Q : Is MacConkey selective or differential?Ī : MacConkey agar is selective and differentialĪ : Crystal violet dye and bile salts halt the growth of gram-positive bacteria. coli appear on MacConkey agar?Ī : E coli will,by fermenting lactose, lower the pH and change the color to pink to dark pinkĪ : Yes, Shigella can grow on MacConkey, Shigella doesn't utilize lactose (Colorless colonies ). Q : Why is MacConkey agar used for urine culture?Ī : Macconkey agar makes it possible to select and differentiate directly from a urine sample the gram-negative rods, which includes most of the bacteria responsible for urinary tract infectionsĪ : Yes, salmonella can grow on MacConkey, salmonella doesn't utilize lactose (Colorless colonies ). In addition, this medium does not contain crystal violet allowing the growth of Staphylococcus, Enterococcus and Mycobacterium spp. Sodium chloride is removed from the medium to provide an electrolyte deficient medium preventing Proteus spp from spreading. coli O157: H7 sorbitol negative (source: Color Atlas of Medical Bacteriology 4ed) ◉ 2- MacConkey without crystal violet and salt MacConkey with sorbitol : on the left an E. Gram negative bacteria that thrive on MacConkey agar but do not ferment lactose appear colorless on the medium and the agar surrounding the bacteria remains relatively transparent : Salmonella, Proteus, Yersinia, Pseudomonas.When the pH drops, the neutral red is absorbed by bacteria, which appear as bright pink to red colonies on the agar : Escherichia coli, Enterobacteria, Klebsiella It also differentiates on the basis of lactose. MacConkey agar is selective for Gram-negative organisms and contain indicators to differentiate lactose fermenters from lactose non-fermenters. Bacteria which ferment lactose decrease the pH of the medium, this decrease is detected by neutral red(become red if pH below 6.8). MacConkey agar supports the growth of Gram-negative bacteria, and inhibits the growth Gram-positive bacteria. MacConkey agar is a commonly used selective and differential medium containing lactose, bile salts, neutral red, and crystal violet. Although the gram-negative rods in this culture are likely insignificant, a clinician might be hard-pressed to remove Pseudomonas coverage once a non-lactose fermenting gram-negative rod is reported.Gram-negative bacteria that grow on MacConkey plate are differentiated by their ability to ferment lactose. This only allows gram-negative species, which have a relatively bile-resistant outer membrane, to form colonies on MacConkey agar. ◉ Selectivity of MacConkey agarĬrystal violet dye and bile salts stop the growth of gram-positive bacteria. In particular members of the Enterobacteriaceae family and the genus Pseudomonas. ◈ MacConkey Agar is a selective and differential medium used for the isolation and differentiation of non-fastidious Gram-negative bacilli, ◉ Principle and interpretation of MacConkey agar ◈ A variant of MacConkey agar without crystal violet and salt can be used to limit the swarming of Proteus spp. ◈ The lactose in the medium can be replaced by other sugars : MacConkey with sorbitol.
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