HACEK-like and Other Fastidious Gram-Negative Rods

General Characteristics (HACEK-like)
  • Gram-negative rods, often coccobacillary.
  • Fastidious, do not grow on MAC.
  • Often normal oropharyngeal flora.
  • Associated with endocarditis.
  • Fermentative or non-fermentative (most ferment glucose, exception: Eikenella corrodens).
  • Non-motile.
  • Catalase and oxidase: Variable.
Capnocytophaga Species
  • Normal oral flora (human and animal, especially dogs).
  • Associated with bacteremia, periodontal disease, and endocarditis.
  • Can cause serious complications (e.g., heart issues) in specific demographics (e.g., middle-aged men with spleen injury) following dog bites.
  • Morphology: Thin, fusiform (pointy-ended) Gram-negative rods.
  • Facultative anaerobes.
  • Gliding motility: Important colonial descriptor on agar (appears melting/spreading, not true flagellar motility).
  • Non-hemolytic.
  • Oxidase and catalase negative.
  • Weakly ferment glucose and maltose.
  • Resistant to trimethoprim and aminoglycosides.
Pasteurella multocida
  • Most commonly isolated species: P. multocida (subspecies: multocida, septica, gallicida). 55 serogroups (A, B, D, E, F).
  • Normal oral flora of cats and dogs.
  • Associated with cat/dog bites and scratches, primarily causing cellulitis.
  • Can grow on BAP and Chocolate agar; no growth on MAC.
  • Gram-negative coccobacillary, pleomorphic.
  • Key descriptor: Bipolar staining (safety-pin appearance).
  • Facultative anaerobe, non-motile.
  • Most are catalase and oxidase positive.
  • Weakly ferment glucose, nitrate positive, indole positive, ODC positive.
  • Urease positive (for BOC purposes, though variable in reality).
Brucella Species
  • Causes brucellosis, most famous manifestation: undulant fever.
  • Zoonotic infection (contact with animals).
    • B.B. melitensis: Goats, sheep (most common).
    • B.B. abortus: Cows.
    • B.B. canis: Dogs.
    • B.B. suis: Pigs.
  • Gram-negative coccobacillary, facultative intracellular pathogens.
  • CDC reportable, possible bioterrorism agent (Category C, Biosafety Level 33).
  • Clinical Phases:
    • Acute: Non-specific, flu-like symptoms.
    • Subchronic: Undulating fevers (episodic, often nocturnal), arthralgia, epididymitis-orchitis in males.
    • Chronic: Chronic depression, chronic fatigue syndrome, internal organ lesions (lymph nodes, spleen, liver, bone).
  • Transmission: Occupational exposure (vets, ranchers), ingestion of contaminated milk/meat (unpasteurized), cutaneous inoculation (animal wastes/hides).
  • Known to cause spontaneous abortion in pregnant women.
  • Obligate aerobes, non-hemolytic, non-motile.
  • Oxidase, catalase, nitrate, and urease positive (urease incubation time can differentiate species).
  • Requires BSL-33 lab for workup; highly infectious; readily aerosolized during processing.
Francisella tularensis
  • Causes tularemia.
  • Zoonotic infection.
  • Type A (more virulent): Associated with rabbits, sheep, ticks.
  • Type B (less virulent): Associated with mosquitoes, rodents.
  • Gram-negative coccobacillus, facultative intracellular parasite.
  • Highly infectious, Type 33 biohazard (Category C bioterrorism agent).
  • Transmission: Aerosols (e.g., mowing over rabbits), animal/insect bites, handling infected animals, contaminated food or water.
  • Most dangerous form: Pneumonic (inhalation).
  • Strict aerobe, non-motile.
  • Fastidious: Requires cysteine, cystine, or thiosulfate for growth.
  • Oxidase negative, weak positive catalase, urease negative.
  • Grows on glucose-cysteine agar, Chocolate agar, BCYE. Growth visible in 24−4824-48 hours.
  • Workup typically requires BSL-33 lab; sent to reference labs for ID.
  • Diagnosis: Direct fluorescent antibody, immunohistochemical stains, PCR, molecular methods.