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A differential medium containing neutral red for the detection of coliform organisms in water and milk examination.
|
Typical Formula* |
gm/litre |
|
Peptone |
20.0 |
|
Lactose |
10.0 |
|
Bile salts |
5.0 |
|
Sodium chloride |
5.0 |
|
Neutral red |
0.075 |
|
pH 7.4 ± 0.2 |
* Adjusted as required to meet performance standards
Directions
To prepare single strength broth, add 40g to 1 litre of distilled water. Mix well and distribute into containers fitted with fermentation (Durham) tubes. Sterilise by autoclaving at 121℃ for 15 minutes.
Description
For the past fifty years, MacConkey Broth has been the standard medium for the primary isolation of coliform bacteria, and has been recommended for this purpose by the Public Health Laboratory Service Water Committee7 and the World Health Organization1. The Oxoid product conforms to their specification for water testing and also to the formulation specified by the Dept. of Health2 for milk grading.
The advantages of MacConkey Broth in the presumptive coliform test are the low proportion of false positive reactions (PHLS Water Subcommittee3) and the fact that most strains of Escherichia coli produce a positive reaction within 24 hours4. Disadvantages, due to variability of the peptone and bile salts contained in the original medium, have been overcome by large scale production, pooling of batches and careful quality control - including titrimetric standardisation of the bile salts by a method described by Burman5.
The neutral red is pre-tested for the absence of toxic substances before inclusion in the Oxoid medium. Childs & Allen6 have shown that some samples of neutral red were inhibitory. For those who prefer, this medium is also available with bromocresol purple as the indicator - for details of this alternative medium and the presumptive coliform test see MacConkey Broth Purple (CM0505).
Storage conditions and Shelf life
Store the dehydrated medium at 10-30℃ and use before the expiry date on the label.
Store the prepared medium at 2-8℃.
Appearance
Dehydrated medium: Straw-pink coloured, free-flowing powder
Prepared medium: Dark red coloured solution
Quality control
|
Positive control: |
Expected results |
| Escherichia coli ATCC®8739 * |
Turbid growth; gas production. |
|
Negative control: |
|
|
Staphylococcus aureus ATCC®6538* |
Turbid growth; no gas production. |
* This organism is available as a Culti-Loop®
Precautions
The neutral red indicator is carefully selected for this formulation and therefore shows no inhibitory effect. However, the more sensitive reaction of bromocresol purple in MacConkey Broth Purple (CM0505) is often preferred.
References
1. World Health Organization (1963) International Standards for Drinking Water 2nd ed., WHO, Geneva.
2. Dept. of Health (1937) Memo 139/Foods, HMSO, London.
3. Public Health Laboratory Service Water Subcommitee (1953) J. Hyg. Camb. 51. 268-277.
4. Windle Taylor E. (1958) `The Examination of Waters and Water Supplies' 7th ed., Churchill Ltd., London.
5. Burman N. P. (1955) Proc. Soc. Water Treat. Exam. 4. 10-20 and discussion 20-26.
6. Childs Eileen and Allen L. A. (1953) J. Hyg. Camb. 51. 468-477.
7. Dept. of Health and Social Security (1969) 4th impression, HMSO London.
ATCC® is a registered trademark of American Type Culture Collection.
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