470 PATHOGENIC BACTERIA. 



be remembered that the bacilli are certain to be present 

 only in the second and third weeks. 



As numerous saprophytic bacteria are present in the 

 feces, the resistance which the typhoid bacillus exhibits 

 to carbolic acid can be made use of in obtaining the pure 

 culture. To each of several tubes of melted gelatin 0.05 

 per cent, of carbolic acid is added. This addition is most 

 easily calculated by supposing the average amount of 

 gelatin contained in a tube to be 10 c.cm. To the aver- 

 age tube y 1 ^- c.cm. of a 5 per cent, solution of carbolic acid 

 is added, and gives very nearly the desired quantity. A 

 minute portion of the feces is broken up with a platinum 

 loop and stirred in the tube of melted gelatin ; a drop 

 from this dilution is transferred to the second tube, a 

 drop from it to a third, and then the contents of each 

 tube are poured upon a sterile plate or into a Petri dish, 



Fig. 112. — Bacillus typhi abdominalis : superficial colony two days old, as 

 seen upon the surface of a gelatin plate; x 20 (Heim). 



or rolled, according to Esmarch's plan, in the manner 

 already described. The carbolic acid present in these 

 cases prevents the great mass of saprophytes from de- 

 veloping, but allows the perfect development of the 

 typhoid bacillus (Fig. 112) and its near congener, the 

 Bacillus coli communis (Fig. 115). 



The deep colonies that develop upon such gelatin plate- 



