Question

“Table 5: MacConkey Agar Results”

“Sample”	“Growth”	“Colony Color”	“Analysis”
Toilet water tank	NO	N/A	N/A
Kitchen sink	NO	N/A	N/A
Shower	YES	CLEAR COLOR	SMALL DOTS

“Post-Lab Questions”

“6. What are some potentially pathogenic bacteria that do not ferment lactose that will grow on MacConkey agar?”

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“7. How would you verify that the colonies that grew on a MacConkey agar plate were Gram-negative?”

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“8. Look up the formulation for MacConkey agar. Is this a chemically defined or complex media? Why is that important?”

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Answer 1

6.Some potentially pathogenic bacteria that do not ferment lactose but will grow on MacConkey agar include: Salmonella, Proteus, and Shigella.

7.The selectivity of MacConkey agar is due to crystal violet and bile salts, which inhibits the growth of gram-positive bacteria. Thus, only gram-negative bacteria can grown on this agar. Further confirmation can be done by performing a gram stain on the bacteria.

8. MacConkey is a complex media. The media contains lactose, e pancreatic digests of gelatin and casein, peptic digest of animal tissue, and bile salts, the exact composition of which are not known. Complex media is important for the growth of organisms whose specific growth requirements are not known. Hence a media which can provide a vide range of nutritional sources must be used in that case.