MORPHOLOGY AND PHYSIOLOGY OF BACTERIA. 27 



Nitrifying bacteria play a very important part by providing 

 plant life with a most necessary food. They occur in the soil, and 

 two kinds have been described — the one kind converting ammonia 

 into nitrous acid, and the other changing nitrous into nitric acid. 

 To Winogradsky and Frankland we are principally indebted for our 

 knowledge of these bacteria. 



Pathogenic bacteria are those which are genetically related to 

 disease. Many organisms have been supposed to be pathogenic, or 

 have been described in connection with diseases, which are only 

 saprophytic associates. By saprophytic we mean organisms which 

 feed upon dead organic matter. They include many forms which 

 are found on the skin, in the intestinal canal, and sometimes in the 

 internal organs, especially the liver and kidneys ; the tissues have 

 lost their vitaUty, and the organisms, through some lesion, have 

 been carried into the circulation. 



That many organisms are causally related to disease, there is 

 strong evidence in proof. No organism can be considered to be pro- 

 ductive of disease unless it fulfils the conditions which have been 

 laid down by Koch. Great stress must be laid upon the importance 

 of successive cultivation through many generations, as the objection 

 that a chemical virus may be carried over from the original source 

 is thus overcome. Any hypothetical chemical poison carried over 

 from one tube to another would, after a great number of such 

 cultivations, be diluted to such an extent as to be inappreciable 

 and absolutely inert. 



Though we may accept as a fact the existence of pathogenic 

 organisms, we are not in all cases in a position to assert the means 

 by which they produce their deleterious or fatal effects. Many 

 theories have been propounded. It has been suggested that the 

 pathogenic organisms may be compared to an invading army. 

 The cells or phagocytes arrayed against them endeavour to as- 

 similate and destroy them, but perish themselves in the attempt. 

 This might explain the breaking down of tissue, and the for- 

 mation of local lesions, but does not assist us in understanding 

 the fatal result in thirty-six to forty-eight hours produced by the 

 inoculation of the bacilli of anthrax. Another view is that the 

 invading army seizes upon the commissariat, appropriating the 

 general pabulum, which is so essential to the Ufe of the tissues. 

 This would hardly account for so acute and fatal a result as in 

 anthrax, but would lead one to expect symptoms of inanition and 

 gradual exhaustion. Moreover, against this theory we have the 

 fact that death may result, in some cases, with the presence 



