MacConkey agar is a selective and differential culture medium formulated to isolate Gram‑negative bacteria and distinguish them based on their ability to ferment lactose.
Explanation
This medium contains crystal violet dye and bile salts that suppress the growth of Gram‑positive organisms. Lactose is the primary carbohydrate; if bacteria ferment lactose they acidify the surrounding medium, causing the pH indicator neutral red to change from yellow to pink. Non‑lactose fermenters do not acidify the medium and therefore form colorless colonies.
MacConkey agar was developed in the early twentieth century and remains a fundamental tool in clinical and environmental microbiology. It is commonly used to culture enteric Gram‑negative bacilli from urine, stool and food samples. By distinguishing lactose‑fermenting organisms such as Escherichia coli, Klebsiella pneumoniae and Enterobacter species from pathogens like Salmonella and Shigella, it guides diagnostic decisions and helps monitor contamination in food and water.
Diagnostic Features and Applications
- Lactose‑fermenting bacteria (e.g., E. coli, Klebsiella spp.) form pink or red colonies due to acid production.
- Non‑lactose fermenters such as Salmonella, Shigella and Pseudomonas produce pale or colorless colonies.
- Gram‑positive organisms are inhibited by the crystal violet and bile salts and therefore do not grow.
- MacConkey agar is routinely employed for screening clinical specimens and testing water and food samples for coliforms.
MacConkey agar continues to be an essential medium for differentiating Gram‑negative bacteria in diagnostic and research laboratories. Its combination of selective agents and pH indicator provides rapid visual identification of lactose fermentation, enabling targeted follow‑up testing.
Related Terms: Culture Medium, Selective Media, Differential Media, Enterobacteriaceae, Lactose