Denitrification capability: Which of the following bacterial genera are NOT known for transforming nitrate to gaseous nitrogen (i.e., do not typically perform denitrification)?

Introduction / Context:
Denitrification is the step in the nitrogen cycle where nitrate or nitrite is reduced to gaseous forms (N2, N2O), typically under low-oxygen conditions. Recognizing which genera are classic denitrifiers versus those that are not helps in environmental microbiology and wastewater engineering.

Given Data / Assumptions:

• The task is to select a genus that does not typically carry out denitrification.
• Candidate genera include Achromobacter, Alcaligenes, Pseudomonas (well-known denitrifiers), and Agrobacterium.
• “None of these” would imply all listed genera denitrify.

Concept / Approach:
Canonical denitrifiers include Pseudomonas, Paracoccus, Alcaligenes, and some Achromobacter species. These possess the enzymatic machinery (nitrate, nitrite, nitric oxide, and nitrous oxide reductases) for the multi-step pathway. Agrobacterium, best known for plant-associated traits (e.g., T-DNA transfer), is not regarded as a standard denitrifying genus in classical references.

Step-by-Step Solution:
List classical denitrifying genera: Pseudomonas, Alcaligenes, Achromobacter. Contrast with Agrobacterium, which is not a textbook denitrifier. Rule out “None of these,” because some listed genera clearly denitrify. Choose Agrobacterium as the non-denitrifier.

Verification / Alternative check:
Environmental engineering texts consistently cite Pseudomonas and Alcaligenes as predominant denitrifiers; Agrobacterium is absent from these lists, supporting the selection.

Why Other Options Are Wrong:

• Achromobacter / Alcaligenes / Pseudomonas: Recognized denitrifiers in anoxic environments.
• None of these: Incorrect because at least three listed genera are classic denitrifiers.

Common Pitfalls:
Assuming all soil-associated genera denitrify; many are plant-associated without the full denitrification pathway.

Final Answer:
Agrobacterium does not typically perform denitrification.