Description
| Intended Use and Principle of Procedure |
| MacConkey Agar conforms with the specifications of The United States Pharmacopeia (USP). MacConkey agars are slightly selective and differential plating media mainly used for the detection and isolation of gram-negative organisms from clinical,1 dairy,2 food,3,4 water,5 pharmaceutical6 and industrial7 sources. MacConkey Agar is used for isolating and differentiating lactosefermenting from lactose-nonfermenting gram-negative enteric bacilli. MacConkey Agar Base is used with added carbohydrate in differentiating coliforms based on fermentation reactions. MacConkey Agar without Crystal Violet is used for isolating and differentiating enteric microorganisms while permitting growth of staphylococci and enterococci. The medium can be used also to separate Mycobacterium fortuitum and M. chelonae from other rapidly growing mycobacteria. MacConkey Agar without Crystal Violet or Salt and MacConkey Agar without Salt are used for isolating and differentiating gram-negative bacilli while suppressing the swarming of most Proteus species.
Peptones are sources of nitrogen and other nutrients. Lactose is a fermentable carbohydrate. When lactose is fermented, a local pH drop around the colony causes a color change in the pH indicator (neutral red) and bile precipitation. Bile salts, bile salts no. 3, oxgall and crystal violet are selective agents that inhibit growth of gram-positive organisms. Agar is the solidifying agent. |
| Summary and Explanation |
| MacConkey Agar is based on the bile salt-neutral red-lactose agar of MacConkey.8 The original MacConkey medium was used to differentiate strains of Salmonella typhosa from members of the coliform group. Formula modifications improved the growth of Shigella and Salmonella strains. These modifications included the addition of 0.5% sodium chloride, decreased agar content, and altered bile salts and neutral red concentrations. The formula improvements gave improved differential reactions between these enteric pathogens and the coliform group. MacConkey Agar contains crystal violet and bile salts that inhibit gram-positive organisms and allow gram-negative organisms to grow. Isolated colonies of coliform bacteria are brick red in color and may be surrounded by a zone of precipitated bile. This bile precipitate is due to a local pH drop around the colony due to lactose fermentation. Colonies that do not ferment lactose (such as typhoid, paratyphoid and dysentery bacilli) remain colorless. When lactose nonfermenters grow in proximity to coliform colonies, the surrounding medium appears as cleared areas. It is recommended in the USP for use in the performance of Microbial Limit Tests.
MacConkey Agar Base is prepared without added carbohydrates, which permits their addition either individually or in combination. It is recommended that carbohydrates such as sucrose or lactose be added in a concentration of 1% to the basal medium. MacConkey Agar without Crystal Violet is a differential medium that is less selective than MacConkey Agar. The lack of crystal violet permits the growth of Staphylococcus and Enterococcus. Staphylococci produce pale pink to red colonies and enterococci produce compact tiny red colonies either on or beneath the surface of the medium. The medium is used also to separate Mycobacterium fortuitum and M. chelonae from other rapidly growing mycobacteria.
MacConkey Agar without Crystal Violet or Salt and MacConkey Agar without Salt (which also lacks crystal violet) are differential media used for isolating and cultivating gram-negative enteric organisms and gram-positive cocci from waters, feces and other sources suspected of containing these organisms, as well as limiting the swarming of Proteus species. |
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