File:LF and NLF Gram Negative Bacteria on MacConkey medium.jpg

English: On MacConkey agar, both lactose-fermenting (LF) and non-lactose-fermenting (NLF) Gram-negative bacteria can be distinguished based on their growth characteristics and colony appearance. Here's a more detailed description of what you might observe for each type: Lactose-Fermenting (LF) Gram-Negative Bacteria-Colony Appearance: LF bacteria typically form pink or red colonies on MacConkey agar. Growth Characteristics- LF bacteria have the ability to ferment lactose, which is one of the components of MacConkey agar. This fermentation produces acid as a metabolic byproduct, which in turn lowers the pH of the agar. The pH indicator (usually neutral red) changes color in response to the drop in pH, resulting in the pink or red coloration of LF colonies. Examples of LF Bacteria- Common LF bacteria that can be identified on MacConkey agar include Escherichia coli, Klebsiella pneumoniae, and Enterobacter aerogenes. These bacteria are often part of the Enterobacteriaceae family. Non-Lactose-Fermenting (NLF) Gram-Negative Bacteria-Colony Appearance: NLF bacteria appear colorless or pale on MacConkey agar. Growth Characteristics: NLF bacteria do not possess the necessary enzymes to ferment lactose, so they do not produce acid during lactose fermentation. As a result, the agar remains its natural color (colorless or pale). Examples of NLF Bacteria: Common NLF bacteria that can grow on MacConkey agar include Salmonella species, Shigella species, and Pseudomonas aeruginosa. These bacteria do not produce the characteristic pink or red color associated with lactose fermentation.