i8o MILK AND THE PUBLIC HEALTH CHAP. 



and at once examine the deposit. This is a useful guide to 

 the best dilutions to add for plating. 



II. ESTIMATION OF THE NUMBER OF B. COLI AND ALLIED 

 ORGANISMS 



A great deal of very valuable work has been carried out 

 upon B. coli and its allies. We now know that there are a 

 large number of bacteria which can be differentiated from the 

 original B. coli communis by cultural tests, but which are 

 nevertheless closely allied and frequently seem to have a very 

 similar origin and distribution in nature. The varieties which 

 are of chief significance in relation to milk contamination all 

 ferment lactose. These lactose fermenters of B. coli type are 

 not present in milk as drawn from the teats, and when found 

 in milk are nearly always derived from cow manure, possibly 

 occasionally from human excreta. 



To isolate this group of organisms, several procedures may 

 be used, but the following is recommended as, in the writer's 

 experience, the most satisfactory. 



Lactose bile-salt broth (conveniently called L.B.B.) in double 

 tubes, is used. To prepare : 



Five grammes each of sodium taurocholate and lactose, 20 

 grammes of peptone, and 1 litre of water are heated together until 

 the solids are dissolved. The mixture is filtered and sufficient 

 strong neutral litmus solution is added to give a distinct colour. 

 The medium is then distributed into fermentation tubes, and 

 sterilised, by steaming for 20 minutes on three successive days. 



Definite fractions of the milk are added by pipette to these 

 tubes. The amount to add will depend upon the suspected 

 degree of pollution of the milk. A usual procedure is to add 

 I/O, O'l, O'Ol c.c. if the sample is byre milk, and if it 

 is ordinary vended milk to add in addition O'OOl, O'OOOl, 

 0-00001, 0-000001, and occasionally even 0-0000001 c.c. The 

 dilutions are obtained in the ordinary way, as described in 

 Chapter IX. 



These dilutions are widely spaced, and consequently the 

 number of B. coli present can only be enumerated between 

 rather wide limits. The liability to accidental differences 

 from faulty or unequal dilution is not inconsiderable. The 



