i 3 2 METHODS OF PURE CULTURE. 



few gelatin tubes being charged with various quantities of the- 

 sample and poured on to plates. This method is of particular 

 importance in the quantitative bacteriological analysis of water, 

 for which reference should be made to Tiemann-Gartner's hand- 

 book. The counting of the colonies grown on the plates is effected 

 by the aid of special counting apparatus, that of Woltf hiigel being 

 used for the Koch plates. For counting the colonies on gelatin 

 plates in Petri dishes the author, in 1893, constructed a cheap 

 counting-plate, obtainable from F. Mollenkopf, of Stuttgart (10 

 Thor Strasse). The number of germs thus found is always smaller 

 than the living cells actually present in the inoculating mixture, 

 since only such as have developed into colonies are enumerated, 

 whereas a number of germs in the original have failed to develop 

 under the conditions prevailing, owing to the medium being un- 

 suitable for some, and the temperature of the incubator, though 

 favourable to the majority, being too hot or too cold for a minority. 

 The medium relatively most suitable for the purpose of ascertain- 

 ing the number of germs is, in most cases, gelatinised meat-juice,, 

 and this is therefore the one most frequently used. Considerable 

 influence on the number of developing germs is exerted by the 

 degree of alkalinity of the medium, a fact first conclusively demon- 

 strated by A. REINSCH (II.) and confirmed by MAX DAHMEN (L). 

 If it is a question not of ascertaining, as nearly as possible, the 

 total germ content of a sample, but only how many of the cells 

 are capable of development in a given medium, then the latter is 

 arranged in a solidified condition as a plate culture. For example, 

 wort gelatin is generally used unless the contrary be expressly 

 stated when determining the number of germs in brewery water. 

 It is important to know for certain whether the colonies in a plate 

 culture are each developed from a single cell, since it is only in 

 such cases that a pure culture can be obtained on re-inoculation.. 

 This aim is attempted by thin sowing and thoroughly shaking the 

 liquefied medium, in order to separate the cells from each other. 

 Nevertheless, there is always some uncertainty, which we must 

 endeavour to remove by discarding the first series of plates and by 

 preparing a second series wherein any impurities may become 

 manifest; then, if the colonies are found to stand this test, the' 

 re-inoculations therefrom may be considered as pure cultures. 

 This can be ensured from the outset if the growth of the colonies, 

 i.e., from the single cells, be followed by the aid of the micro- 

 scope from the beginning. This test is, however, feasible only 

 with large cells (Eumycetes spores, yeast cells, &c.), and will be 

 enlarged upon in a subsequent chapter in connection with the 

 pure culture of yeast. On the other hand, it is, as a rule, imprac- 

 ticable for bacteria, since their examination necessitates the use 

 of such a high (short focus) objective that the latter has to be- 

 brought so near the plate as to impinge on the gelatin stratum. 

 Pure cultivation with solidified media is almost exclusively 



