152 BACTERIA 



plays an active role. In the necrosis of bone and in subphrenic 

 abscess the typhoid bacilli may act as a pus former. Commonly 

 it produces death by (i) profound toxaemia; (2) ulceration of the 

 Peyer's patches, causing perforation and peritonitis; (3) by the 

 destruction of a blood vessel in the floor of an ulcer producing a 

 haemorrhage. 



In animals, as a rule, typhoid bacilli if injected, produce no dis- 

 ease, and the bacilli rapidly die. In chimpanzees, however, it is 

 possible to produce typical typhoid lesions and symptoms. 



Natural and Acquired Immunity. Human blood serum is 

 strongly bactericidal toward the typhoid bacillus. Normal gastric 

 juice, with its hydrochloric acid, destroys the 

 bacillus when ingested and this forms the 

 natural means of protection. Immunity follow- 

 ing an attack of typhoid is generally of long 

 duration. If bacilli do reach the blood stream 

 of an immune individual, the amboceptors 

 originated by a previous infection, together 

 acdon. OnTSrf the with the complement normally present, effect 

 field shows typhoid a solution of the invading organism. Artifi- 



o t a h e \ l 1 ha?f Cl Thows cial immunit y has been effected against typhoid 

 clumping. (Greene's by vaccinating individuals with killed cultures. 

 Anti-toxin for typhoid has been prepared by 

 injecting horses with killed culture of typhoid bacilli, but it has 

 not proved to be effective. 



Agglutinations. One of the most important means of diagnos- 

 ing typhoid fever is by the so-called Widal test, really the Gruber and 

 Durham agglutination reaction. This consists in applying the 

 serum of the blood of a person, supposedly ill with typhoid, to a 

 fresh bouillon culture of typhoid bacilli. If the person has the dis- 

 ease, and it has lasted for five or more days, the bacilli are promptly 

 agglutinated in clumps. Undiluted normal serum, and serum from 

 people suffering other diseases, will bring about the same reaction at 

 times; it is therefore best to dilute the serum with water 150, and 



