6 MASS. EXPERIMENT STATION BULLETIN 383 



The Laboratory Test 



Most uninformed persons seem to think that all the bacteriologist has 

 to do to determine the purity of a water is to put a drop of it on a glass 

 slide and look at it through a microscope, and that if any typhoid or 

 dysentery bacteria are present they would be seen and recognized. Bac- 

 teriologists wish it were as simple as that. But it is not, and for the 

 following reasons: 



In the first place, it would be impossible to concentrate water to the 

 point where a single drop would contain sufificient bacteria to be seen. 



In the second place, under the microscope bacteria can be identified 

 only by the following general shapes and formations: 



1. Spirilla. Here the cells are spiral, or corkscrew-like, in appearance. 



2. Cocci. These cells are spherical in shape and appear in the follow- 

 ing formations: 



a. Staphylococci, bunched like grapes. 



b. Streptococci, in chains of varying length. 



c. Diplococci, in pairs. 



d. Some other formations which need not be mentioned here. 



3. Rods. Here the cells are somewhat rectangular in shape. 



It is true that by certain staining methods further identification within 

 these groups can be made. However, identification by microscopic methods 

 would still be very general, so that within each of these groups would be 

 hundreds of species which must be identified by other than microscopic 

 methods. 



Both the tj'phoid and dysentery organisms belong to the third group; 

 that is, they are rod shaped. To the bacteriologist these forms are known 

 as "bacilli." Under the microscope, even with the use of stains, it is im- 

 possible to distinguish between them. Further, there are normally present 

 in water many other rod-shaped bacteria which are similar in appearance 

 to the typhoid and dysentery bacilli, but which are harmless. Therefore, 

 a direct microscopic examination of a water would supply little if any 

 information regarding its purity. 



The major methods used in identifying and classifying all bacteria are 

 of a chemical and physical nature. They can be very simply explained 

 as follows. Bacteria, like plants and animals, are living organisms. The 

 main difference between the bacteria and higher forms of life is that the 

 former are single-celled while the latter are multicelled. Being alive, 

 bacteria must "breathe," they must "eat," and they must discharge the 

 by-products of that "breathing" and "eating"; even as all animals do. By 

 cultivating the organisms in known kinds of atmospheres, and by "feed- 

 ing" them known kinds and amounts of food, then analyzing the by- 

 products both chemically and physically, it is possible to identify and 

 classify them. Another factor that assists is the source of the organism. 

 Many species of bacteria, like many plants and animals, live and multiply 

 best in certain localities. With bacteria it may be the oral cavity, the in- 

 testines, or the skin uf man or animal; with plants and animals it may be 

 geographic locations. 



It is by these methods then, rather than by the microscope, that the 

 bacteriologist determines the purity of a water supply. It will be noted 

 that the term used was "determines the pttrity of a water supply," not 



