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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.
Non-Lactose-Fermenting (NLF) Colonies: Appearance- NLF colonies appear colorless or pale on MacConkey agar. Explanation: NLF bacteria do not possess the necessary enzymes to ferment lactose. As a result, they do not produce acid during lactose fermentation, and the agar remains its natural color (colorless or pale).
Strong lactose fermenters will appear as dark blue/purple/black, and E.coli (which also ferments lactose) colonies will be dark colored, but will also appear to have a metallic green sheen. Other coliform bacteria will appear as thick, slimy colonies, with non-fermenters being colorless, and weak fermenters being pink. [citation needed]
Burkholderia cepacia complex (BCC) is a species complex consisting of Burkholderia cepacia and at least 20 different biochemically similar species of Gram-negative bacteria. They are catalase-producing and lactose-nonfermenting. [1]
enterocolitica is non-lactose fermenting, and therefore show up on the plates as 2mm translucent pale colonies. On Yersinia Selective Agar plates Y. enterocolitica produces 1.5mm colonies with a dark pink center and translucent border. [6] [5]
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.
Coliform organisms ferment the lactose in this medium, producing a green metallic sheen (i.e. Escherichia coli), whereas non-lactose-fermenting organisms produce clear, colourless colonies, [1] i.e. Salmonella species.
Non-fermenters (also non-fermenting bacteria) are a taxonomically heterogeneous group of bacteria of the phylum Pseudomonadota that cannot catabolize glucose, and are thus unable to ferment. This does not necessarily exclude that species can catabolize other sugars or have anaerobiosis like fermenting bacteria.