342 TYPHOID FEVER. 



disposal, with a view to obtaining one 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 in surroundings very 

 favourable to vegetable activity 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 recommends 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. As the reaction of the medium has also an im- 

 portant effect on the sensitiveness of a culture, he recommends 

 that such bouillon should first be made neutral to phenol-phtha- 

 leine, and then have added to it 3 or 4 per cent of normal hydro- 

 chloric acid. When these precautions are taken a growth occurs 

 which only gives a uniform turbidity in the bouillon without any 

 adhesion of the bacilli in masses. It is usually, however, quite 

 safe to use bouillon prepared in the ordinary way. 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 pos- 

 sible, observations should always be made with dilutions of 

 i-io, 1-30, 1-50, i-ioo. 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 comparatively short time produce a 

 reaction when diluted many hundreds of times. The conditions 

 giving rise to such sera are not known, and the cases from 

 which they are derived are not necessarily of a severe type. 

 With highly diluted sera not only may the reaction be delayed 

 but it may be incomplete. Here, what is usually seen is that the 

 clumps formed are small, many bacilli being left free. These 

 latter may either have been rendered motionless or they may 

 still be motile. No diagnosis is conclusive which is founded on 

 the occurrence of such an incomplete clumping alone. Seeing 



