STANDARD MEDIA 83 



able because of the toxins present and which interfere with the 

 growth of many species of bacteria. 



Other media of a more or less special character will be described 

 or referred to under the discussion of methods. Those described 

 above are the more important ones required in the bacteriological 

 examination of foods and drugs. 



7. Technique for Making Quantitative and Qualitative Estimations 

 by the Plating Methods 



As has been explained, the plating method is intended to de- 

 termine the number of living bacteria present in foods and drugs 

 and the results supplement the results of the method of making 

 the direct counts already described. From this statement it is 

 evident that the quantitative results by the two methods are not 

 the same. For example, the bacterial count of a catsup by the 

 direct method may be very high while the plating method may 

 give negative results, due to the fact that the heat sterilization 

 employed at the cannery killed all of the bacteria present. This 

 also shows why it is absolutely necessary to employ both methods 

 in order to form a correct estimate of the total contamination of 

 the substance. 



The following suggestions on laboratory technique are given 

 with a view to the unification of methods, thereby leading to 

 greater uniformity in comparative results. 



I. Apparatus.-^Test- tubes to be used for the usual cultural 

 purposes shall be of medium weight and thickness, 15 cm. long 

 by 1.6 cm. diam. Petri dishes shall be 10 cm. in diam. Petri 

 dishes with porous covers are preferred. All glass ware must 

 be scrupulously clean and may be sterilized by exposing to a dry 

 heat of about 150° C. for a period of i hr., after being cleaned, 

 wiped dry and plugged with a good grade of commercial cotton. 

 A browning of the free ends of the cotton plugs indicates that 

 the right degree of heating has been attained. A standard wire 

 7 



