MacConkey agar contains lactose as its primary carbohydrate source, which is utilized by lactose-fermenting bacteria, leading to the production of acid and a color change in the medium. Protein sources in MacConkey agar come from peptones, which provide essential amino acids and nitrogen for bacterial growth. This selective and differential medium is used primarily to isolate and differentiate gram-negative bacteria, particularly Enterobacteriaceae.
No, Alcaligenes faecalis will not grow on MacConkey agar. MacConkey agar is selective for gram-negative bacteria that ferment lactose. Alcaligenes faecalis is a gram-negative bacteria that does not ferment lactose, so it will not grow on MacConkey agar.
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MacConkey agar
Lac+ bacteria is acid base color indicator that lowers the pH of the MacConkey agar. The MacConkey agar contains neutral red dye, lactose, peptone, and crystal violet dye.
To test carbohydrate fermentation, common culture media include phenol red broth and MacConkey agar. Phenol red broth contains a specific carbohydrate, and the color change indicates fermentation. MacConkey agar is selective for Gram-negative bacteria and differentiates lactose fermenters, which produce acid, leading to a color change in the medium. Both media are effective for assessing the fermentation capabilities of various microorganisms.
MacConkey agar contains agar as a solidifying agent which solidifies at room temperature to provide a solid surface for bacterial growth and colony isolation. MacConkey broth, on the other hand, does not contain agar and remains liquid at room temperature to allow for easier culture of bacteria in liquid medium for metabolic studies or growth analyses.
MacConkey agar is designed to grow Gram-negative bacteria and also contains crystal violet dye which inhibits the growth of Gram-positive bacteria. Since S. aureus is Gram-positive it should not grow on MacConkey agar. No. MacConkey agar inhibits MOST of the Gram-positive bacteria. There are some Gram-positive bacteria that can tolerate bile-salt are able to grow in MacConkey agar. Enterococcus and some species of Staphylococcus are able to grow in MacConkey. (lack citation though)
Clostridium species do not grow on MacConkey agar because they are anaerobic bacteria that require an oxygen-free environment, while MacConkey agar is designed for the growth of aerobic and facultatively anaerobic Gram-negative bacteria. Additionally, MacConkey agar is selective for lactose fermenters, and Clostridium does not ferment lactose. Therefore, Clostridium cannot be cultured on this medium.
Lactose in MacConkey agar serves as a fermentable carbohydrate that allows for the differentiation of bacteria based on their ability to ferment lactose. Lactose fermentation produces acidic byproducts, which lower the pH of the agar, causing lactose-fermenting bacteria to produce pink/red colonies while non-fermenters appear colorless.
Enterococcus faecalis typically does not grow on MacConkey agar as this selective and differential medium is primarily used for isolating and differentiating members of the Enterobacteriaceae family, which are mostly gram-negative bacteria. Enterococcus species are gram-positive bacteria and do not ferment lactose, the key ingredient in MacConkey agar that allows for differentiation.
E. coli that grow on MacConkey agar typically exhibit lactose fermentation, causing them to produce pink or red colonies. These colonies may also appear mucoid due to the production of a polysaccharide capsule. Additionally, E. coli on MacConkey agar will typically not grow well under acidic conditions, as the agar contains crystal violet which inhibits the growth of Gram-positive bacteria.
Bile salts in MacConkey agar inhibit the growth of Gram-positive bacteria by disrupting their cell membranes. This selective property allows for the preferential growth of Gram-negative bacteria on MacConkey agar, making it a useful medium for the isolation and differentiation of enteric bacteria.