146 BACTERIA. 



forming organisms have all been described ; but as their 

 powers of decomposing different substances and of giving 

 rise to pigment or to certain enzymes necessarily overlap one 

 another, or are met with in the same individual under 

 different conditions, it remains an exceedingly difficult matter 

 to give any classification founded on the above mixed 

 characteristics. 



Some of the organisms which, at present, are supposed to 

 be saprophytic in character, may eventually be found to be 

 parasitic under certain conditions, and it is for this reason 

 that the term non-pathogenic is gradually ousting the older 

 botanical term of saprophytic. 



Parasitic bacteria are those which are able to flourish on 

 or within the substance of plants or animals. In animals 

 they live in the various cavities of the body, or in certain 

 cases in one or other of the nutrient fluids of the body, and 

 they derive their food from these fluids, or from the nutrient 

 materials that are taken in by the host. The ordinary terms 

 of obligate parasites, facultative saprophytes, and facultative 

 parasites, can scarcely be used in connection with these 

 minute organisms, as, with very few exceptions, all the 

 parasitic bacteria have now been cultivated outside the body 

 and under such conditions that it is evident that some of 

 those which were at one time looked upon as purely parasitic 

 have a saprophytic stage of existence ; so that nearly all, if 

 not all, the parasitic bacteria must be looked upon as facul- 

 tative parasites, or parasites that can develop almost as well 

 saprophytically as they can parasitically, or facultative 

 saprophytes, or saprophytes that can develop almost as 

 well parasitically as they can saprophytically. 



We know of course that all parasitic bacteria are not 

 pathogenic, and that we have along the alimentary tract, 

 and even in the respiratory tract, a large number of para- 

 sitic organisms. The Sarcina ventriculi, for example, first 

 described by Goodsir, may be taken as a purely parasitic 

 organism. It is found especially in dilated stomachs. A 

 drop of fluid taken from the contents of such a stomach 

 frequently contains a number of organisms which present a 

 curious appearance under the microscope. They are very 

 like the typical bale of wool or goods tied with a strong 

 cord in three directions. Other sarcinae are saprophytic as 

 well as parasitic, but the Sarcina ventriculi has not yet been 



