go BACTERIOLOGICAL METHODS 



stances may have been subjected to processes of sterilization during 

 some phase of the processing or of manufacture. 



In a general way the quantitative results by the plating method 

 are to be interpreted in a manner similar to the results by the 

 direct count. In some cases the question at issue may be relative 

 to the presence or absence of viable bacteria in substances which 

 presumably do not contain living organisms, such as canned foods 

 generally. Manufacturers of canned products are of the opinion 

 that the methods of heat sterilization employed will kill all of the 

 bacteria which may have been present at the time of canning. 

 This is undoubtedly true in many cases, but in other instances it is 

 only too evident that retarded fermentation processes continue 

 after the cans are sealed, which accounts for the high counts in 

 canned food products which contained only small numbers of bac- 

 teria at the time the cans were sealed. These subdued fermenta- 

 tion processes as a rule do not result in the formation of sufficient 

 gas to produce "swells" and hence the article is not suspected 

 until the container is opened when a more or less disagreeable or 

 peculiar odor is noticeable, which is, as a rule, not sufficiently 

 pronounced to prevent the use of the article as food. 



In addition to the purely quantitative results, the plating 

 method indicates the general quaUtative character of the organ- 

 isms present and conveys some idea as to the course of the infection 

 or contamination as shown by the characters of the colonies de- 

 veloped in the Petri dish or in the tubes. 



9. Qualitative Determinations 



The chief qualitative determinations in food and drugs labora- 

 tories pertain to sewage contamination. The recognition and 

 determination of pathogenic bacteria as the typhoid bacillus, 

 the cholera bacillus, diphtheria bacillus, etc., is an incidental 

 possibility in the food laboratory routine and not a regular part 

 of it. Of far greater significance is the recognition of the evidence 



