CONTAMINATION OF MILK 



I8l 



writer recommends the following dilutions and procedure for 

 routine work. 



Byre milk. — Add 1 c.c. of the milk to each of five L.B.B. tubes. 

 and 0-1 c.c. of the milk to each of two L.B.B. tubes. Incubate all 

 seven tubes at 37" C. for two days, and record as positive all the 

 tubes showing acid and gas. To isolate the lactose fermenting bacilli 

 use one or more of the positive tubes containing the least amount 

 of milk and plate as described below. 



Vended milk. — Winter (October to April inclusive). ]\Iake a 

 series of dilutions of the milk as described in Chapter IX. Inocu- 

 late ten L.B.B. tubes, four being with O'OOOl c.c. each, four with 

 O'OOl c.c. each, one with O'Ol c.c, and one with O"! c.c. Incubate 

 all ten tubes at 37" for two days and record the positive results. 

 Plate from one or more of the positive tubes containing the least 

 amount of milk. 



Summer (May to September inclusive). Dilutions and procedure 

 as for winter samples, but inoculate nine L.B.B. tubes adding the 

 following quantities of milk — O'OOOl c.c. to each of four tubes, O'OOl 

 to each of four tubes, O'Ol to one tube. 



By this method of examination a much closer estimate 

 of the number of B. coli can be obtained than by the ordinary 

 methods and without more expenditure of time and material. 

 The interpretation of the results is dealt with in Chapter XIV. 



To isolate these lactose fermenting bacilli solid media 

 must be used, and there are a number of such media which 

 give good results. The most useful of these are fuchsine-agar, 

 nutrose-agar (Drigalski and Conradi's medium) and lactose 

 bile -salt neutral red agar (conveniently called L.B.A). L.B.A. 

 is, on the whole, both the most uniform and convenient to use, 

 while it gives very reliable results. 



Preparation of L.B.A. — Sodium taurocholate 5 grammes, "Witte's 

 peptone 20 grammes, and distilled water 1 litre, are boiled up 

 together, 20 grammes of agar are added and dissolved in the solution 

 in the autoclave in the ordinary way. The medium is cleared with 

 white of egg and filtered. After filtration, 10 grammes of lactose 

 and 5 c.c. of recently prepared 1 per cent neutral red solution is 

 added. The medium is then tubed and sterilised for 15 minutes 

 on three successive days. 



To isolate, add one loopful of the selected lactose bile- 

 salt broth tube to a tube of sterile water. After mixing 

 well, place two loopfuls of the latter i^pon the surface of the 



