What are the most appropriate screening tests to presumptively differentiate and identify nonfermentative gram-negative bacilli from the Enterobacteriaceae?

The correct choice highlights a set of tests that effectively differentiate nonfermentative gram-negative bacilli from the Enterobacteriaceae family. The use of oxidase testing is crucial here, as nonfermentative gram-negative bacilli are typically oxidase positive, while many members of the Enterobacteriaceae family are oxidase negative. This test helps to quickly categorize bacteria based on their metabolic characteristics.

Additionally, the Triple Sugar Iron (TSI) test provides valuable information regarding the fermentation patterns of sugars present in the medium, which varies between these two groups. Nonfermenters usually show no gas or hydrogen sulfide production in TSI, while Enterobacteriaceae may ferment glucose and produce gas or hydrogen sulfide.

Nitrate reduction is another significant test since many nonfermenters can reduce nitrate to nitrite or other nitrogenous compounds, whereas not all Enterobacteriaceae share this trait. Finally, growth on MacConkey agar supports the differentiation process, as it allows for the selective isolation of gram-negative bacteria and can indicate lactose fermentation, a feature prominent in Enterobacteriaceae.

Together, these tests form a comprehensive screening panel that provides distinct metabolic insights to differentiate nonfermentative gram-negative bacilli from Enterobacteriaceae,