Question

MacConkey agar is selective and differential. Explain why they are selective and differential, and what is in the media that enables this to occur.

What results would you expect from plating the following microorganisms on MacConkey, and why?

A) Staphylococcus aureus

B) Serratia marcescens

C) Escherichia coli

Answer 1

MacConkey agar is selective because it selectively allows the growth of ONLY gram negative and enteric bacilli.

mechanism: it contains crsytal violet and bile salts which inhibit the growth of the gram positive bacteria.

MacConkey agar is differential because out of the enteric bacilli isolates it helps to differentiate those who ferment lactose and those who do not as the media contains lactose .

the media also contains an ph indicator nuetral red which on production of lactic acid makes the ph acidic and changes its color to pink to indicate the presence of lactose fermenting bacilli.

a) no colonies will be produced as it is gram positive and its growth is inhibited by bile salts.

b) it is gram negative bacteria and is slow lactose fermenting in nature and produces a dark red colony against a pink media.

c) it is gram negative will produce a pink colony in the media due to its lactose fermenting ability. the bile salts precipitate in the immediate neighbourhood of the colony, causing the medium surrounding the colony to become hazy.