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Material Safety Data Sheet

Organisms

Organisms this product works with:

Dehydrated Culture Media

MACCONKEY AGAR

Code: CM0007

A differential medium for the isolation of coliforms and intestinal pathogens in water, dairy products and biological specimens.

Typical Formula*

gm/litre

Peptone

20.0

Lactose

10.0

Bile salts

5.0

Sodium chloride

5.0

Neutral red

0.075

Agar

12.0

pH 7.4 ± 0.2

 

* Adjusted as required to meet performance standards

Directions
Suspend 52g in 1 litre of distilled water. Bring to the boil to dissolve completely. Sterilise by autoclaving at 121°C for 15 minutes. Dry the surface of the gel before inoculation.

Description
A differential medium for the detection, isolation and enumeration of coliforms and intestinal pathogens in water, dairy products and biological specimens. MacConkey Agar corresponds to the medium recommended by the World Health Organization1, the Dept. of Health2 and by Windle Taylor3 for the bacteriological examination of water.

Although principally used for coliforms, this medium may also be employed for the differentiation of other enteric bacteria (including pathogens) and is suitable for the differentiation of Pasteurella species4.

Technique
Pathological specimens
Due to its ability to support the growth of pathogenic Gram-positive cocci (e.g. staphylococci and enterococci) as well as Enterobacteriaceae, MacConkey Agar is particularly recommended for the cultivation of pathogens which may be present in a variety of specimens such as urine, faeces and wound swabs. Whilst it is selective it does not suppress a mixed bacterial flora to the same extent as other inhibitory media (including other MacConkey agars). It provides a number of other diagnostic indications in addition to bile tolerance, such as colony morphology and chromogenesis. MacConkey Agar should be used in parallel with other selective indicator media such as Desoxycholate Citrate Agar, Bismuth Sulphite Agar, Brilliant Green Agar and Brilliant Green Bile (2%) Broth, and a non-selective medium such as Blood Agar.

Water Examination2,3
The medium may be used for the direct count of coli-aerogenes bacteria, using pour-plates prepared from known volumes of the water sample, but a more exact role for the medium is for the differentiation of organisms producing acid and gas in MacConkey Broth at 35°C: all positive broth tubes are plated on MacConkey Agar, the plates are incubated for 24 hours at 35°C and examined for typical colonies (see below). Colonies composed of Gram-negative non-sporing rods are subcultured for further identification.

The presence of enterococci in azide or tellurite media may be confirmed by subculture on MacConkey Agar. See below for colonial morphology.

Yersinia and Pasteurella differentiation
MacConkey Agar can be used to differentiate Yersinia species from Pasteurella species4. Yersinia pestis, Yersinia pseudotuberculosis and Yersinia enterocolitica will show growth on MacConkey Agar after 24 hours incubation at 35°C5.

Pasteurella species (including Pasteurella multocida) will not grow on MacConkey Agar.

Pectinolytic Organisms (Stewart6)
Stewart used Oxoid MacConkey Agar as the basis of a selective-diagnostic medium for pectinolytic organisms, in order to isolate soft-rot Erwinia species from specimens containing other Enterobacteriaceae.

MacConkey Agar-calcium chloride plates (5.2g CM0007 powder, 0.4g CaCl2, 75ml distilled water) overlaid with a pectate-EDTA layer (0.1% EDTA containing 2% sodium polypectate) are inoculated and incubated for 48 hours at 25°C. Lactose fermenting Erwinia produce red colonies in shallow pits formed by pectate liquefaction.

Colonial characteristics
After 24 hours at 35-37°C typical colonies are as follows:

Organism

Colour

Remarks

Escherichia coli

red

non-mucoid

Aerobacter aerogenes

pink

mucoid

Enterococcus species

red

minute, round

Staphylococcus species

pale pink

opaque

Pseudomonas aeruginosa

green-brown

fluorescent growth

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 plates of medium at 2-8°C.

Appearance
Dehydrated Medium: Straw pink coloured, free-flowing powder.
Prepared medium: Dark red coloured gel.

Quality control

Positive controls:

Expected results

Enterococcus faecalis ATCC® 29212 *Good growth; red coloured colonies.
Staphylococcus aureus ATCC® 25923 *

Good growth; pink coloured colonies.

Negative control:

 

Uninoculated medium.

No change

* This organism is available as a Culti-Loop®

Precautions
The colonial characteristics described give presumptive identification only of the isolated organisms. It is necessary to subculture and carry out confirmation tests for final identification.

To enhance the pigment of suspected Staphylococcus aureus, hold the plates on the bench at ambient temperature for 12-18 hours.

References
1. World Health Organization (1963) International Standards for Drinking Water 2nd ed. WHO, Geneva.
2. Environment Agency (2002) The Microbiology of Drinking Water 2002. Methods for examination of Waters and Associated Materials.
3. Windle Taylor E. (1958) `The examination of Waters and Water Supplies’ 7th ed., Churchill Ltd., London.
4. Hoogendijk J. L. (1962) Antonie van Leeuwenhoek J. Microbiol. Serol. 28(3) 315-320.
5. Wilson G. S. and Miles A. A. (1964) `Topley and Wilson’s Principles of Bacteriology and Immunity’ 5th Ed., Edward Arnold Ltd., London. vol.2.
6. Stewart D. J. (1962) Nature 195(4845), 1023.

 
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