Which of the following organisms are generally positive for β-galactosidase?

• A. Salmonella spp.
• B. Shigella spp.
• C. Proteus spp.
• D. E. coli

Answer: (D)

β-galactosidase is an enzyme that is important in the fermentation of lactose. The presence of this enzyme can be detected in a variety of organisms, but it is particularly significant in identifying certain bacteria within the Enterobacteriaceae family. Escherichia coli (E. coli) is known for its ability to ferment lactose, indicating the presence of β-galactosidase. This characteristic is often employed in laboratory settings, such as in the MacConkey agar test, where E. coli forms pink colonies due to lactose fermentation, which is facilitated by this enzyme.

In contrast, the other organisms listed, such as Salmonella, Shigella, and Proteus, do not typically produce β-galactosidase. These bacteria may utilize different metabolic pathways and do not ferment lactose, which is why they do not turn pink on MacConkey agar and are often non-lactose fermenters. Therefore, the distinguishing characteristic of E. coli as a lactose fermenter and a producer of β-galactosidase confirms its positive result for the enzyme, making it the correct answer to the question.