330 TYPHOID FEVER. 



made neutral the first medium, being usually naturally acid, 

 by sodium hydrate, the second, being usually alkaline, by citric 

 acid. The medium is then filtered, filled into tubes containing 

 5 c.c., and these are sterilised. After inoculation the character- 

 istic appearances ought to manifest themselves in about twenty 

 hours. 



(6) Growth on Phenolated Media. It was at one time thought the addi- 

 tion of .2 per cent carbolic acid to the ordinary media inhibited the growth of 

 all bacteria but the typhoid bacillus. It has been found, however, that the 

 growth of the B. coli is also unaffected by such a medium, though it prevents 

 the growth of most putrefactive organisms which liquefy gelatin. 



(7) The Application of the Agglutination Test in distinguish- 

 ing B. typhosus front B. coli. The scope of the application of 

 this test will be discussed later (see Immunity). Here we may 

 say that a negative result obtained with a suspected B. typhosus 

 culture is of greater value than a positive result obtained with a 

 suspected B. coli culture. The test is only to be taken in con- 

 junction with the other means of differentiating the two organ- 

 isms and is not strictly a crucial one. 



It will thus be seen that the diagnosis between the B. 

 typhosus and the B. coli is a matter of no small difficulty. The 

 points to be attended to in making such a diagnosis have 

 been given above. There is no evidence that the one or- 

 ganism ever passes into the other. Klein has found that both 

 after prolonged sojourn in distilled and tap water, and also after 

 passage through the bodies of a series of animals, each organism 

 still preserves its original characters. Statements as to their 

 identity usually rest on theoretical considerations, or on purely 

 negative evidence. Great difficulties sometimes arise in con- 

 sequence of a bacillus being found which, while giving a number 

 of the characteristics of either one or the other, fails to give 

 some of the characteristic tests, or only gives them very slowly. 

 This is especially true of organisms related to the B. coli. It has 

 consequently become common to speak of the typhoid group 

 and the coli group in order that such varieties may be included. 

 In the coli group cases may be met with which do not give an 

 indol reaction (the so-called paracolon group), which do not 

 curdle milk, or which do not produce gas, and Gordon even 

 includes varieties producing alkali, or slowly liquefying gelatin. 

 Three of the most important varieties, the bacillus enteritidis 



