9.2 Bite Wounds and Zoonotic Infections
Key Takeaways
- Pasteurella multocida from cat or dog bites is a GNCB that grows on BAP and chocolate, often not MacConkey, and is oxidase positive.
- Eikenella corrodens from human clenched-fist injuries pits agar, has a bleach odor, and is facultative.
- Cutaneous anthrax presents as a black eschar; tularemia as ulceroglandular disease; Brucella as a systemic febrile zoonosis, not typical cellulitis.
- Brucella was removed as a U.S. select agent by the final rule published December 17, 2024 and effective January 16, 2025, but remains highly infectious, reportable, and still requires sentinel rule-out/refer.
- Do not sniff plates; work possible zoonoses in a BSC and refer rather than completing open-bench identification.
9.2 Bite Wounds and Zoonotic Infections
Quick Answer: Cat/dog bites: Pasteurella multocida (GNCB; BAP and chocolate, often not MacConkey; oxidase positive; rapid cellulitis). Human bites: Eikenella corrodens (pits agar, bleach odor, facultative). Cutaneous anthrax is an eschar; tularemia is ulceroglandular; Brucella is systemic, not typical cellulitis. Do not sniff plates. Work possible zoonoses in a BSC. Full sentinel colony rule-out lives in the select-agent chapter. Brucella was removed as a U.S. select agent on December 17, 2024, but remains highly infectious, reportable, and still requires rule-out/refer.
The outline (II.G.3.d–e) splits this topic into bite wounds (Pasteurella multocida, Eikenella corrodens) and zoonotic infections (Bacillus anthracis, Brucella spp., Francisella tularensis, Yersinia spp.). The M exam wants clinical and epidemiologic clues plus first-line Gram stain and colony recognition. Detailed sentinel biochemical algorithms, gamma phage, and referral paperwork belong in the later BSL-3/select-agent chapter. Here, recognize the SST presentation, protect the staff, and know when to stop.
Bite wounds
Bite wounds are mixed by nature (oral flora of the biter plus skin flora of the victim), but two organisms are outline-specific identifiers. Collect tissue or aspirate, not a dry surface swab, and include anaerobic transport because oral anaerobes travel with both animal and human bites.
Pasteurella multocida (cats and dogs)
P. multocida is a small gram-negative coccobacillus. It grows on sheep blood agar and chocolate agar, often as small gray colonies, and often does not grow on MacConkey. It is oxidase positive (also typically catalase positive and indole positive). A musty or mushroom odor is classically described; do not sniff the plate to confirm it.
The clinical clue is rapid cellulitis after a cat or dog bite, often within hours to a day, with pain, swelling, and serosanguineous drainage. Cats inoculate deeper because of narrow teeth. Culture of an abscess aspirate or tissue is far better than a surface swab. Pasteurella is not a MacConkey-growing enteric: if a bite isolate is a fast oxidase-positive GNCB that skipped MacConkey, think Pasteurella before E. coli.
Dogs and cats also inoculate streptococci, staphylococci, Capnocytophaga canimorsus (asplenic fulminant sepsis—know the association even though it is not the outline's named bite pair), and anaerobes. The exam still wants Pasteurella as the named animal-bite pathogen.
Eikenella corrodens (human bite)
E. corrodens is a HACEK-group organism and a classic human bite / clenched-fist (fight-bite) isolate, especially when a fist strikes teeth and inoculates a metacarpophalangeal joint. It is a slender gram-negative rod, facultative (grows aerobically and anaerobically), pits or corrodes the agar, and produces a bleach (hypochlorite) odor. It is oxidase positive, catalase negative, and asaccharolytic, and it usually fails MacConkey. Colonies may be small and easy to overlook under streptococci and staphylococci from the same wound.
Human bite cultures are often mixed with viridans streptococci, S. aureus, Eikenella, Haemophilus, and oral anaerobes (Fusobacterium, Prevotella, anaerobic gram-positive cocci). Recover Eikenella from tissue or joint fluid, not from a dry swab that sat on the bench. Do not confuse the bleach odor of Eikenella with the grape odor taught for P. aeruginosa—and do not sniff either plate as a method.
| Feature | P. multocida | E. corrodens |
|---|---|---|
| Typical exposure | Cat or dog bite/scratch | Human bite, clenched-fist injury |
| Gram stain | GNCB | Slender GNR |
| BAP / chocolate | Grows | Grows (often tiny) |
| MacConkey | Often no growth | Usually no growth |
| Oxidase | Positive | Positive |
| Bench clue | Rapid cellulitis | Pits agar; bleach odor; facultative |
Zoonotic infections: SST clues, then stop and protect
Possible B. anthracis, Brucella, Francisella, and Y. pestis are not routine wound workups. Epidemiologic clues plus a suspicious Gram stain or colony are enough to move the plates into a biosafety cabinet (BSC), stop automated identification and susceptibility testing that could aerosolize the organism, and begin the sentinel rule-out/refer pathway taught in the select-agent chapter. Do not sniff plates. Do not wave plates under the nose for odor.
Bacillus anthracis — cutaneous anthrax eschar
Cutaneous anthrax is the SST form. After inoculation of spores (animal hides, wool, livestock, intentional powder), a painless papule becomes a vesicle and then a black necrotic eschar with striking edema. The patient may not look as toxic as the skin lesion suggests.
B. anthracis is a large boxcar gram-positive rod, often in chains; spores may be seen from culture but are not always obvious in clinical material. Colonies are nonhemolytic and nonmotile, with irregular medusa-head edges and a tenacious beaten-egg-white consistency. Those colony details are confirmed in the select-agent chapter. At the SST bench, a nonhemolytic Bacillus from an eschar plus livestock or powder history is a stop-work event—not a C. perfringens look-alike (which is hemolytic with double zones) and not an environmental Bacillus to identify on the open bench.
Francisella tularensis — ulceroglandular tularemia
Ulceroglandular disease is the SST presentation: a punched-out ulcer at a tick, deer-fly, or rabbit-handling site plus regional lymphadenopathy. Hunters, landscapers, and tick-endemic regions (south-central United States) are classic.
F. tularensis is a tiny, poorly staining gram-negative coccobacillus, fastidious, often requiring cysteine (chocolate, BCYE; poor or no growth on unsupplemented blood agar and usually none on MacConkey). It is a select agent. Slow-growing tiny GNCB from an ulcer-node syndrome worked on the open bench is a classic laboratory exposure. Work in a BSC as soon as the differential includes tularemia. Do not force the isolate onto a routine automated GNR panel.
Brucella spp. — not typical cellulitis
Brucella is a zoonosis of livestock (cattle, goats, swine, dogs depending on species) and unpasteurized dairy. It causes undulant fever, night sweats, arthralgia, and hepatosplenomegaly—a systemic illness. It is not the usual cause of garden-variety cellulitis or a drainable SST abscess. Occupations: veterinarians, slaughterhouse workers, hunters, and laboratorians.
Colonies are tiny gram-negative coccobacilli, slow-growing, oxidase and catalase positive, and urea positive; they can be mistaken for Haemophilus or Moraxella if the history is ignored. Laboratory-acquired brucellosis is the historic cautionary tale: sniffing plates and aerosolizing blood-culture isolates.
Select-agent status versus bench reality (teach both). ASM guidance (August 2025) states that Brucella was removed as a U.S. select agent; the HHS/USDA final rule was published December 17, 2024 and took effect January 16, 2025. It remains highly infectious, reportable, and still requires sentinel laboratory rule-out and refer. The current ASCP M outline still lists Brucella among BSL-3/select-agent examples. Do not treat removed from the select-agent list as permission to work the isolate on the open bench or to skip public-health referral.
Yersinia spp.
Yersinia pestis is the SST-relevant zoonosis: bubonic plague after flea bite or handling infected rodents, rabbits, or prairie dogs (southwestern United States). Patients present with a painful, swollen bubo and systemic toxicity; skin may show a pustule or necrosis at the inoculation site. Gram stain may show bipolar safety-pin gram-negative rods. This is a select-agent rule-out. Y. enterocolitica and Y. pseudotuberculosis are more enteric or mesenteric than SST; do not let a GI Yersinia vignette replace plague when the stem is a zoonotic SST or bubo.
Bench safety rules the exam will test
- History first. Animal bite, eschar, ulcer-plus-node, livestock/dairy fever, or prairie-dog/flea exposure changes the entire workup.
- Do not sniff plates. Textbook odor descriptions are not a method. Aerosolizing Brucella or Francisella is a staff-exposure event.
- Use a BSC for any isolate that might be a zoonotic select-agent pathogen, including Brucella after its delisting.
- Do not place suspicious isolates on automated ID/AST platforms until they are ruled out—those instruments lyse organisms and generate aerosols.
- Refer rather than complete the ID when sentinel criteria are met. Colony-level rule-out tables (motility, hemolysis, gamma phage, urea timing, cysteine dependence) are taught in the later select-agent chapter.
On the exam
A cat bite plus a MacConkey-negative oxidase-positive GNCB is Pasteurella. A fight bite that pits the agar and smells like bleach is Eikenella. A black eschar is cutaneous anthrax. An ulcer with lymphadenopathy after a rabbit or tick is tularemia. A febrile livestock worker is Brucella, not cellulitis. A bubo in the Southwest is Y. pestis. All four zoonoses: BSC, no sniffing, rule-out/refer.
A cat-bite cellulitis isolate is a gram-negative coccobacillus that grows on blood and chocolate agar but not MacConkey and is oxidase positive. The most likely identification is:
Which statement correctly describes Eikenella corrodens recovered from a human clenched-fist injury?
Which statement about Brucella is correct for current U.S. laboratory practice?