MacConkey Broth

(Purple)

LAB 5

Description

This medium is used in the detection and enumeration of faecal coliforms (37˚C) and E. coli (44˚C). The replacement of neutral red used in the original formulation by bromocresol purple makes the colour change caused by acid producing organisms easier to read.

Method for reconstitution

Weigh 35 grams of powder, disperse in 1 litre of deionised water. Mix well and dispense into tubes or bottles with inverted Durham tubes. Sterilise by autoclaving for 15 minutes at 121˚C. Prepare double strength broth (70g/l) if 50ml or 10ml amounts of inoculum are to be added to equal volumes of broth. Prepare single strength broth (35g/l) if 1ml or 0.1ml amounts of inoculum are to be added to 10ml of broth.

Appearance: Purple, clear.

pH: 7.3 ± 0.2

Minimum Q.C. organisms: E. coli NCIMB 50034 B. subtilis (inhibition)

Storage of Prepared Medium: Capped containers - up to 1 month at 15-20˚C in the dark.

Incubation: 37˚C aerobically for coliforms, 44˚C aerobically for E. coli. Use Durham tubes to detect gas production for E. coli.

Growth Indicators: Turbidity, gas production. Lactose-fermenting organisms cause a colour change from purple to yellow.

References

Ministry of Health (1937). Bacteriological Tests for Graded Milk, Memo 139/Foods. H.M.S.O., London.

Minister of Health, Public Health Laboratory Service Water Committee (1969). The Bacteriological Examination of Water Supplies, 4th Edn. report No. 71. H.M.S.O., London.

World Health Organisation (1971). International Standards for Drinking Water, 3rd Edn, W.H.O., Geneva.