MacConkey Agar, Isolation Techniques and Selective Media, A Photographic Atlas for the Microbiology Laboratory,
Purpose
MacConkey Agar is used to isolate and differentiate members of the Enterobacteriaceae based on the ability to ferment lactose. Variations on the standard medium include MacConkey Agar w/o CV (without crystal violet) to allow detection of Gram-positive cocci or MacConkey Agar CS to control swarming bacteria (such as Proteus) that interfere with other results.
Principle
MacConkey Agar is a selective and differential medium containing lactose, bile salts, neutral red, and crystal violet. Bile salts and crystal violet inhibit growth of Gram-positive bacteria. Neutral red dye is a pH indicator that is colorless above a pH of 6.8 and red at a pH below 6.8. Acid accumulating from lactose fermentation turns the dye red. Lactose fermenters turn a shade of red on MacConkey agar whereas lactose nonfermenters remain their normal color or the color of the medium (Figures 2-19 and 2-20). Formulations without crystal violet allow growth of Enterococcus and some species of Staphylococcus, which ferment the lactose and appear pink on the medium
2-19 MACCONKEY AGAR MacConkey Agar
inoculated with (clockwise from top) Escherichia
coli, Enterobacter aerogenes, Shigella sonnei, and
Proteus mirabilis. E. coli and E. aerogenes produce
pink color from acid-producing lactose fermentation.
S. sonnei and P. mirabilis, both lactose nonfermenters,
remain their normal color. Note the precipitated bile salts
around the E. coli, also shown in Figure 2-20.
2-20 MACCONKEY AGAR STREAKED FOR
ISOLATION MacConkey Agar inoculated with
Escherichia coli and Shigella flexneri. Note the
precipitated bile salts around the E. coli
caused by acid from lactose fermentation.
Suggested Reading
- Michael J. Leboffe & Burton E. Pierce. A Photographic Atlas for the Microbiology Laboratory 4th edition 2011
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