A gram negative bacillus with bipolar staining isolated from a wound infection caused by a bite from a pet cat. The following characteristic reactions were seen: oxidase +; OF test (O+/F+); nonmotile; no growth on MAC

A gram negative bacillus with bipolar staining isolated from a wound infection caused by a bite from a pet cat. The following characteristic reactions were seen: oxidase +; OF test (O+/F+); nonmotile; no growth on MAC

• A. Pseudomonas aeruginosa
• B. Vibrio cholerae
• C. Aeromonas hydrophila
• D. Pasteurella multocida

Answer: (D)

In cat bite wound infections, Pasteurella multocida is the classic culprit. It is a small gram-negative coccobacillus that often shows bipolar (safety-pin) staining, which helps distinguish it under the microscope. Lab features fit this organism well: it is oxidase positive and nonmotile, and it typically does not grow on MacConkey agar because it is fastidious and prefers enriched media like blood or chocolate agar. The OF test result showing both oxidative and fermentative metabolism aligns with Pasteurella’s ability to metabolize carbohydrates in aerobic conditions and ferment them as well, reflecting its flexible energy use without lactose fermentation. These traits help separate it from other organisms listed: Pseudomonas aeruginosa is motile and generally grows on MacConkey; Aeromonas and Vibrio are also motile and have different ecological associations and growth patterns. The combination of the cat bite context, bipolar gram-negative coccobacilli, oxidase positivity, nonmotility, and no growth on MacConkey strongly points to Pasteurella multocida.

In cat bite wound infections, Pasteurella multocida is the classic culprit. It is a small gram-negative coccobacillus that often shows bipolar (safety-pin) staining, which helps distinguish it under the microscope. Lab features fit this organism well: it is oxidase positive and nonmotile, and it typically does not grow on MacConkey agar because it is fastidious and prefers enriched media like blood or chocolate agar. The OF test result showing both oxidative and fermentative metabolism aligns with Pasteurella’s ability to metabolize carbohydrates in aerobic conditions and ferment them as well, reflecting its flexible energy use without lactose fermentation.

These traits help separate it from other organisms listed: Pseudomonas aeruginosa is motile and generally grows on MacConkey; Aeromonas and Vibrio are also motile and have different ecological associations and growth patterns. The combination of the cat bite context, bipolar gram-negative coccobacilli, oxidase positivity, nonmotility, and no growth on MacConkey strongly points to Pasteurella multocida.