Technique
The presumptive coliform examination consists of the inoculation of measured volumes of water into tubes of MacConkey Broth (Purple) which are incubated at 35°C for 48 hours. Choice of volumes for inoculation will depend on the bacteriological grade of the water being tested; for `medium’ waters the Public Health Laboratory Service Water Committee (1961) recommend one 50ml, five 10ml and five 1ml quantities of water - 50ml and 10ml amounts being added to their own volume of double-strength MacConkey Broth while the 1ml amounts are each added to 5ml of single-strength MacConkey Broth.

Acid formation is indicated by a yellow colouration of the broth, and gas formation is indicated by an amount of gas at least sufficient to fill the concavity at the top of the Durham tube. From the number of tubes showing the presence of acid and gas, the most probable number of (presumed) coliform bacteria present in 100ml of the original water may be estimated by reference to probability tables; these tables based on McCrady’s computations, are included in Report No.71: `The Bacteriological Examination of Water Supplies’² and in many other publications dealing with this subject.

For the differential coliform test, each MacConkey tube showing acid and gas is then subcultured into a fresh tube of MacConkey Broth and incubated at 44°C. Formation of gas within 48 hours is practically specific for Escherichia coli and indicative of faecal pollution of the original water sample.

MacConkey Broth (Purple) is also suitable for the bacteriological examination of milk, as described by Davis⁴. This method, which is basically similar to that used for the examination of water, consisting in the inoculation of suitable dilutions of the milk into tubes of this medium followed by incubation and inspection, was originally recommended by the Dept. of Health, London⁵.

The addition of 4-methylumbelliferyl-ß-D-glucuronide (MUG) BR0071 to this medium will enhance the detection of Escherichia coli.

Storage conditions and Shelf life
Store the dehydrated medium at 10-30°C and use before the expiry date on the label.
Store the prepared medium at room temperature.

Appearance
Dehydrated medium: Green coloured, free-flowing powder
Prepared medium: Purple coloured solution

Quality control