McConkey agar is a complex growth medium for culturing bacteria. MacConkey agar is selective and differential media. This medium is selective because, it selects and allows growth of only gram-negative bacteria. It is differential because it differentiates lactose fermenters and lactose non fermenters in two distinct colors. This differentiation is on the basis of lactose fermentation property. let’s talk about in what way McConkey agar selective and differential and what that means so in detail
Alfred Theodore MacConkey was first person to develop the McConkey medium in the year 1900.
McConkey agar principle:
Bile salts and crystal violet, present in MacConkey agar interferes the growth of many Gram-positive bacteria. So on MacConkey agar gram positive bacteria doesn’t grow. MacConkey agar favor the growth of gram-negative bacteria only. The bacteria present in Enterobacteriaceae family are gram negative bacteria. These bacteria are also called as the enetrics i.e. those bacteria which live in the intestine. In intestinal environment bile salts are present. Therefore enteric bacteria are adapted to grow in presence of bile salts. MacConkey agar also distinguishes between lactose-fermenter gram-negative bacteria and lactose-non-fermentor gram-negative bacteria. MacConkey agar distinguishes between lactose-fermenter gram-negative bacteria and lactose-non-fermentor gram-negative bacteria. On this MacConkey agar plate, the lactose-fermenter E. coli colonies are bright pink.
Composition of MacConkey agar medium:
MacConkey agar medium contains peptone, lactose, agar, neutral red as MacConkey agar pH indicator, and taurocholate (Bile Salt), crystal violet ,
Lactose non fermenter bacteria (pale yellowish color) on right
Serratia marcescens is gram-negative bacteria which do not ferment lactose. On MacConkey agar Serratia marcescens produces yellowish cream-colored colonies.
E. coli is a gram-negative bacterium that is a strong lactose-fermenter. E. coli colonies grow in bright pink color on MacConkey agar.
Several pathogenic bacteria such as Shigella and Salmonella species do not ferment lactose and are colorless on MacConkey agar
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MacConkey agar selective
Macconke agar is selective for gram-negative bacteria. It contains crystal violet and bile salts . In McConkey agar substances are embedded such as Crystal violet dye and bile salts. Both the Crystal violet dye and bile salts collectively inhibit the growth of gram-positive bacteria. Therefore Gram positive bacteria doesn’t grow on McConkey agar. Whereas there is no effect of Crystal violet dye and bile salts on growth of gram-negative bacteria. Since it selects for growth of gram-negative bacteria, MacConkeys agar is selective.
McConkey agar differential
McConkey agar is differential because it differentiates gram negative bacteria in two groups. These two groups are
1. lactose fermenting gram-negative bacteria and
2. lactose non-fermenting gram-negative bacteria.
lactose is a disaccharide sugar. Bacteria utilize lactose by fermenting it. During the fermentation of lactose, there is the production of acidic by-products. Because of these acidic by-products, the pH of the agar get low. McConkey agar contains an indicator dye known as Neutral Red. At acidic pH, the color of neutral red is pink/red. And at alkaline pH color is yellowish. A sterile McConkey plate is yellowish in color. When bacteria ferment the lactose then there is production of an acid byproduct. the acid produced lowers the pH and that causes the neutral red to change to this pink color. Therefore lactose fermentor gram-negative bacteria will appear pink in color. And Lactose
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