of yeasts, though one not possessed 

 by all to the same degree, is that of producing alcoholic fermentation. 

 In practice we distinguish between the yeasts that can be employed 

 practically, "culture yeasts," and those which often act as disturbing 

 factors, so-called "wild" yeasts. 



The shape of most of the culture yeasts is oval or elliptical (Fig. 139). 

 Round or globular forms are more often met with among the wild 

 species and such as excite only a slight degree of fermentation. They 

 are known as " torula " forms. But sausage-shaped and thread forms 

 are also met with. 



The individual yeast cells are strongly refractive, so that under the 

 microscope at times they have almost the lustre of fat droplets. This 

 is important because in examining fresh tissues the yeast cells may be 

 hard to distinguish from fat droplets, often requiring the aid of certain 

 reagents for their identification. 



The size of the individual yeast cells varies enormously, even in 



