The laboratory would like to selectively eliminate gram posi

The laboratory would like to selectively eliminate gram- positive bacteria from a mixture of gram- positive and gram- negative bacteria. Which of the following procedures would be more appropriate and why or why not?

Solution

MacConkey’s (MAC) medium selectively allows the growth of gram-negative bacteria. It is used to differentiate the enterobacteria by their lactose fermentation ability. Gram-positive bacteria cannot grow in MacConkey’s medium due to the presence of bile salts and crystal violet. E.coli is an enterobacter species and can ferment lactose. So, it produces pink colonies on MacConkey’s medium. While the Salmonella typhimurium does not ferment lactose, so they produce yellow to light pink color colonies.