Bromocresol Purple Lactose Agar

(Drigalski agar)

Description

A modification of MacConkey's medium, formulated in 1926 by Dunham and Schoenlein, for the recovery of coliform bacteria in foodstuffs and water. The brilliant green and bile inhibit most Gram positive organisms thus overcoming the problem of some Clostridium spp. fermenting lactose and giving false positive results.

Method for reconstitution

Weigh 40 grams of powder, disperse in 1 litre of deionised water. Allow to soak for 10 minutes, swirl to mix then warm to dissolve. Dispense into tubes or bottles with inverted Durham tubes. Sterilise by autoclaving at 115˚C for 15 minutes.

Appearance: Green, clear.

pH: 7.4 ± 0.2

Minimum Q.C. organisms: Salmonella sp. NCIMB 50076 E. coli NCIMB 50034

Storage of Prepared Medium: Capped containers - up to 1 month at 2-8˚C in the dark.

Inoculation: Serial 1:10 dilutions of homogenised sample are inoculated into the broth in the proportion of 1ml sample to 9ml broth. Ensure the Durham tube is free from gas bubbles before commencing inoculation. B.G.B. broth can be used at double strength if required but cannot be sterilised by autoclaving, pasteurisation must be used instead.

Incubation: E. coli and thermotrophs 44˚C for 18 hours aerobically. Mesopholic coliforms 32˚C for 24-48 hours aerobically. Psychrotrophic coliforms 4˚C for 10 days aerobically.

Interpretation: Turbidity, colour changes (to yellow or yellowish green) and production of gas are all presumptive evidence of the growth of organisms of the coli-aerogenes group. Confirmation by indole production in Tryptone Water LAB 129 (44˚C for E. coli).

References

Pharmacopoeia of Culture Media for Food Microbiology (1987). Int. J. Food Microbiol. 5:3:206-207.

American Public Health Association, American Water Works Association and Water Pollution Control Federation, (1975), Standard Methods for the Examination of Water and Wastewater, 14th ed., Washington D.C.

Association of Official Analytical Chemists (AOAC). Bacteriological Analytical Manual, 5th ed., Washington, D.C. Association of Official Analytical Chemists. 1978.

Hausler, W. J. (ED) (1972). Standard Methods for the Examination of Dairy Products. 13th ed., Washington. D.C. American Public Health Association.

Shane, M.S. (1947). Studies on false confirmed test using B.G.B. and comparison studies on Lauryl Sulfate Tryptose Broth as presumptive medium. J. Am. Water Works Assoc., 39: (4), 337.