PREPARATION OF MEDIA 67 



many are unsatisfactory); sterilize 12-15 min at 120°C; cool tubes 

 immediately by immersing in cold water. For anaerobic organisms, 

 Spray's system of classification is based upon use of this medium (tubed 

 in a deep column) to which is added 0.05 g of reduced iron or a thin strip 

 of iron. The reactions determined for aerobic bacteria on this medium 

 are explained in Chap. VII; those for anaerobes in Spray's (1936) original 

 paper. 



Pigment production. This character may be observed on a variety of 

 media, and reports describing characteristics of new species should indicate 

 the medium used. Starch agar (as described above) is often satisfactory, 

 and if the organism will grow on potato slants, this medium may be used. 

 Potato slants are prepared as follows: Peel white potatoes and cut plug 

 from center, using cork borer of appropriate size. Slice plugs obliquely 

 to make slants. Wash slants overnight in slowly running cold tap water. 

 Place plugs in tube supporting them with small glass rod, stick of wood, 

 or potato slice; add 1 ml of water to keep slants moist during incubation, 

 and sterilize. Pigment production by Clostridia may be observed in 

 corn-meal infusion medium (page 49) prepared without addition of liver 

 or in potato infusion. 



Lipolysis. For media to be used to detect Hpolytic action and for 

 methods of proper preparation of fat emulsions, consult the papers of 

 Castell (1941), Castell and Bryant (1939), Collins and Hammer (1934), 

 Eisenberg (1939), Knaysi (1941), and Starr (1941). 



MEDIA FOR SPECIFIC BACTERIOLOGICAL PROCEDURES 



In the preceding sections a variety of media have been presented which 

 are suitable for the isolation, cultivation, characterization, and mainte- 

 nance of bacteria. Many of these media can be employed for multiple 

 purposes, and conversely, in some instances, several different formula- 

 tions may prove satisfactory for the same purpose. The established per- 

 formance of a medium and the personal preference of the laboratory 

 worker result in the adoption of one formula in place of another. 



However, there are numerous bacteriological procedures which require 

 that media of a designated composition be used. Listed in Table 3 are 

 several such microbiological procedures, together with references which 

 specify the composition of media required for performance of the tests. 



MEDIA FOR SPECIAL PURPOSES 



Voluminous experimental evidence attests to the fact that the composi- 

 tion of the culture medium has a profound influence on the microbial cell 

 with respect to formation of enzymes, toxins, antibiotics, and other 



