The membrane filter technique is used to test relatively large volumes of samples. It is extremely useful in monitoring drinking water and a variety of natural waters (1). The earlier medium used to detect coliforms in water employed bile salts as the selective agent. This was replaced with Teepol by Burman (2). The effectiveness of teepol was demonstrated earlier (3, 4). MLauryl Sulphate Broth is similar to this medium, the only difference being the use of sodium lauryl sulphate as the inhibitory agent instead of teepol. M-Lauryl Sulphate Broth is recommended for enumeration of Escherichia coli and coliforms using membrane filtration technique (5, 6).
An absorbent pad is saturated with M-Lauryl Sulphate Broth. The filter, through which the water sample is passed, is aseptically placed on this saturated absorbent pad, with face upwards.
Burman (7) 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.
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.
References:
1. Eaton A. D., Clesceri L. S. and Greenberg A. E., (Eds.), 1995, Standard Methods for the Examination of Water and Wastewater, 19th Ed., American Public Health Association, Washington, D.C.
2. Burman N. P., 1967, Proc. Soc. Wat. Treat. Exam., 16:40.
3. Jameson J. E. and Emberley N.W., 1956, J. Gen. Microbiol. 15:198-204
4. Jebb W. H. H., 1959, J. Hyg. Camb. 57. 184-192
5. Joint Committee of PHLS and the Standing Committee of Analysts, 1980, J. Hyg. Camb. 85.181
6. Department of the Environmental Health and Social Security and PHLS, 1982, The Bacteriological Examination of Drinking Water Supplies, Report on Public Health and Medical Subjects No. 71, HMSO, London.
7. Burman N. P., 1967, Rec. Adv. in Bacteriological Examination of waters; C.H. Collins (Ed.), Butterworth, London.