Could the two gram negative cultures in this investigation be differentiated by the way they grew on MAC agar? Explain your response.
The correct answer and explanation is:
Correct Answer: Yes, the two gram-negative cultures could be differentiated by the way they grew on MacConkey (MAC) agar.
MacConkey agar is a selective and differential medium specifically designed to isolate gram-negative bacteria and distinguish them based on lactose fermentation. It contains bile salts and crystal violet, which inhibit the growth of gram-positive organisms, allowing only gram-negative bacteria to grow. Additionally, it contains lactose and a pH indicator (neutral red) that help differentiate between lactose fermenters and non-fermenters.
When gram-negative bacteria grow on MAC agar, their ability or inability to ferment lactose determines the color of their colonies. Lactose-fermenting bacteria, such as Escherichia coli or Klebsiella pneumoniae, produce acid from lactose. This acid lowers the pH and causes the pH indicator to turn the colonies pink or red. On the other hand, non-lactose fermenters like Pseudomonas aeruginosa or Salmonella species do not ferment lactose, so the colonies appear colorless or pale, and the medium remains its original color.
Therefore, if the two gram-negative cultures in the investigation showed different colony appearances on MAC agar—such as one producing pink colonies and the other producing colorless colonies—this would clearly indicate that one is a lactose fermenter while the other is not. This difference provides a reliable method for preliminary differentiation and classification of gram-negative bacteria in laboratory diagnostics.
This property of MAC agar is extremely useful in clinical microbiology for the identification of enteric pathogens, many of which are non-lactose fermenters. Differentiation based on lactose fermentation is one of the first steps in the identification process, and it significantly narrows down the list of possible organisms. Hence, observing the growth characteristics on MAC agar can effectively distinguish between two gram-negative bacterial cultures.