Presumptive tests used for identification of Pseudomonas species and to differentiate them from Enterobacteriaceae family

Presumptive tests used for identification of Pseudomonas species and to differentiate them from Enterobacteriaceae family

• A. Catalase, lysine decarboxylase
• B. Oxidase, OF test and pigment production
• C. Growth on MAC and BAP
• D. Growth on BAP and EMB

Answer: (B)

The main idea is using quick biochemical cues that separate Pseudomonas from Enterobacteriaceae by how they metabolize and what they produce pigment-wise. Pseudomonas typically shows an oxidase-positive reaction, meaning it uses cytochrome c oxidase in its respiratory chain and yields a positive result on the oxidase test. In contrast, Enterobacteriaceae are usually oxidase negative, so this test immediately differentiates many of the common Gram-negative rods. The oxidative-fermentative (OF) test adds another layer: Pseudomonas tends to oxidize carbohydrates without fermenting them, so you see an acid reaction in the open (oxygen-present) tube but not in the closed tube, indicating oxidative metabolism rather than fermentation. Enterobacteriaceae, on the other hand, commonly ferment glucose, producing acid in both open and closed conditions. This pattern helps distinguish non-fermenting, oxidase-positive Pseudomonas from fermenting, oxidase-negative Enterobacteriaceae. Pigment production is a practical clue as well. Many Pseudomonas species produce distinctive pigments (like pyocyanin and pyoverdin), resulting in characteristic blue-green or fluorescent colonies on agar. Such pigments are not typical of Enterobacteriaceae, which helps reinforce the separation. While these tests are not definitive on their own, together they provide a strong, rapid presumptive screen to identify Pseudomonas and differentiate it from the Enterobacteriaceae family, guiding further confirmatory testing.

The main idea is using quick biochemical cues that separate Pseudomonas from Enterobacteriaceae by how they metabolize and what they produce pigment-wise. Pseudomonas typically shows an oxidase-positive reaction, meaning it uses cytochrome c oxidase in its respiratory chain and yields a positive result on the oxidase test. In contrast, Enterobacteriaceae are usually oxidase negative, so this test immediately differentiates many of the common Gram-negative rods.

The oxidative-fermentative (OF) test adds another layer: Pseudomonas tends to oxidize carbohydrates without fermenting them, so you see an acid reaction in the open (oxygen-present) tube but not in the closed tube, indicating oxidative metabolism rather than fermentation. Enterobacteriaceae, on the other hand, commonly ferment glucose, producing acid in both open and closed conditions. This pattern helps distinguish non-fermenting, oxidase-positive Pseudomonas from fermenting, oxidase-negative Enterobacteriaceae.

Pigment production is a practical clue as well. Many Pseudomonas species produce distinctive pigments (like pyocyanin and pyoverdin), resulting in characteristic blue-green or fluorescent colonies on agar. Such pigments are not typical of Enterobacteriaceae, which helps reinforce the separation.

While these tests are not definitive on their own, together they provide a strong, rapid presumptive screen to identify Pseudomonas and differentiate it from the Enterobacteriaceae family, guiding further confirmatory testing.