INTRODUCTION 35 



quantitative or qualitative determinations made directly under the 

 suitable power of the compound microscope; or, certain quantities 

 of the substances are placed in or upon certain culture media 

 (Petri dish cultures, tube cultures, etc.) in order to bring out the 

 biological and biochemical characteristics of the contaminating 

 organisms, whereupon the cultural products are examined micro- 

 scopically. In this latter instance the microscopical examination 

 may even be entirely omitted. 



The direct microscopical method has some very marked ad- 

 vantages and should be carried out whenever feasible, particularly 

 when purely quantitative results or estimates is the main object 

 sought after. In other instances the direct method must be 

 combined with cultural tests and the two are often checks upon 

 each other. 



2. Direct Bacteriological Examinations Quantitative Tests 



Substances to be examined include waters and liquids of all 

 kinds; sewage; milk 1 and cream; ice cream, liquid pharmaceuti- 

 cals and medicamenta, oils, catsups, beverages of all kinds, all 

 semisolids such as pastes, jams, jellies, etc., other semiliquids and 

 semisolids which may be readily diluted with water if necessary; 

 solids as powders, pills, tablets, soils, clays, meats, starches, 

 dextrins, flours, meals, dried fruits, dried eggs, dried albumen, 

 sugar, etc. In fact all substances which are in any way liable to 

 contamination by micro-organisms. 



The following is an outline of the methods of making determina- 

 tions of the number of organisms in food and drugs. 



a. Substances Requiring Concentration. Certain substances 

 which contain comparatively few micro-organisms, as drinking 

 waters, mineral waters, beverages generally, tinctures, fluid ex- 



1 In the case of milk, the centrifuge is first used to separate out the fat as much 

 as may be necessary to make the ready counting of the organisms possible. (See 

 also Chapter on Milk Analysis.) 



4 



