56 DIFCOMANUAL 



Bacto-Koser Citrate Medium is a liquid medium recommended for the differ- 

 entiation of Escherichia coli and Aerohacter aero genes. It conforms to the 

 formula specified in Appendix I of "Standard Methods for the Examination of 

 Water and Sewage"^ for this purpose. In this medium an ammonium salt serves 

 as the sole source of nitrogen, and sodium citrate as the only source of carbon. 



The desirability for a means of differentiating coli-aerogenes organisms in 

 sanitary studies has long been recognized and a wide variety of media have been 

 devised to accomplish the same. Koser^ showed that either citric acid or its 

 sodium salt, when employed in a synthetic medium as the only source of carbon, 

 is readily utilized by A. aero genes, while E. coli fail to develop. This property 

 was quite constant and not quickly acquired or lost. 



In using this medium, coli-like colonies from Endo or Eosin Methylene Blue 

 Agar plates are inoculated into tubes of Koser Citrate Medium and, after 24 to 

 48 hours incubation, tubes showing marked turbidity may be assumed to contain 

 organisms of the aerogenes group. Inasmuch as coli-type organisms fail to grow 

 in this medium all tubes inoculated from coli-like colonies on Endo or Eosin 

 Methylene Blue Agar plates and remaining clear after 36 hours incubation may 

 be considered as coli. 



Bacto-Koser Citrate Medium is prepared according to Koser's^ original for- 

 mula. Chemically pure salts are used in the preparation of the medium and it is 

 carefully tested to be sure that no sources of carbon, other than the citrate 

 radical, or nitrogen other than ammonium salts, are present. 



To rehydrate the medium, dissolve 5.7 grams of Bacto-Koser Citrate Medium 

 in 1000 ml. of distilled water. The medium is distributed in tubes and sterilized 

 by autoclaving for 15 minutes at 15 pounds pressure (121°C.). The final reac- 

 tion of the medium will be pH 6.7. 



One pound of Bacto-Koser Citrate Medium will make 79.6 liters of medium, 



1 Standard Methods for the Examination ^ j, Bact., 8:493:1923. 



of Water and Sewage, 9th Edition : 231 : 1946. 



BACTO 



MacGONKEY BROTH (B20) 



DEHYDRATED 



Bacto-Peptone 20 g. 



Bacto-Lactose 10 g. 



Bacto-Oxgall 5 g. 



Bacto-Brom Cresol Purple 0.01 g. 



Bacto-MacConkey Broth is prepared according to a modification of the bile 

 salt broth recommended by MacConkey^ which contained 0.5 per cent sodium 

 taurocholate and litmus as an indicator. In later publications^'^ MacConkey 

 suggested other variants of this medium and employed neutral red instead of 

 litmus as an indicator. 



In recent investigations of the original formula, taking into consideration its 

 probable use as a presumptive medium in the detection of the coliform group 

 in water and milk examination, it was demonstrated that Bacto-Oxgall was well 

 adapted for use in this medium instead of sodium taurocholate. Medium pre- 

 pared from Bacto-MacConkey Broth is clear and does not require filtration or 

 adjustment of the reaction. Bacto-MacConkey Broth is now prepared with the 

 sulfonephthalein indicator Bacto-Brom Cresol Purple. The color of the uninocu- 

 lated medium is the same as when the indicator litmus was used, but after 

 incubation, acid is demonstrated by the color of the medium changing to yellow. 



To rehydrate the medium, dissolve 35 grams of Bacto-MacConkey Broth in 



