60 DIFCOMANUAL 



BACTO 



FORMATE RIGINOLEATE BROTH (B9) 



DEHYDRATED 



Bacto-Peptone 5 g. 



Bacto-Lactose 5 g. 



Sodium Formate 5 g. 



Sodium Ricinoleate 1 g. 



Bacto-Formate Ricinoleate Broth is prepared according to the formula speci- 

 fied in "Standard Methods for the Examination of Dairy Products"^ and 

 "Standard Methods for the Examination of Water and Sewage."^ It is used for 

 the presumptive test for the members of the coliform group in the bacteriological 

 examination of milk according to "Standard Methods."^ The procedure for use 

 of this medium in water examination is discussed on page 39. 



In research on the development of a medium which would be sufficiently- 

 selective, yet at the same time not inhibitive to any member of the coliform 

 group which might be present in the water or milk sample, Stark and England* 

 devised Formate Ricinoleate Broth. The sodium formate present in the medium 

 accelerates growth and gas production of coli and related organisms, while 

 sodium ricinoleate inhibits the development of Gram-positive bacteria and other 

 organisms which give positive presumptive tests that do not confirm. It is inter- 

 esting to note that gas production appears earlier in this medium than in other 

 media under the same conditions, and that it is present in larger proportions at 

 the completion of the test. The buffer action of the medium, due largely to the 

 destruction of the sodium formate, maintains a more constant reaction, as shown 

 by the fact that, even after 48 hours incubation with coli, the reaction of the 

 medium is rarely more acid than pH 6.0. Noble and White,*^ in their report on 

 the relative productivity of various fermentation media, state that media pre- 

 pared from Bacto-Formate Ricinoleate Broth had a higher productivity rating 

 than other media with which they were compared. 



Tiedeman and Smith,^ in a comparison of several methods for the detection 

 of coliform organisms in pasteurized milk stated that in a survey of 896 samples 

 of pasteurized milk, showing no fermentation in Formate Ricinoleate Broth, 82 1 

 or 91.6 per cent gave no colonies on Desoxycholate Agar. An additional 56 or 

 6.2 per cent gave a count of between 1 and 3 colonies per ml. A total of 97.8 

 per cent negative fermentation tests gave less than 3 colonies on the plating 

 medium, which was considered good correlation. They further state that the 

 Formate Ricinoleate Broth fermentation test is a critical one but does not give 

 information as to the number of coliforms present as does the use of Violet Red 

 Bile Agar or Desoxycholate Agar. 



In the presumptive test for members of the coliform group in the examination 

 of dairy products a series of tubes of Formate Ricinoleate Broth are inoculated 

 with appropriate dilutions of the sample. Use 5 tubes of each dilution. Select 

 dilutions to provide at least one positive and one negative tube in the series 

 inoculated. Incubate tubes for 48 hours at 35-3 7 °C. Gas formation constitutes 

 a positive presumptive test. 



When the medium is to be used in water filtration plant control work'' where 

 the inoculum is greater than 1 ml., particular care must be taken to preserve the 

 correct concentration of the ingredients after dilution with the sample. The table 

 given below indicates the quantity of dehydrated medium to use per 1000 ml. 



To rehydrate the medium, dissolve 16 grams of Bacto-Formate Ricinoleate 

 Broth in 1000 ml. of distilled water. The medium is distributed in fermentation 

 tubes and sterilized by autoclaving for 15 minutes at 15 pounds pressure 

 (121°C.). The final reaction of the medium will be pH 7.4. 



