CULTURE MEDIA 



93 



the filtrate in the usual way. This medium is designated as 

 sugar-fiee broth. Various sugars or other substances are added 

 to such hroth in order to test the ability of bacteria to ferment 

 them. Acetic acid, 0.5 per cent, is added to broth to make a 

 selective medium for acid-resisting bacteria. Glycerin, 5 to 7 

 per cent, is added to broth for the cultivation of the tubercle 

 bacillus. Naturally sterile ascitic fluid or blood is 

 added to broth to promote the growth of certain types 

 of microbes, and to encourage anaerobes. Bits of 

 naturally sterile tissue are added to broth for similar 

 purposes. 



Gelatin is modified by the addition of various sugars, 

 especially dextrose and lactose, often with the further 

 addition of litmus. The production of acid by fer- 

 mentation of the sugar is at once evidenced by the 

 reddening of the litmus. Glucose litmus gelatin is also 

 a useful medium for anaerobes. It is best to sterilize 

 the litmus separately and add it from a sterile pipette 

 at the time the medium is used. 



Agar is modified by the addition of 5 to 7 per cent 

 of glycerin, and such glycerin-agai is used extensively 

 for cultivation of the tubercle bacillus and several 

 other pathogenic bacteria. Various sugars, supple- 

 mented by the addition of litmus, are dissolved in p^^ 

 agar to test the fermentation properties of bacteria. Potato in 

 Glucose agar is extensively employed as such for the 

 cultivation of anaerobes. Agar also forms the gelatinizing base 

 for a number of more or less complex special media. 



Steeilizable Special Media 



Potato. — Potatoes were perhaps the first soHd medium, em- 

 ployed in the cultivation of micro-organisms. Boiled or steamed 

 potatoes kept in a moist place, such as a large covered glass dish, 

 may well be employed as an illustration of primitive technic, and 

 excellent cultures of the common chromogenic bacteria may be 



