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MacConkey agar is a selective and differential culture medium for bacteria. It is designed to selectively isolate gram-negative and enteric (normally found in the intestinal tract) bacteria and differentiate them based on lactose fermentation. [1] Lactose fermenters turn red or pink on MacConkey agar, and nonfermenters do not change color.
This is important because gut bacteria, such as Escherichia coli, can typically ferment lactose, while important gut pathogens, such as Salmonella enterica and most shigellas are unable to ferment lactose. Shigella sonnei can ferment lactose, but only after prolonged incubation, so it is referred to as a late-lactose fermenter. During ...
In fact, lactic acid bacteria contain the needed enzymes to digest lactose, and their populations multiply strongly during the fermentation. Therefore, milk fermented even a short time contains enough enzymes to digest the lactose molecules, after the milk is in the human body, which allows adults to consume it.
The species C. amalonaticus, C. koseri, and C. freundii can use citrate as a sole carbon source. Citrobacter species are differentiated by their ability to convert tryptophan to indole (C. koseri is the only citrobacter to be commonly indole-positive), ferment lactose (C. koseri is a lactose fermentor), and use malonate.
Lactose fermentation In one study that sought to prove that some fermentation produced by L. lactis can hinder motility in pathogenic bacteria, the motilities of Pseudomonas, Vibrio, and Leptospira strains were severely disrupted by lactose utilization on the part of L. lactis. [34]
Leuconostoc lactis is a Gram-positive, non-motile, lactic acid bacterium that thrive best in acidic conditions and moderate temperatures. [2] [1] L. lactis is capable of acidifying culture media through lactose fermentation to pH levels of 4.0-4.1, and milk to levels below 5.4. [4]
This strain is similar to Subsp. Mesenteroides, the optimum temperature of 20 and 30 °C. It can also ferment Glucose, fructose, lactose, maltose, sucrose and trehalose. There is also a variation by strain for requirements of riboflavin, pyridoxal and folic acid. Some strains also required a combination of uracil, guanine, adenine and xanthine. [5]
Lactobacillus bulgaricus colonies grown on China Blue Lactose Agar, after anaerobic incubation. Lactobacillus bulgaricus is the main bacterium used for the production of yogurt. It also plays a crucial role in the ripening of some cheeses, [3] as well as in other processes involving naturally fermented products.