436 BACTERIOLOGY. 



only properties possessed by it that may be said to be 

 constant are its motility ; its inability to cause gaseous 

 fermentation of glucose, lactose, or saccharose ; its inca- 

 pacity for coagulating milk; and its growth on gelatin 

 plates ; but there are other bacilli which possess these 

 same characteristics to a degree that renders their differ- 

 entiation from the typhoid organism often a matter that 

 requires the careful application of all the different tests. 



THE AGGLUTINATION REACTION. An interesting 

 reaction of the typhoid bacillus is seen when it is 

 brought in contact with the blood-serum from human 

 beings sick of typhoid fever, or from animals that 

 have survived inoculation with cultures of this organ- 

 ism. This reaction consists of a peculiar alteration 

 in the relation of the organisms to one another in 

 the fluid. As ordinarily seen in a hanging drop of 

 bouillon, the typhoid bacilli appear as single, act- 

 ively motile cells ; when to such a drop a drop of di- 

 lute serum from a case of typhoid fever is added the 

 motility of the organism gradually becomes lessened, 

 and finally ceases, and the bacteria congregate in 

 larger and smaller clumps. The reaction may also be 

 produced in another way, viz., by adding to about 4 or 

 5 c.c. of a twenty-four-hour-old bouillon culture of 

 typhoid bacilli in a narrow test-tube about eight drops 

 of serum from a case of typhoid fever, after which the 

 tube is placed in the incubator. After a few hours the 

 normally clouded culture is seen to have undergone a 

 change ; instead of the diffuse cloud caused by the growth, 

 the fluid is found clear and contains within it flocculent 

 masses of the bacteria that have agglutinated together 

 as a result of the specific action of the serum used. 



For the hanging-drop test, sufficient serum may be 

 obtained from a needle-prick in the finger, while for 



