What biochemical reaction would indicate the presence of a non-lactose fermenting gram-negative rod?

• A. TSI = A/A
• B. DNase positive
• C. Urease positive
• D. Indole negative

Answer: (B)

The presence of a non-lactose fermenting gram-negative rod can be indicated by various biochemical tests. In this context, being DNase positive is a significant indicator of certain non-lactose fermenters, particularly members of the Enterobacteriaceae family such as Shigella or Salmonella. When an organism produces DNase, it can break down DNA, demonstrating its ability to utilize nucleic acids for nourishment. This characteristic is especially helpful in differentiating enteric bacteria, as typical lactose fermenters such as Escherichia coli are typically DNase negative.

While other biochemical reactions may indicate specific characteristics of certain bacteria, they may not specifically point to non-lactose fermenting gram-negative rods as effectively as demonstrating DNase production. The other options may relate to a variety of bacterial traits, but they do not directly correlate to the identification of the non-lactose fermenters in the same way that a positive DNase test would.