Microbiology & Bacteria
Gram negative bacilli
Photo Credit: CDC/Dr. William A. Clark from the Centers for Disease Control and Prevention's Public Health Image Library
Gram positive bacilli
By Riraq25 (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
Gram positive cocci
Gram negative spirilla
By Dr. Graham Beards - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=18420815
Mannitol Salts
Mannitol Salts: This medium functions as both a selective and a differential medium. It is selective because not many types of bacteria can grow on this high-salt (7.5%) medium. It is differential because different species of bacteria use different nutrients in the medium, and therefore produce waste products of different pH. Because this medium has a pH indicator, this difference is indicated by a color change.
Left: Staphylococcus aureus uses mannitol and produces an acidic waste, which turns the plate yellow.
Right: Staphylococcus epidermidis does not use mannitol (a sugar). Medium does not turn yellow.
EMB with E. coli
Eosin Methylene Blue (EMB): This medium is another selective and differential medium. EMB selects for Gram negative bacteria. Growth of Escherichia coli (E. coli) causes a green metallic sheen.
