138 BACTERIOLOGY FOR NURSES 



are arranged in a rack and in them are placed equal amounts 

 of serum dilution and bacterial suspension, each one of the tubes 

 receiving a dilution of serum higher than the preceding one. 

 It should be remembered that the addition of the bacterial sus- 

 pension increases the serum dilution; thus a 1 in 10 dilution of 

 serum when mixed with an equal volume of bacterial suspension 

 becomes a 1 in 20 dilution. 



The mixture of serum and bacteria is placed in the incubator 

 for a few hours and then left at room temperature or placed in 

 the ice chest for several hours. When agglutination takes place 

 the clumps of bacteria fall to the bottom of the tube, leaving the 

 fluid entirely or partially clear according to the amount of agglu- 

 tinin present in the serum. 



In typhoid fever a positive agglutination reaction may be given 

 as early as the third day of the disease ; ordinarily it does not 

 appear before the seventh or eighth day. Occasionally the reac- 

 tion may be absent or occur only during convalescence ; as a rule 

 it is strongest during convalescence, remains positive several 

 weeks, and then disappears. 



The agglutinins were regarded for a long time as absolutely 

 specific. That is, that dysentery bacilli were agglutinated by serum 

 of an individual or animal immune to dysentery and by no other. 

 Later it was found that group agglutinins exist ; that is, an immune 

 serum will agglutinate closely related species though in a less 

 degree. For instance, the serum of a typhoid patient may agglu- 

 tinate typhoid bacilli in a 1 in 80 dilution; the same serum may 

 agglutinate the closely related colon bacilli in a 1 in 10 dilution and 

 may have no effect whatever on the unrelated diphtheria bacilli. 



In addition to their diagnostic value the agglutinins may serve 

 as an aid in the differentiation of bacterial species. Thus if 

 serum obtained from an animal highly immunized against the 

 typhoid bacillus agglutinate an unknown organism that has the 

 cultural characteristics of the typhoid bacillus, the unknown 

 organism is undoubtedly B. typhosus. Especially may this be 

 regarded as a proof if the unknown strain is agglutinated by the 

 same dilution of the serum as a known strain. 



