Which group of bacteria is characterized by its ability to ferment lactose?

• A. Pathogenic strains only
• B. Non-lactose fermenters
• C. Lactose fermenters
• D. All Enterobacteriaceae

Answer: (C)

Lactose fermenters are a specific group of bacteria known for their ability to metabolize lactose, a sugar found in milk and dairy products, as a source of energy. This characteristic is particularly important in the context of the family Enterobacteriaceae, which includes many genera such as Escherichia, Klebsiella, and Enterobacter. These organisms possess the enzyme β-galactosidase, which allows them to break down lactose into glucose and galactose, resulting in the production of acid and gas. This fermentation process can be easily detected on culture media that contain lactose, as acid production leads to a color change in the media.

Understanding which bacteria can ferment lactose helps in clinical microbiology to differentiate between various enteric pathogens. For instance, while some Enterobacteriaceae members, like Escherichia coli, are strong lactose fermenters and often produce a characteristic pink colony on MacConkey agar, other members like Salmonella and Shigella do not ferment lactose and remain colorless.

The other categories mentioned—pathogenic strains only, non-lactose fermenters, and all Enterobacteriaceae—do not accurately encompass the specific ability to ferment lactose in the same way that lactose fermenters do,