MacConkey agar is most suitable for the cultivation of which of the following? Choose one: A. lactic acid bacteria (such as Lactobacillus and Streptococcus) B. enteric pathogens (such as Salmonella and Shigella) C. endospore formers (such as Bacillus) D. respiratory pathogens (such as Mycobacterium) E. nutritionally fastidious organisms (such as Neisseria)
The correct answer and explanation is:
Correct Answer: B. enteric pathogens (such as Salmonella and Shigella)
MacConkey agar is a selective and differential culture medium designed primarily for the isolation and differentiation of Gram-negative enteric bacteria, especially members of the Enterobacteriaceae family. These include enteric pathogens such as Salmonella, Shigella, and Escherichia coli. The medium contains bile salts and crystal violet, which inhibit the growth of Gram-positive organisms. This selectivity makes it particularly useful for isolating enteric Gram-negative rods from specimens that may contain a mixture of bacterial types.
In addition to being selective, MacConkey agar is differential based on the organism’s ability to ferment lactose. It contains lactose as the sole carbohydrate source and neutral red as a pH indicator. Lactose fermenters, such as E. coli, produce acid during fermentation, lowering the pH and causing the colonies to appear pink to red. Non-lactose fermenters like Salmonella and Shigella do not produce acid and form colorless or pale colonies. This differentiation is useful for preliminary identification and diagnosis of intestinal infections.
The other options are incorrect for the following reasons:
- Lactic acid bacteria (option A) are Gram-positive and do not grow well on MacConkey agar due to the presence of bile salts and crystal violet.
- Endospore formers like Bacillus (option C) are also Gram-positive and typically inhibited by MacConkey agar.
- Respiratory pathogens such as Mycobacterium (option D) require special media like Lowenstein-Jensen due to their unique growth requirements.
- Nutritionally fastidious organisms such as Neisseria (option E) require enriched media like chocolate agar and would not grow effectively on MacConkey agar.
Therefore, MacConkey agar is best suited for isolating and differentiating enteric pathogens.