M-Lauryl Sulphate Agar
M-Lauryl Sulphate Agar is used for enumeration of Escherichia coli and coliforms in water, using membrane filter
technique.
Composition**
Ingredients Gms / Litre
Peptic digest of animal tissue 39.000
Yeast extract 6.000
Lactose 30.000
Sodium lauryl sulphate 1.000
Phenol red 0.200
Agar 15.000
Final pH ( at 25°C) 7.4±0.2
**Formula adjusted, standardized to suit performance parameters
Directions
Suspend 91.2 grams in 1000 ml distilled water. Heat to boiling to dissolve the medium completely. Sterilize by autoclaving
at 15 lbs pressure (121°C) for 15 minutes.
Principle And Interpretation
Burman (1) substituted bile salts with teepol in Membrane Enriched Teepol Broth, the membrane filtration test medium used
to detect coliform organisms in water. M-Lauryl Sulphate Agar is prepared by substituting teepol with sodium lauryl sulphate.
The water samples are filtered through sterile membrane filter and then placed face upward on agar plates contains M-Lauryl
Sulphate Agar. Burman (2) recommended the following incubation temperatures and durations.
Unchlorinated waters:
Coliform organisms :4 hours at 30°C followed by 14 hours at 35°C
Escherichia coli :,,4 hours at 30°C followed by 14 hours at 44°C
Non-chlorinated organisms benefit from 4 hours incubation at 30°C but chlorinated organisms require 6 hours incubation at
25°C. After incubation, yellow colonies are formed which should be confirmed further.
Peptic digest of animal tissue and yeast extract act as a source of nitrogen, carbon and amino acids. Lactose is the source of
fermentable carbohydrate. Phenol red serves as an indicator. Sodium lauryl sulphate inhibits gram positive bacteria
Quality Control
Appearance
Light yellow to pink homogeneous free flowing powder
Gelling
Firm, comparable with 1.5% Agar gel
Colour and Clarity of prepared medium
Red coloured clear to slightly opalescent gel forms in Petri plates
Reaction
Reaction of 9.12% w/v aqueous solution at 25°C. pH : 7.4±0.2
pH
7.20-7.60
Cultural Response
Organism Inoculum
(CFU)
Growth at
35-37ºC
Growth at
44ºC
Colour of
Colony on
Membrane
Enterobacter aerogenes
ATCC 13048
50-100
50-100
luxuriant inhibited yellow
Escherichia coli ATCC
25922
luxuriant luxuriant yellow
Bacillus subtilis ATCC 6633 =10³ inhibited inhibited
Staphylococcus aureus
ATCC 25923
=10³ inhibited inhibited
Enterococcus faecalis ATCC
29212
=10³ inhibited inhibited
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
