138 DIFCO MANUAL 



of the plates partially removed. The final reaction of the medium will be pH 7.0. 

 One pound of Bacto-S S Agar will make 7.4 liters of the final medium. 



1 Apartado De Los Archives Uruguayos De ^^ Am. J. Pub. Health, 36:51:1946. 

 Medicina, 18:485:1941. "Am. J. Digestive Diseases, 7:229:1944. 



2 Public Health Reports, 57:521, 524:1942. is Standard Methods for the Examination of 

 8 J. Lab. Clin. Med., 27:1081:1942. Dairy Products, 9th Edition: 166: 1948. 



* Am. J. Pub. Health, 31:363:1941. ^^ Diagnostic Procedures and Reagents, 3rd Edi- 

 5 The Lancet, Vol. I, (XXIII) :677: 1942. tion:23:i950. 



. Med. Assoc, 119:1402:1942. 20 Pub^ Health Reports, 65:1075:1950. 



Health Reports, 56:2415:1944. 21 p^per read at Microbiological Congress, 1950. 



* Public Health Reports, 60:1355:1945. 22 j, Bact., 54:1:1947. 



8 Am. J. Diseases Children, 66:539:1943. ^3 guU, Johns Hopkins Hospital, 83:367:1948. 



10 Rev. Chileana Hyg. Med. Prev., 4:353:1942. ^ J. Milk and Food Tech., 13:226:1950. 



^ J. Pediatrics, 26:39:1945. ^ Proc. 22nd Ann. Meet. Northeastern Conf. 

 ^ Can. J. Pub. Health, 36:401:1945. Lab. Workers in Pullorum Disease Control, 



Am. Vet. Med. Assoc, 58:20:1946. Burlington, Vermont, June 20-21, 1950. 



«J. Am. 

 'Public 1 



"J. 

 "Pul 



iblic Health Reports, 62:806:1947. ^e j^ Infectious Diseases, 18:596:1916. 



15 Am, J. Pub. Health, 33:127:1943. 



BACTO 



DESOXYGHOLATE CITRATE AGAR (B274) 



DEHYDRATED 



Pork, Infusion from 330 g. 



Proteose Peptone No. 3, Difco .... 10 g. 



Bacto-Lactose 10 g. 



Sodium Citrate 20 g. 



Ferric Ammonium Citrate 2 g. 



Sodium Desoxycholate 5 g. 



Bacto-Agar 13.5 g. 



Bacto-Neutral Red 0.02 g. 



Bacto-Desoxycholate Citrate Agar, a modification of the original Leifson^ 

 formula, is a selective medium for use in the isolation of enteric pathogens. It is 

 recommended for use in all cases wherein a selective Desoxycholate Citrate Agar 

 is specified or desired. The use of Desoxycholate Citrate Agar is recommended 

 as a plating procedure for examination of specimens for evidence of infection 

 with Salmonella and Shigella as given in "Diagnostic Procedures and Reagents"^ 

 of the American Public Health Association. 



On Desoxycholate Citrate Agar the growth of coliform bacteria is inhibited 

 or greatly suppressed. Gram-positive bacteria are generally inhibited. Salmonella 

 and Shigella organisms grow quite unrestricted. The selectivity of this medium 

 permits the use of fairly heavy inocula without danger of overgrowth of the 

 Shigella and Salmonella by extraneous organisms. Occasionally, however, coli- 

 form strains are encountered that persist on Desoxycholate Citrate Agar. Such 

 strains, if present in large numbers, produce acid from the lactose, precipitate the 

 bile salt, and give an opaque red medium which makes it diflficult to detect the 

 pathogens. Distribution of the inoculum over the surface of the medium to give 

 a sparsely populated section helps to insure against complete masking of the 

 pathogens by such coliform organisms. Inoculated plates are incubated at 37 °C. 

 for 24 hours. 



Organisms which grow on Desoxycholate Citrate Agar but which do not 

 ferment lactose produce colorless raised colonies. Salmonella typhosa produces 

 translucent colonies with a bluish cast. Other Salmonella colonies are large, 

 opaque and may possess a brownish center. Shigella produce opaque ground- 

 glass appearing colonies with even margins. Coliform organisms which persist 

 on Desoxycholate Citrate Agar form raised, even, red colonies that are often 

 surrounded by a red halo of precipitated bile salt. 



