Trevor A. Crowell, M.D.
- 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.
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