E. coli is actually green on tryptic soy agar. The agar itself is a white, amber color.
The uninoculated Kligler Iron Agar media is typically a red color, reflecting the presence of peptones and sugars in the medium.
Simmons citrate agar is a differential agar used to determine if a sample bacteria can utilize citrate as its only carbon source. The agar is initially a green color due to the bromo thymol blue pH indicator in it. If a bacteria uses the citrate, the by-products are ammonia and ammonium hydroxide both of which will alkalize the agar and increase the pH to the point of changing the indicator's color to blue, so the whole agar turns from green to blue.
E. coli forms dark blue colonies on Chromocult agar due to its ability to metabolize certain substrates present in the agar, resulting in the production of dark blue pigments such as indole. This metabolic activity is specific to E. coli and helps to differentiate it from other bacteria based on colony color.
The hemolysis is called green hemolysis because of the color change in the agar.
Salmonella appears as black colonies on bismuth sulfite agar due to the formation of hydrogen sulfide gas, which reacts with bismuth ions in the medium to produce insoluble black bismuth sulfide.
Escherichia coli will appear as dark purple colonies with a green metallic sheen on EMB agar due to its ability to ferment lactose and produce acid.
EMB agar is useful for the selective isolation of gram-negative bacteria, especially fecal coliforms such as E. coli, from clinical and environmental samples. It is also helpful in differentiating lactose-fermenting bacteria (which produce dark colonies with green metallic sheen) from non-lactose fermenters based on colony color and appearance.
Agar agar does not produce collagen. It is a plant-based alternative to gelatin and is commonly used as a vegan substitute in cooking and baking. Collagen is a protein found in animals, especially in their connective tissues.
The peptones in EMB agar provide nitrogen for bacterial growth, while the lactose is a source of carbon. These ingredients support the growth of certain coliform bacteria, which ferment lactose to produce characteristic color changes on the agar.
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.
Nutrient agar is typically a pale yellowish color when prepared and solidified.