198 MICROSCOPIC AND CULTURAL STUDY OF BACTERIA 



Calcium Carbonate Nutrient Sugar Media. Bacteria grown in 

 sugar media frequently form acid products from the fermentation of 

 the sugars the amount of acid products may be sufficient to inhibit 

 the development of the organisms even after one or two days' 

 growth. The addition of insoluble carbonates as calcium carbonate 

 neutralizes the acid as it is formed and thus maintains automatically 

 a favorable reaction for prolonged development. Bolduan 1 has shown 

 that pieces of marble about 0.5 centimeters square in 100 c.c. of broth 

 not only restrain the development of free acid the marble appears to 

 create a somewhat more favorable medium, especially for the pneumo- 

 coccus and streptococcus as well. The bits of marble should be 

 sterilized in the hot-air sterilizer before they are introduced into the 

 broth. 



Nutrient Glycerin Broth. To 1 liter of sugar-free broth add 3 to 

 5 per cent, pure, redistilled glycerin immediately before filtering. 

 Sterilize in the autoclave fifteen minutes at fifteen pounds pressure. 

 Glycerin broth is extensively used for the cultivation of the tubercle 

 bacillus 2 and it is frequently employed in the culture of bacteria which 

 are susceptible to desiccation the glycerin conserves the moisture 

 and retards evaporation. 



The various sugar-broths may be prepared with meat extract as 

 a basis; pathogenic bacteria develop less luxuriantly as a rule in 

 extract media than in meat infusion media, however. 



Dunham's Solution. Five grams of common salt and 10 grams of 

 Witte peptone are added to one liter of water and heated to boiling 

 until the peptone is completely dissolved. Pass through filter paper 

 until perfectly clear, tube, using 10 c.c. to each test tube, and sterilize 

 in the autoclave. The reaction does not require adjustment. 



This medium is frequently used to test the ability of bacteria to 

 form indol. Indol is formed in the absence of utilizable sugars by 

 Bacillus coli; members of the cholera group and other bacteria form 

 tryptophan by the splitting off of alanin : 



CH 2 .CHNH 2 .COOH / \ 



V NH 



Tryptophan Indol 



+ Alanin. The alanin is decom- 

 posed by the bacteria. 



1 New York Medical Journal, May 13, 1905. 



2 The reaction of glycerin broth designed for the cultivation of tubercle bacilli should 

 be +1.0 acid. The organism does not develop well in media neutral to phenolphthalein. 



