MACROSCOPIC COLONY COUNT 



and sterilized before it is used again. 

 If the sample is taken from a bottle, 

 the bottle should be first shaken to 

 ensure thorough mixing and the milk 

 may be poured into the sample bottle, 

 although it is better here also to use 

 a sampling tube. 



If the temperature of the milk is 

 desired, it should be taken from a 

 different container from that used for 

 the bacteriological sample, or after the 

 bacteriological sample has been with- 

 drawn. All records- shall be made 

 immediately after taking the sample. 

 The milk sample shall be placed in a 

 properly labeled bottle. The most 

 convenient kinds of sample bottles are 

 glass stoppered, or those closed with a 

 cork lined screw cap. Cotton plugs 

 are not a satisfactory method of closure. 

 The sample bottles shall be placed 

 at once in a carrying case containing 

 cracked ice, so that the milk is 

 promptly cooled to near the freezing 

 point. 



The samples shall be transferred to 

 the laboratory as quickly as possible 

 and shall be plated with as little delay 

 as possible. The samples placed in 

 cracked ice and water may be kept for 

 several hours (12) without an appre- 

 ciable increase in bacteria. If the 

 plates are not made within four hours 

 from the time of collection, the number 

 of hours that did elapse should be 

 stated in the report. If the milk is 

 kept at 40° C. a slight and somewhat 

 variable increase may be found in 

 twelve to twenty hours. Up to twenty 

 hours this will not be more than 20 

 per cent in normal cases. The larger 

 increases may be expected in milk 

 which has been stored at low tem- 

 peratures for some time previous to 

 sampling.^ Continued shaking of the 

 milk during its transit to the laboratory 

 tends to break up the clumps into 



smaller masses and so increases slightly 

 the number of colonies. 



In the case of samples to be used for 

 direct microscopic examination, icing 

 of the samples may be dispensed with 

 under some conditions where it is 

 possible to add preservatives (formaline 

 2 to 3 drops of a 40 per cent solution 

 of formaldehyde for each 10 cc. of 

 milk)' to the samples as taken. Samples 

 containing preservatives that have been 

 allowed to stand until the cream is 

 compact are not satisfactory, and are 

 likely to give a lower count than 

 fresh samples. 



(a) macroscopic colony count (petei 

 plate met50d) 



Composition of medium. 



Standard beef extract agar* shall be 

 used for all routine work and shall 

 contain the following ingredients:** 



Agar (oven dried) 1-2% 



The beef extract shall be Liebig's 

 where this is obtainable, or some other 

 brand giving comparable results. 



Witte peptone, if available, can be 

 used with assurance that the reaction 

 of the medium will be neutral (pH = 

 7.0); other brands — such as Ar- 

 mour's, Digestive Ferments Co.'s, Park, 

 Davis Co.'s, — although more acid can 

 often be used for milk analysis without 

 necessitating change of reaction; and 

 nearly any good commercial peptone 

 may be used with comparable results 



*Beef infusion may be substituted for beet extract in those 

 laboratories where past records are based on the use of beef 

 infusion agar; but in the interest of uniformity, it is urged 

 that beet extract be used. 



**This medium is essentially the same as that recom- 

 mended in the last edition of the Standard Methods of 

 Water Analysis except for the reaction preferred. 



