CHAPTER VI. 



THE QUALITATIVE BACTERIOLOGICAL ANALYSIS. 



THE most important part of the bacteriological examination of 

 water supplies consists in the isolation and recognition of the 

 various kinds of bacteria present ; unfortunately, much confusion 

 exists as to the species of micro-organisms which normally 

 inhabit waters from various sources. Many organisms have been 

 described as different species which are really only varieties, the 

 variation being caused by differences in food, habitat, tempera- 

 ture, &c. Marshall Ward believes that many of the bacteria 

 isolated from impure waters behave as weakened forms, and 

 considers that the description of these enfeebled forms explains 

 many of the species found in the various works on water- 

 bacteria. Of late years, however, there has been a great increase 

 in the number of tests to which an organism must be submitted 

 in order to determine the species to which it belongs. Conse- 

 quently it is only quite lately that bacteria have been separated 

 into distinct species, characterised by Avell-defined biological and 

 chemical reactions. Following on the elaboration of these dif- 

 ferential tests, there has been a marked diminution in the 

 reports of the successful isolation of pathogenic organisms, such 

 as the B. typhosus, from water supplies supposed to have been 

 infected with the germs of specific disease. 



Bacteriologists now realise that the isolation of such an 

 organism as the B. typhosus is a matter of considerable difficulty, 

 and that it can only be accomplished under exceptionally 

 favourable circumstances. Admitting this to be the case, and 

 considering from a hygienic point of view that it is highly 

 desirable, in the absence of the discovery of a specific pathogenic 

 organism, to be able to state that a suspected water has been 

 polluted with sewage, many bacteriologists have recently studied 



