Which organism is non-lactose fermenting, oxidase negative, nitrate positive, PAD positive, citrate positive, and indole positive?

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Multiple Choice

Which organism is non-lactose fermenting, oxidase negative, nitrate positive, PAD positive, citrate positive, and indole positive?

This item tests how a specific set of biochemical traits can identify a non-lactose–fermenting enteric bacterium. The pattern described—non-lactose fermenting, oxidase negative, nitrate positive, PAD positive, citrate positive, and indole positive—fits Providencia rettgeri extremely well. Being non-lactose fermenting rules out the lactose-fermenters; oxidase negative places it with the Enterobacterales family; nitrate positive shows it reduces nitrate, a common trait in many enterics. The PAD positive result indicates phenylalanine deaminase activity, a characteristic shared by Providencia (and some related genera) that helps distinguish it from other non-lactose fermenters. Citrate positive reveals it can use citrate as a carbon source, another feature aligned with Providencia rettgeri. Indole positive confirms tryptophanase activity, which together with the previous traits narrows the identification to Providencia rettgeri rather than other non-lactose–negative, oxidase–negative organisms.

This item tests how a specific set of biochemical traits can identify a non-lactose–fermenting enteric bacterium. The pattern described—non-lactose fermenting, oxidase negative, nitrate positive, PAD positive, citrate positive, and indole positive—fits Providencia rettgeri extremely well. Being non-lactose fermenting rules out the lactose-fermenters; oxidase negative places it with the Enterobacterales family; nitrate positive shows it reduces nitrate, a common trait in many enterics. The PAD positive result indicates phenylalanine deaminase activity, a characteristic shared by Providencia (and some related genera) that helps distinguish it from other non-lactose fermenters. Citrate positive reveals it can use citrate as a carbon source, another feature aligned with Providencia rettgeri. Indole positive confirms tryptophanase activity, which together with the previous traits narrows the identification to Providencia rettgeri rather than other non-lactose–negative, oxidase–negative organisms.

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