SERUM DIAGNOSIS 383 



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

 another, and ultimately become completely immobile and form 

 clumps by their aggregation. 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. 121). 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 

 which gives the best result in the greatest number of undoubted 

 cases of typhoid fever, and which gives as little reaction as 

 possible with normal sera or sera derived from other diseases. 

 This latter point is important, as some races react very readily 

 to non-typhoid sera. Again, care must be taken as to the state of 

 the culture used. The suitability of a culture may be impaired 

 by varying the conditions of its growth. Continued growth 

 of a race at 37 C. makes it less suitable for use in the test, as 

 the bacilli tend naturally to adhere in clumps, which may be 

 mistaken for those produced by the reaction. Wyatt Johnson, 

 recommended that the stock culture should be kept growing on 

 agar at room temperature and maintained by agar sub-cultures 

 made once a month. For use in applying the test, bouillon 

 sub-cultures are made and incubated for twenty-four hours at 

 37 C. The relation of the dilution of the serum to the 

 occurrence of clumping is most important. It has been found 

 that if the degree of dilution be too small a non-typhoid serum 

 may cause clumping. If possible, observations should always be 

 made with dilutions of 1:10, 1 : 30, 1 : 60, 1 : 100. To speak 

 generally, the more dilute the serum the longer time is necessary 

 for a complete reaction. Some typhoid sera have, however, 

 very powerful agglutinating properties, and may in a compara- 

 tively short time produce a reaction when diluted many hundreds 

 of times. With a too dilute serum not only may the reaction 



