22 



DAIRY BACTERIOLOGY 



nearly reaches the agar surface. A plug of absorbent cotton wool 

 soaked in alkaline pyrogallol is then inserted a short distance 

 above the first plug, and the tube is closed by a rubber stopper or 

 a cotton wool plug soaked in melted paraffin wax (see Fig. 22). 

 As soon as the cultures have made growth they must be kept at 

 a low temperature or they will quickly die. The acid-producing 

 bacteria keep better, the smaller the amount of sugar contained 

 in the agar medium. Lactic acid bacteria may be preserved 

 alive for years if not more than 0-25 per cent, of dextrose is 

 employed, but it is safer to sow into fresh media every month. 

 If it is desired to keep lactic acid bacteria in perfect condition 

 with respect to their action on milk, they 

 a should always be cultivated in milk, and 



reinoculated into fresh sterilised milk as fre- 

 " quently as possible. 



Inoculation is carried out by means of a 

 platinum wire sealed into a glass rod or fixed 

 into a screwed aluminium holder ; the end of 

 G the wire may be shaped as desired ; when 



dealing with liquids it is looped. The wire is 

 always flamed immediately before use. 

 d In order to demonstrate the presence of 



certain organisms which may be greatly out- 

 numbered by other species, the method of 

 plating fails ; it will be necessary to use the 

 enrichment method of inoculating a small 

 quantity of the material into a medium which 

 Method for the Cul- f avours the growth of the organism in question 

 tivation of Anae- to the disadvantage of others ; the cumulative 

 (After Salomons*) effect of several reinoculations into the same 

 medium will often be the production of a pure 

 culture of the desired organism, or at any rate a culture in which 

 the organism can easily be recognised. Such methods are 

 employed, among others, for demonstrating the presence of 

 pathogenic organisms. 



METHODS OF EXAMINATION 



Of all the methods in use for the determination of the species 

 of microorganisms, microscopic examination is one of the most 

 important, for until the shape and size of the cells have been 

 noted it will not even be possible to say definitely whether one is 

 dealing with a yeast or a bacterium. The more highly developed 

 the organism, the completer will be the information to be gained 

 by microscopic examination, especially if the various stages of 



