An oxidase-positive, gram-negative straight rod associated with gastroenteritis that yields an A/A reaction in TSI is most consistent with which organism?

An oxidase-positive, gram-negative straight rod associated with gastroenteritis that yields an A/A reaction in TSI is most consistent with which organism?

• A. Pseudomonas
• B. Enterobacter
• C. Aeromonas
• D. Plesiomonas

Answer: (C)

An oxidase-positive, gram-negative straight rod that causes gastroenteritis and yields an A/A result on TSI fits Aeromonas. Aeromonas species are environmental Gram-negative rods found in water and seafood and are known to cause gastroenteritis. They are oxidase-positive and ferment carbohydrates, typically giving an A/A (acid/acid) pattern on triple sugar iron, often with gas. This combination helps distinguish them from others: Pseudomonas is also oxidase-positive but usually a nonfermenter (yielding K/A or K/K on TSI) and isn’t a classic enteric pathogen; Enterobacter is a lactose fermenter but oxidase-negative; Plesiomonas can cause similar GI symptoms but is usually curved rather than a straight rod.

An oxidase-positive, gram-negative straight rod that causes gastroenteritis and yields an A/A result on TSI fits Aeromonas. Aeromonas species are environmental Gram-negative rods found in water and seafood and are known to cause gastroenteritis. They are oxidase-positive and ferment carbohydrates, typically giving an A/A (acid/acid) pattern on triple sugar iron, often with gas. This combination helps distinguish them from others: Pseudomonas is also oxidase-positive but usually a nonfermenter (yielding K/A or K/K on TSI) and isn’t a classic enteric pathogen; Enterobacter is a lactose fermenter but oxidase-negative; Plesiomonas can cause similar GI symptoms but is usually curved rather than a straight rod.