338 TYPHOID FEVER 



diagnosis is based on the fact that living and actively motile 

 typhoid bacilli, if placed in the diluted serum of a patient suffer- 

 ing from typhoid fever, within a very short time lose their 

 motility and become aggregated into clumps. We shall find 

 (see Immunity) that in many diseases the serum has this property 

 of causing agglutination of cultures of the causal bacterium. 

 The principles on which the possession of the faculty depends and 

 also its significance, are obscure, and in the case of the typhoid 

 bacillus we do not know the true interpretation of some of the 

 facts which have been observed. 



The methods by which the test can be applied have already 

 been described (p. 109). 



(1) It will be there seen that the loss of motility and clumping 

 may be observed microscopically. If a preparation be made by 

 the method detailed (typhoid serum in a dilution of, say, 1 : 30 

 having been employed), and examined at once under the micro- 

 scope, the bacilli will usually be found actively motile, darting 

 about in all directions. In a short time, however, these move- 

 ments gradually become slower, the bacilli begin to adhere to one 

 another, and ultimately become completely immobile and form 

 clumps by their aggregation, so that no longer are any free 

 bacilli noticeable in the preparation. When this occurs the 

 reaction is said to be complete. If the clumps be watched still 

 longer a swelling up of the bacilli will be observed, with a 

 granulation of the protoplasm, so that their forms can with 

 difficulty be recognised. In a preparation similarly made with 

 non-typhoid serum the individual bacilli can be observed separate 

 .and actively motile for many hours. 



(2) A corresponding reaction visible to the naked eye is 

 obtained by the " sedimentation test," the method of applying 

 which has also been described (p. 111). Here at the end of 

 twenty-four hours the bacilli form a mass like a precipitate at 

 the bottom of the mixture of bacterial emulsion and diluted 

 typhoid serum, while the upper part remains clear. A similar 

 preparation made with normal serum shows a diffuse turbidity 

 at the end of twenty-four hours. The test in this form has the 

 disadvantage of taking longer time than the microscopic method, 

 but it is useful as a control ; in nature it is similar. 



Such is what occurs, in the case of a typical reaction. The 

 value of the method as a means of diagnosis largely depends on 

 attention to several details. The race of typhoid bacillus 

 employed is important. All races do not give uniformly the 

 same results, though it is not known on what this difference of 

 susceptibility depends. A race must therefore be selected 



