428 A MANUAL OF BACTERIOLOGY 



hours, and the bacteria are killed by heating to 53 C. 

 for one hour, higher temperatures having proved to be 

 deleterious, and after cooling 0-25 per cent, of lysol is 

 added ; it is not necessary to employ a virulent bacillus. 

 The immunising power of a typhoid vaccine depends 

 partly upon the number of bacilli it contains, and partly 

 on the particular strain of bacillus used. The vaccine is 

 standardised by counting the number of bacilli it contains 

 by Wright's method. In the early days the symptoms 

 produced by the inoculation were often severe, but with 

 more moderate doses are now hardly appreciable. Two 

 doses of the vaccine should be given, with an interval 

 of about seven to ten days between the two, the doses 

 being 500 and 1,000 millions respectively. The vaccine 

 deteriorates on keeping. Emulsions of agar cultures 

 and autolysed cultures have also been used for preparing 

 vaccines. Polyvalent vaccines were introduced by 

 Castellani, and are now much used, e.g. Typhoid and 

 Paratyphoid A and B (see p. 437). Cholera may also be 

 introduced into it. 



The voluminous data now collected with regard to the 

 protective power of anti-typhoid vaccination against 

 attack indicate that the incidence of typhoid fever 

 among the inoculated is not more than one-fifth that 

 among the uninoculated, and the case mortality is 

 halved. 



Variation of the B. typhosus. Allusion has already been 

 made to Twort's work on the " education " of B. typhosus 

 to ferment lactose, and on the apparent conversion of 

 B. typhosus into B. alkaligenes by Horrocks (p. 6). 

 Penfold also records variations in the fermentive powers 

 of B. typhosus (Journal of Hygiene, vol. xi, 1911, p. 30). 



B. pyogenes fetidus, obtained by Passet from a rectal abscess, 

 has all the fermentation reactions of B. typhosus, but it rapidly 

 liquefies gelatin and casein. 



