A 35 year old man comes to the clinic with a skin lesion following a cat bite 1 day back. On examination he was found to have cellulitis of the affected area. Which of the following is the most likely causative organism?
First, I remember that cat bites can lead to infections because the cat's mouth has various bacteria. Common pathogens associated with animal bites include Pasteurella, Staphylococcus, and Streptococcus. But which one is most likely?
Pasteurella multocida is a gram-negative coccobacillus commonly found in the oral flora of cats and dogs. It's known to cause rapid-onset cellulitis after a bite. The infection usually presents within 24-48 hours, which matches the patient's timeline here. So that's a strong candidate.
Other options might include Staphylococcus aureus or Streptococcus pyogenes, which are more common in general skin infections. However, in the context of a cat bite, the primary suspect should be Pasteurella. Also, anaerobes like Bacteroides might be present in mixed infections, but they're less likely to be the sole pathogen here.
Wait, the question is about the most likely causative organism. So the answer should be Pasteurella multocida. Let me confirm the key points: it's gram-negative, causes rapid cellulitis after cat/dog bites, and often requires antibiotic treatment. The other options are either less common in this context or not primary pathogens for cat bites.
**Core Concept** The question assesses knowledge of pathogens implicated in infections following animal bites, focusing on **Pasteurella multocida**, a gram-negative coccobacillus commonly found in the oral flora of cats and dogs.
**Why the Correct Answer is Right** *Pasteurella multocida* is the most likely causative organism in this scenario. It causes rapid-onset cellulitis (within 24–48 hours) after cat or dog bites, characterized by erythema, swelling, and pain. The organism’s lipopolysaccharide endotoxin and proteolytic enzymes contribute to local tissue destruction and inflammation. Prompt treatment with beta-lactam antibiotics (e.g., amoxicillin-clavulanate) is critical to prevent complications like abscess formation or septic arthritis.
**Why Each Wrong Option is Incorrect**
**Option A:** *Staphylococcus aureus* is a common skin pathogen but not the primary cause of cat-bite cellulitis.
**Option B:** *Streptococcus pyogenes* may cause cellulitis but is less specific to animal bites compared to *Pasteurella*.
**Option C:** *Bacteroides fragilis* is an anaerobe associated with mixed infections, not typical for isolated cat-bite cellulitis.
**Clinical Pearl / High-Yield Fact** Remember **“Pasteurella = cat/dog bite”** for NEET/USMLE exams. Differentiate it from *Capnocytophaga canimorsus* (dog bites) and *Eikenella corrodens* (human bites). Always consider mixed infections in animal bites and tailor antibiotics accordingly.
**Correct Answer: C. *Pasteurella multocida***