/ 276 BACTERIA PATHOGENIC TO MAN 



even years. From time to time one of these is taken out and used to 

 tart a fresh series of bouillon cultures. 



.DILUTION OF THE BLOOD SERUM TO BE EMPLOYED AND TIME 

 INQUIRED FOR THE DEVELOPMENT OF REACTION. The serum test, 

 as has been pointed out, is quantitative and not qualitative. By this 

 it is not meant to assert that all the agglutinating substances produced 

 / in the blood of a patient suffering from typhoid infection are the same 

 /'as those present in small amount in normal blood, or those produced 

 'in the blood of persons sick from other infections. It is true, however, 

 that the apparent effect upon the bacilli is identical, the difference 

 being that in typhoid fever, as a rule, substances which cause this 

 reaction are usually far in excess of the amount which ever appears in 

 non-typhoid blood, so that the reaction occurs after the addition to the 

 culture of far smaller quantities of serum than in other diseases, or 

 when the same dilution is used it occurs far more quickly and completely 

 with the typhoid serum. The agglutinins which develop in animals 

 after immunization with many bacilli comprise those which are specific 

 and those which have affinity fcg., widely differing varieties. (See 

 chapter on Agglutinins.) It is most important to remember that it is 

 purely a matter of experience tp determine in any type of infection 

 what degree of agglutinating^treng^h in a serum is of diagnostic 

 value. 



The results obtained in the Health Department Laboratories, as 

 well as elsewhere, have shown that in a certain proportion of cases 

 not typhoid fever a slow reaction occurs in a 1 : 10 dilution of 

 serum or blood; but very rarely does a complete reaction occur in! 

 this dilution within fifteen minutes. When dried blood is used the 

 slight tendency of non-typhoid blood in 1 : 10 dilution to produce 

 agglutination is increased by the presence of the fibrinous clumps, 

 .and perhaps by other substances derived from the disintegrated blood 

 cells. 



From many cases examined it has been found that in dilutions] 

 of 1 : 20 a quick reaction is almost never produced in any febrile dis- 

 ease other than that due to typhoid or paratyphoid bacillus infection,! 

 while in typhoid fever such a distinct reaction often occurs with dilu-j 

 lions of 1: 100 or more. It is possible that some cases of paratyphoid; 

 infection give a prompt reaction in 1 : 20 dilutions, but if this is so, it 

 is not a serious drawback. 



The mode of procedure, therefore, as now employed is as follows: 

 The test is first made with the typhoid bacillus in a 5 per cent, solution [ 

 of serum or blood. In the case of serum, one part of a 1 : 10 dilution isj 

 added to one of the bouillon culture. With dried blood, a solution of] 

 the blood is first made, and the dilution guessed from the color. Toj 

 obtain an idea of the dilution by the color, known amounts of blood 

 are dried and then mixed with definite amounts of water; the colors 

 resulting are fixed in the memory as guides for future tests. If there isj 

 no reaction that is to say, if within five minutes no marked change] 

 is noted in the motility of the bacilli, and no clumping occurs nothing; 



