The only gram positive, catalase negative, lactose fermenter, aerobic bacilli, that is H2S positive and shows bottle brush or pipe-cleaner growth in gelatin media?

The only gram positive, catalase negative, lactose fermenter, aerobic bacilli, that is H2S positive and shows bottle brush or pipe-cleaner growth in gelatin media?

• A. Listeria monocytogenes
• B. Corynebacterium diphtheriae
• C. Erysipelothrix rhusiopathiae
• D. Lactobacillus acidophilus

Answer: (C)

Focus on the combination of Gram stain, catalase reaction, hydrogen sulfide production, and the distinctive growth pattern in gelatin. Erysipelothrix rhusiopathiae is a slender Gram-positive rod that is catalase negative and, among the commonly tested organisms, uniquely produces hydrogen sulfide. In gelatin media, it shows a characteristic bottle-brush or pipe-cleaner pattern along the stab line, a visual clue that's often taught for this organism. While some other candidates may be catalase positive or lack H2S production, the long filamentous, brush-like growth in gelatin together with the H2S result points most strongly to Erysipelothrix rhusiopathiae. Note that this organism is not typically a lactose fermenter, which helps differentiate it from other Gram-positive rods listed.

Focus on the combination of Gram stain, catalase reaction, hydrogen sulfide production, and the distinctive growth pattern in gelatin. Erysipelothrix rhusiopathiae is a slender Gram-positive rod that is catalase negative and, among the commonly tested organisms, uniquely produces hydrogen sulfide. In gelatin media, it shows a characteristic bottle-brush or pipe-cleaner pattern along the stab line, a visual clue that's often taught for this organism. While some other candidates may be catalase positive or lack H2S production, the long filamentous, brush-like growth in gelatin together with the H2S result points most strongly to Erysipelothrix rhusiopathiae. Note that this organism is not typically a lactose fermenter, which helps differentiate it from other Gram-positive rods listed.