MacConkey Agar is based on the bile salt-neutral red-lactose agar of MacConkey.3 the original MacConkey medium was used to differentiate strains of Salmonella typhosa from members of the coliform group. Formula modifications improved growth of Shigella and Salmonella strains. These modifications include the addition of 0.5% sodium chloride, decreased agar content, altered bile salts, and neutral red concentrations. The formula modifications improved differential reactions between enteric pathogens and coliforms. MacConkey Agar is recommended for the detection and isolation of Gram-negative organisms from clinical,4 dairy,5 food,6,7 water,8 pharmaceutical,1,2 and industrial9 sources.
Peptone is the nitrogen and vitamin sources in MacConkey Agar. Lactose is the fermentable carbohydrate. During Lactose fermentation a local pH drop around the colony causes a color change in the pH indicator, Neutral Red, and bile precipitation. Bile Salts Mixture and Crystal Violet are the selective agents, inhibiting Gram-positive cocci and allowing Gram-negative organisms to grow. Sodium Chloride maintains the osmotic environment. Agar is the solidifying agent. 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.
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.
1. United States Pharmacopeial Convention, Inc. 1995. The United States pharmacopeia, 23rd ed. The United States Pharmacopeial Convention, Rockville, MD.
2. European Parliament (Ph. Eur.). 2005. 5th ed. European Pharmacopoeia Commission.
3. MacConkey, A. 1905. Lactose-fermenting bacteria in feces. J. Hyg. 5:333-379.
4. Murray, P. R., E. J. Baron, M. A. Pfaller, F. C. Tenover, and R. H. Yolken (eds.). Manual of clinical microbiology, 6th ed. American Society for Microbiology, Washington, D.C.
5. Marshall, R. T. (ed.). Standard methods for the examination of dairy products, 16th ed. American Public Health Association, Washington, D.C.
6. U.S. Food and Drug Administration. 1995. Bacteriological analytical manual, 8th ed., AOAC International, Gaithersburg, MD.
7. Vanderzant, C., and D. F. Splittstoesser (eds.). 1992. Compendium of methods for the microbiological examination of food, 3rd ed. American Public Health Association, Washington, D.C.
8. Eaton, A. D., L. S. Clesceri, and A. E. Greenberg (eds.). 1995. Standard methods for the examination of water and wastewater, 19th ed. American Public Health Association, Washington, D.C.
9. Association of Official Analytical Chemists. 1995. Official methods of analysis of AOAC International, 16th ed. AOAC International. Arlington, VA.
10. Mazura-Reetz, G. T. Neblett, and J. M.Galperin. 1979. MacConkey Agar: CO2 vs. ambient incubation. Abst. Ann. Mtg. American Society for Microbiology. C179.