Which test differentiates Klebsiella spp. from Enterobacter spp.?

• A. Hydrogen sulfide test
• B. Indole test
• C. ONPG test
• D. Voges-Proskauer test

Answer: (C)

The correct choice is the ONPG test, which stands for ortho-nitrophenyl-β-D-galactopyranoside test. This test is used to differentiate between Klebsiella species and Enterobacter species based on their ability to ferment lactose. Klebsiella species are typically positive for this test, meaning they can ferment lactose, while Enterobacter species also generally ferments lactose but can produce varying results due to other metabolic differences.

The ONPG test specifically assesses the ability of the bacteria to cleave the ONPG substrate, which is a lactose analog. Klebsiella spp. consistently demonstrate a positive result for this test due to their active production of β-galactosidase. In contrast, while Enterobacter spp. can also be lactose fermenters, this test helps pinpoint the more specific metabolic capabilities that differentiate the two genera.

Understanding this distinction is vital in clinical microbiology and helps in the accurate identification of bacteria within clinical specimens, facilitating effective treatment strategies for infections caused by these organisms.