68 BACTERIOLOGICAL METHODS 



added ingredients (spices, sugar, vinegar, etc.), of the canning 

 product. The analyst can thus determine approximately how 

 much of the hyphal tissue present is derived from the use of 

 moldy fruit and how much is traceable to unsanitary factory 

 conditions. Again, the presence of one or more ova or larvae of 

 intestinal parasites, as the tape worm, would indicate sewage 

 contamination or contamination with fecal matter. Sand and 

 dirt particles indicate insufi&cient washing, etc. It is self evident 

 that the value of the report by the analyst depends upon his 

 knowledge of the subject and the range of his experience. Until 

 the work is weU under way and the methods are perfected, there 

 is no place for inexperienced analysts in our food and drugs 

 laboratories. 



4. Quantitative Estimations by the Cultural Methods 



Estimating the number of bacteria per cc. in foods and drugs, 

 etc., by planting or plating definite amounts of the substances 

 into plate (Petri dish) culture media, is a well-known and standard 

 procedure. The general and special technique of the plating 

 method is described in the various text-books and manuals on 

 bacteriology. Some of the details of the method are standard, 

 in so far as they are generally adopted by investigators, such 

 as the preparation of certain culture media, making the dilutions, 

 counting, etc.; in other regards there is anything but uniformity. 

 It is generally admitted that the results of different investigators 

 differ widely but there appears little unanimity of opinion as to 

 the factors which are responsible for these variations in quanti- 

 tative results. 



Micro-organisms are sensitive to a degree and they respond 

 readily to the slightest variations in moisture, temperature and 

 food' supply. A failure to recognize this fully in laboratory 

 practice leads to confusion and erroneous results. The following 

 are some of the more important factors which are responsible for 

 errors and variations in results. 



