MICROORGANISMS AND FERMENTATIONS 17 



introduction of a substance of which the composition is not quite 

 constant ; only the best brands, prepared under standard condi- 

 tions, should be used. Most milk bacteria thrive in the following 

 solution : Tap water, 1 litre ; sodium chloride, 2 grams ; dipo- 

 tassium phosphate, 2 grams ; magnesium sulphate, 1 gram ; 

 dextrose, 20 grams ; peptone, 20 grams. 



This solution has an alkaline reaction, and phosphoric acid 

 should be added until it only just turns litmus paper blue. For 

 the cultivation of water bacteria and most pathogenic organisms, 

 the medium should be distinctly alkaline ; for yeasts and moulds 

 it should be acid. The media are distributed in flasks or test 

 tubes closed with plugs of non-absorbent cotton wool and sterilised 

 by heating in an autoclave for a quarter of an hour at 1 10 to 120 C. 



The Freudenreich flask is the most convenient vessel to use, as 

 the contents are not so easily dried up or infected as in test tubes. 

 If the nutrient solution is to be used for the investigation of acid 



FIG. 18. -Freudenreich Flask. FIG. \<d.Petruschlcy Flask. 



formation, then 10 c.c. are introduced into each test tube, and the 

 inoculated solution is titrated when the maximum acidity is 

 certain to have been reached, say, after fourteen days at 30 C. 

 The amount of acid produced by lactic acid bacteria increases 

 with the amount of nitrogenous nutrient material in the medium ; 

 these organisms thrive better on casein peptone l than on Witte's 

 peptone, but most of the rod-shaped lactic acid bacteria thrive 

 best on an extract of autolysed yeast 2 . 



In order that the cultures may be kept at definite temperatures 

 the bacteriological laboratory must be equipped with several 

 incubators heated by gas or electricity, and, if necessary, cooled 

 by water circulation, in such a way that the temperature is 

 automatically kept constant. 



In order to examine microorganisms, to obtain them as pure 

 cultures or to determine their numbers, they must first be isolated. 

 This may be accomplished by Koch's method of plating, which 

 consists in distributing a definite small quantity of the liquid 

 containing the organisms in a solid transparent medium which 

 J , 2 See footnotes at end of this Chapter. 



DB. 2 



