122 BACTERIOLOGY. 



tiation between closely related bacteria is prepared by 

 mixing one part of blood-serum (either horse or bovine) 

 and three parts of distilled water. This is neutralized, 

 and heated in a water-bath or an Arnold steam steril- 

 izer until it becomes opalescent. A 5 per cent, aqueous 

 solution of litmus is then added in the proportion of 1 

 per cent. Any one of the carbohydrates, as dextrose, 

 lactose, saccharose, levulose, mannite, etc., is then added 

 in the proportion of 1 per cent. The finished medium 

 is then placed in test-tubes. The medium must be ster- 

 ilized in an Arnold steam sterilizer, and it is advisable 

 to allow the sterilizer to remain uncovered during the 

 process of sterilization to avoid excessive heating of the 

 medium. 



The relative degree of acidity produced, with or with- 

 out coagulation, with or without gas-production, and 

 with or without reduction of the litmus, in a series of 

 tubes of this medium containing the different carbohy- 

 drates serves to differentiate between related species of 

 bacteria. For instance, the colon bacillus produces an 

 acid reaction with coagulation and gas-formation with 

 some of the carbohydrates, while the typhoid bacillus 

 produces a lower degree of acidity with coagulation, but 

 without gas-production. Similarly, the different types 

 of the dysentery bacillus may be differentiated by means 

 of their effects on the different carbohydrates in this 

 medium. 



GUARNIARI'S GELATIN AGAR MIXTURE. For special 

 work, particularly with the organism of pneumonia 

 (bacterium pneumonise) the gelatin-agar mixture recom- 

 mended by Guarniari is of very great service. It should 

 be exactly neutral in reaction, and should possess the 

 following ingredients : 



