AMERICAN STANDARD METHODS 125 



per c.cm. has no real bearing on its hygienic quality. This 

 series of comparative results is not so important as that reported 

 by Conn because of the psychological factor; both the person 

 plating out the samples (A. J. S.) and the one counting the 

 plates (J. R.) were aware that these determinations were being 

 made, and although every endeavour was made to honestly 

 record the actual conditions found, it is recognised that the 

 results are subject to these limitations. 



The detailed technique for the plate method as adopted by 

 the American Public Health Association in 1916 is as follows: 



Dilutions. For samples of unknown character dilutions 

 of 1 to. 100, 1 to 1000, 1 to 10,000 shall be made, using sterile 

 water and pipettes after the ordinary method. In case the 

 character of the milk is known, less than three dilutions may be 

 made; but in no case shall less than two plates for each sample 

 be made. Grade A,* or its equivalent, should be plated in 

 duplicate, and a dilution lower than 1 to 100 may be used. 



Shaking. Samples must be shaken twenty-five times. 

 Shaking is defined as meaning a rapid up and down motion 

 with an excursion of not less than 1 foot. 



Pipettes. Pipettes must be made to deUver between grad- 

 uation marks, not simply to deliver. 



Pouring Plates. The melted agar must be poured promptly 

 after measuring out the proper quantities of milk. Not more 

 than twelve plates must be allowed to accimiulate after the 

 distribution of the milk into the plates before pouring the 

 agar. 



Incubation and Counting. One standard temperature only 

 is recognised — ^forty-eight hour incubation at 37° C. 



If possible count those plates containing between 30 and 

 200 colonies. If there are none such, count those plates con- 

 taining nearest to 200 colonies. The whole number of colonies 

 on the plate shall be counted where the plates contain less than 

 200 colonies. 



* Milk usually containing less than 10,000 bacteria per c.cm. 



