MICROBIOLOGY

• Pleomorphic, nonencapsulated, branching Gram-negative bacillus [Fig 1].
  • Stains irregularly and, given its highly pleomorphic nature, may be mistaken for other Gram-positive rods.
    • It may be straight or fusiform, occasionally in chains or clumps.
  • It grows slowly, so the lab should be notified to hold cultures if there is clinical suspicion for this pathogen.
  • Can be identified by a characteristic fatty acid profile on gas chromatography.
• Normal commensal of rodent oropharynx, even healthy pets and laboratory rats.
  • The risk of infection after a rat bite may be as high as 10%.
  • It may also be transmitted to humans by bite/scratch from mice, squirrels, cats, dogs, or pigs.
• A significant cause of rat bite fever (RBF; most common cause in the U.S.)
  • Spirillum minus is also a cause, primarily in Asia (spirillary RBF, also known as sodoku); it cannot be cultured on synthetic media; short, thick, tightly coiled spiral may be seen with Giemsa stain, Wright stain, or dark-field microscopy.
  • Streptobacillus notomytis also is a cause rarely reported.