14 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



Differentiation of Bacteria, Yeasts, and Molds. The 



bacteria and yeasts are readily differentiated from molds in 

 that they are always unicellular while the molds, in gen- 

 eral, are multicellular. Bacteria always multiply by fis- 

 sion, that is, the cells divide at right angles to their longest 

 axis. Yeasts, in general, multiply by budding, although a 

 few yeasts are known to multiply by fission. When bac- 

 teria produce spores, usually but one spore is found in a 

 single cell, while with yeasts there is usually more than 

 one. This is always true in those yeasts which multiply by 

 fission. There are also some minute structural differences 

 which may usually be observed between yeasts and bac- 

 teria. The yeast' cell shows a definite nucleus while the 

 presence of a definite nucleus is not so clearly established 

 for bacteria. In most cases the yeast cells are decidedly 

 larger than those of bacteria. 



Botanists do not recognize a distinct group which they 

 term molds. Organisms belonging to this group, which is 

 convenient for the purposes of the bacteriologist, are really 

 members of different groups of fungi. They all resemble 

 each other, however, in forming more or less cottony or vel- 

 vety masses of fungus threads from which spore stalks in 

 various shapes and spores in various groupings are pro- 

 duced. A few of the molds show forms which intergrade 

 with the bacteria. Multiplication in molds is usually by 

 means of spores which are produced in large numbers by 

 single plants. 



The Protozoa. The protozoa are the one-celled animal 

 forms existing in a great variety of shapes, sizes, and meth- 

 ods of life in soil, in water, and occasionally producing dis- 

 eases in man and animals. The group is a very large one. 

 From the standpoint of agricultural bacteriology we are 

 interested in the group because of the possible importance 

 of some forms in soils and the undoubted significance of 

 others in the production of diseases. 



