CHAPTER II. 



THE QUANTITATIVE BACTERIOLOGICAL 

 ANALYSIS. 



THE determination of the number of micro-organisms in a known 

 volume of water is mainly effected at the present day by cultures 

 on solid media. Miquel, however, has suggested a dilution 

 method, in which a known volume of water is diluted amongst a 

 series of broth tubes, the object being to obtain ultimately a 

 series of broth tubes, each of which shall not receive more than 

 one microbe. This method is exceedingly laborious, and quite 

 unsuitable for hygienic work. Most workers now employ what 

 is known as Koch's method of plate cultivation. In this method 

 a small quantity of water is intimately mixed with fluid gelatine 

 or agar-agar, which is then spread out in a thin layer on a glass 

 plate, or preferably in a Petri dish. Sometimes the mixture of 

 water and fluid gelatine is simply rolled round the sides of a test- 

 tube and solidified, producing what is known as an Esmarch roll 

 culture. The frequent presence in water of micro-organisms which 

 rapidly liquefy gelatine renders the Esmarch tube less serviceable 

 than Petri dishes. Much difference of opinion exists as to the 

 method of preparation of the solid media. The committee of 

 American bacteriologists recommended that gelatine and agar- 

 agar should have such a reaction as would be obtained by adding 

 1-5 per cent. N acid to the media after they had been rendered 

 neutral to pheriol-phthalein. Sedgwick and Prescott stated that 



the best results were obtained by adding 0'2 of ^ acid to each 



cubic centimetre of nutritive gelatine. They also found that more 

 colonies developed on nutritive gelatine containing 15 to 20 

 grammes of peptone per litre than on gelatine containing 

 5 grammes of peptone per litre. Reinsch's experiments with 



