Cat Bite - Microbiology

Why Cat Bites Are Especially Dangerous

Microorganisms

1. Pasteurella multocida - THE Key Pathogen

• Present in 70-75% of infected cat bite wounds - the dominant organism
• A normal commensal in the oropharynx of 70% of all cats
• Morphology: Small, gram-negative coccobacillus (facultatively anaerobic, fermentative)
• Gram stain: Shows bipolar staining ("safety pin" appearance)
• Culture characteristics:
  • Grows well on blood agar and chocolate agar
  • Grows poorly on MacConkey agar (key distinguishing feature)
  • After overnight incubation: large, buttery colonies with a characteristic musty odor (due to indole production)
  • Virulence factor: polysaccharide capsule composed of hyaluronic acid
• Clinical behavior: Infections advance rapidly - often within hours - causing severe inflammation, purulent drainage, and lymphadenitis

Fig. Pasteurella multocida in a respiratory specimen (Gram stain)

2. Polymicrobial Flora (Mixed Aerobic + Anaerobic)

3. Special Pathogens - Late/Systemic Consequences

Clinical Infection Profile

• 90% of infected cat bites become symptomatic within 48 hours
• Pasteurella infections: onset within hours - a key clinical clue
• Wound breakdown by type:
  • Cellulitis: 50%
  • Tenosynovitis: 20%
  • Osteomyelitis / Septic arthritis: 15%
  • Abscess: 10%

Antibiotic Susceptibility of P. multocida

Key point: P. multocida is resistant to penicillinase-resistant penicillins and first-generation cephalosporins - drugs that are commonly used for skin infections. This is why amoxicillin-clavulanate is specifically required, not just any beta-lactam. - Goodman & Gilman's Pharmacological Basis of Therapeutics

High-Risk Groups for Systemic Infection

• Age >50
• Diabetes mellitus
• Liver disease / alcoholism
• Immunosuppression (corticosteroids, HIV)
• Chronic renal insufficiency
• Peripheral vascular disease

• Parallel tool calls kept it efficient
• Pulled from 5+ authoritative textbooks simultaneously
• Included the actual microscopy image of P. multocida
• Tables made the polymicrobial flora, susceptibility, and clinical data scannable
• Covered the full spectrum - immediate wound flora, late consequences, and systemic risks
• The antibiotic resistance point (first-gen cephalosporins fail) is a high-yield clinical pearl that's easy to miss

• The Bartonella henselae section could have gone deeper - inoculation papule, incubation period, lymphadenopathy timeline, Warthin-Starry stain, distinction between immunocompetent vs. immunocompromised presentation
• Could have included a summary mnemonic or high-yield exam table at the end
• Tularemia and sporotrichosis were listed but not elaborated - a student might want more on those
• No explicit mention of Capnocytophaga canimorsus (more dog than cat, but relevant in bite wound context)
• The "one-third are aerobic only" stat from Rosen's was buried and deserved highlighting

Rat Bite - Microbiology

Overview

The Two Causative Organisms of Rat Bite Fever

Streptobacillus moniliformis - Detailed Microbiology

• Long, thin gram-negative rod (0.1-0.5 × 1-5 μm)
• Stains poorly and is highly pleomorphic
• Characteristic bulbous swellings ("string of beads" appearance - hence moniliformis = necklace-shaped)
• Extremely long filaments (up to 150 μm) with fusiform swellings and large round bodies in older cultures
• Name origin: Latin monile = necklace

Fig. Gram stain of S. moniliformis - note the pleomorphic forms and bulbous swellings (Medical Microbiology 9e)

• Fastidious - requires enriched media
• Media must be supplemented with: 15% blood, 20% horse/calf serum, or 5% ascitic fluid
• IMPORTANT: Growth is inhibited by sodium polyanethol sulfonate (SPS) - the anticoagulant present in most commercial blood culture bottles. Lab must be informed to use SPS-free bottles
• Broth growth: characteristic "puffball" colonies (or "bread crumb" clumps)
• Agar colonies: small, round; cell-wall-defective L-form variants grow as "fried egg" colonies (heaped center with spreading edges)
• Slow growth: minimum 3 days to isolate
• Identification: biochemically relatively inert; most reliable method is 16S rRNA gene sequencing
• Microaerophilic

Spirillum minus - Detailed Microbiology

• Gram-negative spirochete with 2-5 tight coils
• Cannot be cultured in vitro - key distinguishing feature
• Diagnosis by: darkfield microscopy of blood/exudate, or animal inoculation (mice/guinea pigs)
• Serology: false-positive VDRL (syphilis test) common

Clinical Disease - Streptobacillary RBF

1. Irregular fever (relapsing pattern), chills, headache, myalgia
2. Migratory polyarthralgia / polyarthritis - large joints, asymmetric, resembles rheumatoid arthritis but rarely suppurative; can involve liver, kidneys, meninges, heart valves
3. Maculopapular or petechial rash - palms, soles, acral - appears 2-3 days after fever onset

• Bite site heals before systemic illness - patients may not recall the bite
• Rash can show desquamation of palms and soles ~1 week after resolution (mimics Kawasaki disease)
• Hemorrhagic acral vesicles can resemble leukocytoclastic vasculitis or gonococcemia
• Untreated mortality: ~10%

Other Pathogens Transmissible via Rat Bite

Differential Diagnosis of RBF

• Rocky Mountain Spotted Fever
• Lyme disease
• Meningococcemia / Disseminated gonococcemia
• Secondary syphilis
• Leptospirosis
• Acute rheumatoid arthritis / Acute rheumatic fever
• Endocarditis
• Viral exanthems (Coxsackievirus)
• Ehrlichiosis/Rickettsioses
• Pasteurella multocida infection

Treatment

• Tetanus immunization status must be checked
• Always cleanse wound promptly and thoroughly
• Rat bites: low risk for classic wound infection; main concern is systemic RBF