Fig. 19: Serratia marcescens
Growing on MacConkey Agar
MacConkey agar is a selective medium used for the isolation of non-fastidious Gram-negative rods, particularly members of the family Enterobacteriaceae and the genus Pseudomonas, and the differentiation of lactose fermenting from lactose non-fermenting Gram-negative bacilli. MacConkey agar contains the dye crystal violet well as bile salts that inhibit the growth of most Gram-positive bacteria but do not affect the growth of most Gram-negatives.
If the bacteria do not ferment lactose, the colonies and confluent growth appear colorless and the agar surrounding the bacteria remains relatively transparent (arrow).
Microbiology Laboratory Manual by Gary E. Kaiser, PhD, Professor of Microbiology
is licensed under a Creative Commons Attribution 4.0 International License.
Last updated: September, 2017