![]() ![]() These bacteria also take up the dye, giving a pink appearance to colonies. The drop in pH is just enough to change the color of the indicator dye. These species change the pH of media to acidic but not as much as the strong fermenters do. Weak lactose fermenters: Serratia and Enterobacter aerogenes are an example of this group. A ‘Pink halo’ is a narrow white region at the pink color junction (produced by colonies) and the pH-unaffected area. The dye is also taken up by the bacterial cells resulting in the pink colonies, and the precipitation of bile salts results in a ‘pink halo’. These bacteria drastically change the pH of the surrounding media to acidic, changing the color of neutral red and precipitating the bile salts. Strong lactose fermenters: Escherichia coli is a typical example of this group. For more details on pH changes, read EMB Agar. The color of colonies in MacConkey agar is based on their ability to change the pH of the media. ![]() When the acids are released into surrounding media, media pH drops. Some bacteria consume lactose and produce acids (lactose fermenters). Lactose: A sugar that works as a carbon source. Turns to red/pink when pH drops below 6.8. Neutral red: A pH indicator that acts as a litmus paper. Gram negative species that live in the colon can resist bile salt's negative effects.Ĭrystal violet: Inhibits the growth of gram positive bacteria by inducing DNA lesions. Peptone: Nitrogen source, also works as a carbon source for bacteria that can not metabolize sugars.īile salts: Inhibit the growth of gram positive bacteria by disrupting the membranes. To understand how MacConkey agar works, let's first see the role of different ingredients. ![]()
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