BACTERIA I NUMBER AND KIND 307 



They all ferment glucose and lactose with the produc- 

 tion of acid and gas ; they acidify and throw down 

 casein in milk in three or four days ; none of them 

 liquefy gelatine except Bact. cloaca which induces lique- 

 faction only when the cultures are kept for some time 

 (four to thirty days). 



Ex. 146. To determine if acid and gas-producing organisms 

 are in a sample of milk. 



Prepare the following medium : 



MacConkey's Bile-salt Peptone Water. 



Sodium taurocholate (bile salt) . . 2.5 gr. 



Glucose . . . . . . 2.5 gr. 



Peptone . . . . . . 10.0 gr. 



Water . . . . ! 500 c.c. 



Heat gently to assist solution of ingredients ; filter, and then 

 add sufficient amount of boiled Kubel-Tiemann's neutral litmus 

 solution to colour the medium. 



Pour it into Durham's fermentation tubes, 10 c.c. in each, and 

 steam for twenty minutes on three successive days. 



Inoculate some of these tubes with i c.c. of the sample of 

 milk to be tested for acid and gas producing organisms ; also 

 inoculate with i c.c. of the milk diluted to y 1 ^-, y-, T1J Vo> TOWOJ 



To<jWo> and To^Jooo- ( see Ex - I2 5)- 



Incubate at 37 C. for forty-eight hours. 



Note in which dilutions acid and gas are produced. 



Ex. 147. Separation and identification of glucose- and 

 lactose-fermenting (coliform) organisms in milk. 



Prepare the following medium : 



MacConkeys Bile-salt Lactose A gar. 



Sodium taurocholate . . . . 2.5 gr. 



Peptone . . . : . . . 10 gr. 



Agar 7.5 gr. 



Water . . ... . 500 c.c 



