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MacConkey agar

Catalog Number: 1006017
MacConkey agar

For the selective isolation and identification of enterobacteria from feces, urine, wastewater and foods. MacConkey Agar is a selective and differential medium for the isolation and differentiation of lactose-fermenting organisms from lactose non-fermenting enteric gram-negative bacteria.

Approximate formula g/l:

    Gelatin Peptone
17.0Polipeptone 3.0
10.0Bile Salts #3 1.5
    Sodium Chloride
5.0Agar 13.5
    Neutral Red
0.03Crystal Violet0.001
Final pH 7.1 + 0.2


Suspend 50 g of the medium in one liter of distilled or deionized water. Mix well and heat with frequent agitation. Boil for one minute. Sterilize in an autoclave at 121o C (15 lbs) for 15 minutes. Cool to 45 - 50°C and pour into Petri dishes, 20 ml in each dish. Allow to solidify and later invert the dishes to avoid excess moisture on the surface of the medium.


The specimen can be streaked directly on the medium or inoculated first into an enrichment broth such as Tetrationate Broth, Selenite Cystine Broth, or GN Broth. Incubate the plates and broth tubes at 35oC for 18 to 24 hours. Subculture the broth tubes onto MacConkey Agar and reincubate.

It is recommended to streak samples onto other selective media such as Eosin Methylene Blue Agar, SS Agar, Tergitol 7 Agar, XLD Agar, Hektoen Enteric Agar, Bismuth Sulfite Agar (especially for Salmonella typhi), and/or Brilliant Green Agar, especially for salmonellas. Gram-positive organisms are inhibited by the bile salts and crystal violet. The lactose fermenting enterobacteria lower the pH of the medium which is detected by the neutral red indicator, producing red or rose colonies.

Other organisms not belonging to the enterobacteria such as Pseudomonas and Aeromonas grow on MacConkey Agar. Enterococci can also grow as small pinpoint red colonies as well as some. This medium can also be used for the differentiation of mycobacteria.

Sucrose in 1% concentration may be added to the MacConkey Agar to permit the detection of certain members of the coliform group which ferment sucrose more readily than lactose. This principle was described by Holt-Harris and Teague9 and has been employed by many other bacteriologists. In some laboratories pathogenic significance is assigned to these organisms, and under such conditions sucrose should not be added to the medium.

Due to the selective properties of this medium, some strains of gram-negative enteric bacilli may be encountered that fail to grow or grow poorly on this medium; similarly, some strains of gram-positive organisms may be encountered that are not inhibited or only partially inhibited on this medium. Some strains of enterococci may grow on this medium after prolonged incubation. Incubation of MacConkey Agar plates under increased CO2 has been reported to reduce the growth and recovery of a number of strains of gram-negative enteric bacilli.19 For optimal performance, therefore, it is recommended that plates prepared from MacConkey Agar be incubated under aerobic conditions.


MacConkey Agar is based on the bile salt-neutral red-lactose agar of MacConkey and has been generally used for the selective isolation of gram-negative enteric bacilli from clinical and nonclinical sources.1 MacConkey Agar is recommended for use in the microbiological examination of foodstuffs,2 for direct plating of water samples for coliform counts3 and for the isolation of pathogenic bacteria from cheese and other dairy products.4 It has been specified by such references as: Standard Methods for the Examination of Water and Sewage, Standard Methods for the Examination of Dariy Products, Diagnostic Procedures and Reagents,5 and Compendium of Methods for the Microbiological Examination of Foods6 of the American Public Health Association.

Over a period of years, particularly in Great Britian, the neutral red bile salt agar of MacConkey7 has been quite generally used for differentiating strains of Salmonella typhi from members of the coliform group. MacConkey Agar as modified by the addition of 0.5% sodium chloride, decreasing the agar content to 1.35% and by altering the concentration of bile salts and neutral red, has the added advantage of supporting excellent growth of Shigella and Salmonella. It also gives a more clear cut differential between these enteric pathogens and the coliform group, making it easier to read than the original medium. Block and Ferguson,8 investigating an outbreak of Shiga dysentery, found MacConkey Agar satisfactory in the isolation of this fastidious strain.

The fact that this medium promotes development of these organisms, and at the same time differentiates them from lactose-fermenting gram-negative bacilli, makes it an excellent substrate fro the cultural detection of dysentery, typhoid and other Salmonella organisms in stools and other infected material. Gram-positive bacteria are inhibited.


The differential action of MacConkey Agar is based on fermentation of lactose. Colonies of organisms capable of fermenting lactose produce a localized pH drop which, followed by absorption of the neutral red, imparts a red color to the colony. A zone of precipitated bile may also be present due to this localized drop in pH. Colonies of organims which do not ferment lactose remain colorless and translucent.

When growing in proximity to coliform colonies, they have the appearance of clearing the areas of precipitated bile. On plates which are not overcrowded, the differentiation is exceptionally distinct. A plate crowded with coli will appear red and opaque, yet, if not too crowded, Salmonella or other lactose-nonfermenting organisms may easily be detected by transmitted light. On such plates they will appear as small transparent areas against the red background. A plate showing discrete colonies is to be desired for isolation purposes. Selectivity of the medium is due to the presence of crystal violet and bile salts which markedly to completely inhibit the growth of gram-positive organisms.

Typical Cultural Response on MacConkey Agar After 18 - 24 Hours at 35oC

OrganismGrowthColony Color
Enterobacter aerogenes ATCC 13048good to excellentpink to red
Escherichia coli ATCC 25922good to excellentpink to red with bile ppt.
Proteus vulgaris ATCC 13315good to excellentcolorless
Salmonella enteritidis ATCC 13076good to excellentcolorless
Shigella dysenteriae ATCC 13313fair to goodcolorless
Staphylococcus aureus ATCC 25923inhibited--

  1. MacConkey, "Lactose-fermenting bacteria in feces." J. Hig., v. 5:33 (1905).
  2. Leininger, H.V., "Equipment, media, reagents, routine tests, p. 10 - 94. In: M.L. Speck (ed.), Compendium of methods for the microbiological examination of foods. American Public Health Association, Inc., Washington, D.C. (1976).
  3. Standard methods for the examination of water and wastewater, 15th ed., American Public Health Association, Washington, D.C. (1980).
  4. Standard methods for the examination of dairy products, 9th ed., American Public Health Association, Washington, D.C. (1948).
  5. Diagnostic procedures and reagents, 3rd ed. (1950).
  6. Speck, M.L., Compnedium of methods for the microbiological examination of foods, American Public Health Association, Washington, D.C. (1976).
  7. MacConkey, J. Hyg., v. 5:333 (1905).
  8. Block and Ferguson, Am. J. Pub. Health, v. 130: 42 (1940).
  9. Holt, Harris, and Teague, J. Infectious Dis., v. 18: 596 (1916).
  10. Jeffries, C.D. and H.E. Rogers, "Enhancing effect of agar on swarming of Proteus." J. Bacteriol., v. 95: 732-733 (1968).
  11. Sandys, G.H., "A new method of preventing swarming of Proteus ssp. with a description of a new medium suitable for use in routine laboratory practice." J. Med. Lab. Tech., v. 17: 224-233 (1960).
  12. Jones, H.A. and R.W.A. Park, "The influence of medium composition on the growth and swarming of Proteus." J. Gen. Microbiol., v. 47: 369-378 (1967).
  13. Brogan, T.D., J. Nettleton, and C. Reid, "The swarming of Proteus on semisynthetic media." J. Med. Microbiol., v. 4: 1 - 11 (1971).
  14. Kopp, R., J. Mueller, and R. Lemme, "Inhibition of swarming of Proteus by sodium tetradecyl sulfate, b-phenethyl alcohol, and p-nitrophenylglycerol." Appl. Microbiol., v. 14: 873-878 (1966).
  15. Williams, F.D., "Abolition of swarming of Proteus by p-nitrophenyl glycerin: general properties." Appl. Microbiol., v. 25: 745-750 (1973).
  16. Cowan, S.T. and K.J. Steel, Manual for the identification of medical bacteria. Cambrigde University Press, Cambridge, U.K. (1970).
  17. Lennette, E.H., A. Balows, W.J. Hausler, Jr., and J.P. Truant (eds.), Manual of clinical microbiology, 3rd, ed. American Society for Microbiology, Washington, D.C. (1980).
  18. Speck, M.L. (ed.), Compendium of methods for the microbiological examination of foods. American Public Health Association, Washington, D.C. (1976).
  19. Mazura-Reetz, G., T.R. Neblett, and J.M. Galperin, "MacConkey Agar: CO2 vs. ambient incubation." Abst. Ann. Mtg. Amer. Soc. Microbiol., C179 (1979).
  20. Ministry of Health: Public Health Laboratory Service Water Committee. The bacterilogical examination of water supplies, 3rd ed., HMSO, London (1956).
  21. Windle, T.E., The examination of waters and water supplies, 7th ed., Churchill Ltd., London (1958).
  22. Joseph Md. State Dept. Health Procedures, 1960.


Catalog NumberDescriptionSize
1006017MacConkey Agar500 g
1006117MacConkey Agar without crystal violet500 g
1006217MacConkey Agar without salt500 g
1006317MacConkey Agar Base500 g
1006417MacConkey Agar CS for controlling swarming500 g
1006517MacConkey Broth500 g
1006617MacConkey Sorbitol Agar500 g