Skill Sharpener for the Week of 2017 February 20

The images above demonstrate a before and after incubation of a urine sample on MacConkey agar.  What general characteristics can be stated regarding the organism that grew?​

​Fun fact?  You will notice that where the "before" image shows transparency, the image with growth is more translucent, even where colonies are not growing.  What is causing this phenomenon?

The organism seen growing is most likely a Gram negative organism as MacConkey agar is formulated with crystal violet and bile salts that inhibit the growth of most Gram positive organisms.  The pink colonies indicate the organism ferments lactose.  This media contains lactose as the sole carbon source and the fermentation of the lactose results in the formation of acids which cause the neutral red indicator dye in the media to turn red and dye the colonies pink (as opposed to turning yellow in basic conditions causing the colonies to be colorless).

The occlusion of the media is a result of the bile salts precipitating in an acidic environment.  The precipitation of bile salts is not indicative of any particular bacteria, but is more an indicator of the amount of acid produced i.e. early in incubation, colonies may be pink due to lactose fermentation but the agar may be relatively clear due to the lower concentration of acid.


MacConkey agar package insert

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