Translations available in Spanish. Lactose Fermentation on MacConkey Agar Plates Resource Type: Visual: Image Publication Date: Prior to 1/1/2002 Figure 1 Lactose fermenters & nonfermenters (Labeled view) Lactose fermenters & nonfermenters (Enlarged view) Authors Neal Chamberlain Department of Microbiology/Immunology A.T. Still University of Health Sciences/KCOM Kirksville, Missouri 63501 USA Email: nchamberlain@atsu.edu MacConkey agar medium is commonly used to differentiate between various gram-negative rod-shaped organisms. Many facultative anaerobes in the intestine are lactose fermenters (e.g., Escherichia coli). Several well-known pathogens are unable to ferment lactose (e.g., Shigella, Salmonella). MacConkey agar is a medium that is both selective and differential. It is useful in demonstrating to students these concepts. This medium can differentiate between the lactose fermenters and nonfermenters. It is also a selective medium that only allows the growth of gram-negative rod-shaped organisms. Age of culture is 16 h.