DEHYDRATED CULTURE MEDIA 147 



has been in routine use, but for the isolation of members of the Salmonella and 

 Shigella groups we recommend the following media : 



Bacto-MacConkey Agar, as discussed on page 131. 

 Bacto-S S Agar, as discussed on page 134. 

 Bacto-Bismuth Sulfite Agar, as described on page 139. 

 Bacto-Tetrathionate Broth, as discussed on page 157. 

 Bacto-Selenite Broth as discussed on page 158. 



The differential restraining action exerted by certain dyes on the growth of 

 bacteria is a well-known property, and is utilized frequently in culture media. 

 Crystal violet, for example, when present in a dilution of 1:250,000 in agar 

 media, inhibits quite generally the development of Gram-positive organisms, 

 but has no appreciable effect on the growth of Gram-negative bacteria. This 

 property is employed in the medium of Conradi and Drigalski^ for the isolation 

 of the Gram-negative intestinal bacteria from contaminated material such as 

 water, feces, etc. 



Bacto-Conradi Drigalski Agar is a dehydrated culture medium, prepared to 

 duplicate the original formula. Crystal violet is retained because of its selective 

 action, but the litmus which was originally used as the indicator has been re- 

 placed with Bacto-Brom Cresol Purple. This indicator is much more satisfactory 

 than litmus. Colonies fermenting the lactose in the medium are surrounded by 

 a yellow zone. Bacto-Isoelectric Casein, prepared in our own laboratories, has 

 been substituted for the Nutrose specified in the original formula. 



To rehydrate the medium, suspend 50 grams of Bacto-Conradi Drigalski Agar 

 in 1000 ml. of cold distilled water and heat to boiling to dissolve the medium 

 completely. Distribute in tubes or flasks and sterilize by autoclaving for 15 

 minutes at 15 pounds pressure (121°C.). The final reaction of the medium will 

 be pH 6.8. 



One pound of Bacto-Conradi Drigalski Agar will make 9 liters of medium. 

 ^Zeit. Hyg., 39:283:1902. 



BACTO 



DEXTROSE PROTEOSE NO. 3 AGAR (B68) 



DEHYDRATED 



Proteose Peptone No. 3, Difco 20 g. 



Bacto-Dextrose 2 g. 



Sodium Chloride 5 g. 



Bacto-Agar 13 g. 



Bacto-Dextrose Proteose No. 3 Agar, used with Bacto-Tellurite Blood Solution 

 is suggested as a selective medium for the detection and isolation of Coryne- 

 bacterium diphtheriae. The medium is of particular value in the release of 

 patients and in the detection of diphtheria carriers. This medium inhibits the 

 streptococci and staphylococci, but permits C. diphtheriae to grow. Without 

 added tellurite and blood this medium is recommended as a general laboratory 

 medium containing 0.2 per cent dextrose. 



In 1912 Conradi and Troch^ described a selective serum medium, contain- 

 ing tellurite, used for the isolation of C. diphtheriae. On their medium C. diph- 

 theriae colonies were coal black due to the reduction of the tellurite. Since then 

 many methods for the use of potassium tellurite in media have been described 

 for the isolation and identification of C. diphtheriae. Clauberg^ in 1929 de- 

 scribed a serum medium containing glycerol and potassium tellurite for growth 

 of C. diphtheriae from nose and throat smears. Anderson, Happold, McLeod, 



