Escherichia coli on Eosin-Methylene-Blue (EMB) agar (+) Greenish metallic sheen Note: Citrobacter freundii also gives a greenish metallic sheen To differentiate, Biochemical testing E. coli gives a negative result for Citrate test (green)
In this Lab students will be able to observe the following organelles: Nucleus, cell wall, cytoplasm, and the nucleolus within the cells of an onion and a cheek. Iodine and methylene blue are required for this experiment and are common in any schools science lab department.
EMB Agar: is a selective AND a differential medium. The selective components are the eosin and methylene blue dyes that inhibit the growth of Gram-positive bacteria and permit the growth of Gram-negative enteric rods. The differential component of the medium is lactose, which is a sugar that some organisms can ferment as an energy source. Organisms that ferment lactose produce dark purple (sometimes shiny) colonies and those that cannot, produce white or very light pink colonies.
Preview Eosin-methylene blue (EMB) agar plate inoculated with Escherichia coli (a gram-negative coliform bacterium) showing good growth of dark blue-black colonies with metallic green sheen indicating vigorous fermentation of lactose and acid production which precipitates the green metallic pigment. (Naowarat Cheeptham, Thompson Rivers University, Kamloops, BC, Canada)