SF Broth

Composition**

Final pH ( at 25°C) 6.9±0.2

**Formula adjusted, standardized to suit performance parameters

Directions

Suspend 36.03 grams in 1000 ml distilled water. For double strength broth use 72.06 grams in 1000 ml distilled water. Heat if necesary to dissolve the medium completely. Dispense in tubes and sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes.

Warning: Sodium azide has a tendency to form explosive metal azides with plumbing materials. It is advisable to use enough water to flush off the disposables.

Principle And Interpretation

SF Broth is prepared according to the formula of Hajna and Perry (1) for the detection of faecal Streptococci in the swimming pools, water and milk samples. SF Broth is also recommended for detection of faecal streptococci in water and other samples (2). Streptococci grow luxuriantly at 45.5°C with an acidic reaction, seen as colour change from purple to yellow.

Casein enzymic hydrolysate provides essential growth nutrients. Dextrose is the fermentable carbohydrate. Sodium azide inhibits gram-negative organisms making it selective for Enterococci. Bromo cresol purple is the pH indicator. Phosphates buffer the medium while sodium chloride maintains osmotic equilibrium.

In this medium, the indicator turning yellow in presence of Enterococci is evident after 18-20 hours but to proceed for the isolation, a supplementary incubation in Petri plates is recommended. Turbidity and a yellow colour of the medium indicate positive reaction, while no change in the colour of the medium indicates negative reaction.

Quality Control

Appearance Cream to light green homogeneous free flowing powder

Colour and Clarity of prepared medium Purple coloured clear solution without any precipitate

Reaction Reaction of 3.6% w/v aqueous solution at 25°C. pH : 6.9±0.2

pH 6.70-7.10

Cultural Response Cultural characteristics observed after an incubation at 45-46°C for 18-48 hours.

Storage and Shelf Life

Store below 30°C in tightly closed container and the prepared medium at 2 - 8°C. Use before expiry date on the label.

Reference

1. Hajna and Perry, 1943, Am. J. Publ. Hlth., 33:550.
2. Eaton A. D., Clesceri L. S., Rice E. W., and Greenberg A. W., (Eds.), 2005, Standard Methods for the Examination of Water and Wastewater, 21st Ed., APHA, Washington, D.C.