You are working up a stool culture On MacConkey agar you see

You are working up a stool culture. On MacConkey agar you see many bright pink colonies and many clear colonies. Which colonies are potential pathogens that require further testing?

Clear colonies, lactose fermenters

Bright pink colonies, lactose fermenters

Bright pink colonies, non lactose fermenters

Clear colonies, non lactose fermenters

Solution

Clear colonies non lactose fermenters. Ex. Salmonella sps.

This is a differential plating medium, selective for Gram negative organisms, used primarily for detection and isolation of enteric bacteria.
Gram positives are generally inhibited by crystal violet in the medium.

Isolated colonies of lactose-fermenting bacteria are brick red in color and may be surrounded by a zone of precipitated bile. This reaction is due to the action of the acids, produced by fermentation of lactose, upon bile salts present in the medium, and subsequent absorption of neutral red. Ex. E. coli

Non-lactose fermenters are colorless and transparent.