496 QUANTITATIVE ESTIMATION OF BACTERIA IN MILK 



culture medium with a definite amount of diluted milk, pouring plates 

 and counting the colonies which have developed after a certain period 

 of time. While very superior to making stains from the sediment of 

 10 c.c. of centrifuged milk, this method, however, does not furnish 

 absolutely true values. In the first place only live bacteria yield 

 colonies, but as they naturally are the chief concern in the count, the 

 dead bacteria are of no great significance. Conditions, however, in 

 which all live bacteria present will develop on the plates can never 

 be created, as some microorganisms are aerobic, others anaerobic, and 

 as different species vary in their optimum temperatures of growth, in 

 their requirements as to the exact reaction of the culture medium, etc. 

 Even counts made of various specimens of milk under absolutely 

 identical conditions may furnish quite diverse results; for instance, 

 one sample may contain a large number of anaerobic bacteria and 

 another very few. Since the plates, however, are prepared for aerobic 

 growth, nothing concerning the number of anaerobes is learned. 

 Furthermore, bacteria generally adhere to each other in little groups 

 which cannot be entirely separated into individual microbes even by 

 energetic shaking. Notwithstanding all these defects the method of 

 preparing plates and counting the colonies which have developed after 

 a certain period gives a relatively accurate estimate of the bacterial 

 content of the milk at the time when the specimen was used in the 

 preparation of the plates. 



Steps in the Quantitative Bacterial Analysis of Milk. 1. Collect a 

 sample of milk under all possible aseptic precautions from the speci- 

 men to be examined. Samples obtained from so-called loose milk in 

 a big can must generally be removed by the ordinary dipper, but 

 should be at once poured into a sterile bottle, the cork stopper or 

 cotton plug of which is to be removed only long enough to permit the 

 introduction of the milk and then to be at once replaced. Samples 

 taken from bottles or smaller containers should be procured with sterile 

 pipettes, into which the milk is drawn up by suction either with a 

 rubber bulb or by the mouth. If the latter method is used the upper 

 end of the pipette must be closed with a cotton plug, so that no trace 

 of saliva can run into the pipette. In general, however, this method, 

 even when the pipette is protected, is not to be recommended. In 

 the examination of certified milk it is best to take one of the original 

 small bottles without opening it. In whatever manner the sample is 

 procured it must immediately be packed in ice to avoid the danger 

 of a great multiplication of bacteria during the period of time elapsing 

 between the collection of the specimen and its inoculation into culture 

 media in the laboratory. The can or bottle from which the specimen 

 is taken should first be well shaken and agitated. It is also well to 

 take the temperature of the milk finally, because a large number of 

 bacteria in cold milk would generally indicate improper collection, 

 while a large number in warmer milk may simply point to a multi- 

 plication of lactic-acid bacteria in milk collected cleanly. After 



