Which of the following organisms is lactose fermenting?

• A. Salmonella spp.
• B. Shigella spp.
• C. Klebsiella spp.
• D. Proteus vulgaris

Answer: (C)

Klebsiella spp. is a lactose fermenting organism, which is a key characteristic of this genus within the Enterobacteriaceae family. During lactose fermentation, these bacteria utilize lactose as a primary carbon source, leading to the production of acid and gas. This biochemical activity is crucial for distinguishing Klebsiella from other non-lactose fermenters in diagnostic settings, especially when using selective media such as MacConkey agar, on which lactose fermenters will produce pink colonies due to acid production.

In clinical microbiology, lactase-positive organisms like Klebsiella are important, as they can contribute to various infections, notably urinary tract infections and pneumonia. The presence of gas bubbles may also indicate an active fermentation process.

In contrast, Salmonella, Shigella, and Proteus species do not typically ferment lactose and would not produce the same color change on a medium designed to detect lactose fermenters. This differentiating feature is essential for accurate bacterial identification in laboratory settings.