i2 4 METHODS OF PURE CULTURE. 



bacteria rapidly increase in such a solution, they are therefore also 

 endowed with the faculty of effecting the synthesis of albumin. 

 Comparative researches instituted by E. CRAMER (II.) with cholera 

 vibrio showed, however, that the percentage content of albumin 

 (calculated to dry substance) in the cells cultivated in Uschinsky's 

 solution is lower than in the case of cultures grown in media con- 

 taining albumin. 



In the sixth and seventh decades of the present (nineteenth) 

 century the preparation of a medium suitable as a universal 

 nutrient medium for all possible bacteria formed the object of 

 the repeated exertions of many bacteriologists. Such an attempt 

 is now regarded as hopeless on account of the knowledge which 

 has been gained of the very opposite conditions governing the 

 vitality of the several species. 



83. The Dilution Method and Fractional Cultivation. 



It has already been remarked in Chapter viii., that it is 

 quite the exception for a natural bacterial growth to consist of 

 merely a single species, but that, as a rule, we have to deal with a 

 mixture of several. To separate these from one another, and to 

 further multiply each species by itself, so as to obtain therefrom a 

 pure culture, forms the aim of the methods of pure cultivation. 



We start with the assumption that we have to deal with a 

 number of different bacteria inhabiting a liquid, inasmuch as there 

 is a second condition possible, i.e. when the organisms are dis- 

 tributed within a solid body (such as cheese, butter, soil, &c.). 

 In the latter case a finely divided suffusion of the sample must 

 be made with sterilised water and treated in the same manner as 

 liquid bacterial samples. 



Very often the mycologist is set the task of determining the 

 germ content, i.e. ascertaining how many individual cells are 

 contained per unit of space in a sample. This contingency is 

 often met with in fermentation experiments with yeasts, in order 

 that, from the result of the counting, the extent of the cell 

 multiplication occurring during the fermentation may be ascer- 

 tained. For such purposes a so-called counting chamber, such as 

 supplied, e.g. by Carl Zeiss of Jena, is used. The arrangement of 

 this appliance is shown in Fig. 32, in plan at A and in vertical 

 section at B. On a thick glass slide there is mounted a cover- 

 glass (a) with a circular hollow, within which is cemented a second 

 glass disc (c), o.i m.m. thinner. On the upper side of this 

 latter are etched two systems (crossing each other at right angles) 

 of twenty-one parallel lines at regular intervals of 0.05 m.m. and 

 therefore enclosing compartments each of which has an area of 

 0.0025 sq. m.m. 



If now a sufficiently large droplet of the sample to be counted 

 be laid on the centre of c and covered with a cover-glass (b) about 



