METHODS FOR THE MICROSCOPIC STUDY OF BACTERIA 217 



The value of the method as a convenient means of comparison of the 

 bacterial content of various samples of milk, water, sewage, and the 

 like depends largely upon the supposition that the same types of bac- 

 teria present in different samples will grow quantitatively under like 

 conditions. The comparison of bacterial counts is therefore a com- 

 parison of a section of the total bacterial flora, not an absolute measure 

 of the number of living organisms. The method of counting bacterial 

 colonies has been highly developed for the regulation of water and milk 

 supplies of cities. (See section Water and Milk.) 



(c) Growth of Bacteria in Gelatin. Gelatin is added to cultural 

 media both to confer upon the media the property of solidifying, and 

 to enrich the content in nitrogenous substances. 



Pure gelatin does not contain tyrosine and it is relatively rich in 

 diamino acids; according to Hausmann, 1 nearly 36 per cent, of the 

 nitrogen in gelatin is diamino nitrogen about 63 per cent, in the form 

 of mono-amino acids. Chemically, gelatin media are convenient for 

 the demonstration of soluble, proteolytic enzymes. 2 In the absence of 

 utilizable carbohydrate, several types of bacteria "liquefy" gelatin, 

 that is, through the activity of their proteolytic enzymes the gelatin 

 molecule is split by hydrolytic cleavage to molecules so simple in 

 their state of aggregation that they can no longer produce a "gel." 

 The presence of utilizable carbohydrate prevents the liquefaction of 

 gelatin by many bacteria. 3 



Formerly the morphology of the liquefied zone in gelatin stab cul- 

 tures was regarded as distinctive for individual organisms; thus, the 

 napiform liquefaction produced by cholera vibrios was supposed to 

 be sufficiently constant to possess diagnostic value. It is now generally 

 conceded that this morphological characteristic is of comparatively 

 little importance for the identification of the organism. On the other 

 hand, the liquefaction of gelatin and of coagulated blood serum and 

 casein as well is important from a chemical viewpoint, because it 

 indicates the activity of a soluble proteolytic enzyme. 4 



II. Growth of Bacteria in Fluid Media. (a) Plain Broth. Plain 

 broth, prepared from meat infusion and peptone in the usual manner, 



1 Zeit. f. physiol. Chem., 1899, xxvii, 95. 



2 Kendall, Boston Med. and Surg. Jour., 1913, olxviii, 825. 



3 Kendall and Walker, Jour. Inf. Dis., 1915, xvii, 442. 



4 The enzyme may be obtained sterile and in an active state in the filtrates of liquefied 

 gelatin, blood serum, casein, and from plain broth cultures as well, if the bacteria are 

 removed by filtration through unglazed porcelain: Auerbach, Arch. f. Hyg., 1897, 

 xxxi, 311; Berghaus, ibid., 1906, Ixiv, 1; Kendall and Walker, Jour. Inf. Dis., 1915, 

 xvii, 442. 



