[5] In contrast, the spontaneously occurring L-form, which lacks a cell wall and whose colonies grow in a "fried egg" formation, is non-pathogenic.

[5] The bacterium S. moniliformis is a gram-negative pleomorphic rod occurring frequently in chains and tangled filaments with bulbous or Monilia-like swellings.

The organism presents phenotypically as being facultatively anaerobic, non-motile, weakly ferments glucose and maltose, is catalase and oxidase-negative, does not reduce nitrate, and exhibits no growth on MacConkey agar.

Morphologically, colonies of S. moniliformis are 1–2 mm in size, smooth, convex, non-hemolytic (varied presentation with an α-hemolysis is not uncommon) and gray in color.

[3] The fastidious nature of Streptobacillus moniliformis makes it difficult to culture, with current published recommendations stating it requires media supplemented with 20% serum, ascitic fluid, or whole blood in order to grow.

Numerous published reports also state that the organism is inhibited by the polyanionic detergent sodium polyanethole sulfonate (SPS-trade name; Liquoid; Hoffman-La Roche, Inc., Nutley, N.J.), the main anticoagulant in modern commercially available blood culture bottles, used in automated continuous blood culture instruments.

Bottles should be incubated minimally for 7 days to ensure the best chance for organism recovery when working with low levels of bacteremia.

Automated continuous blood culturing instrumentation growth and fluorescent detection data should be checked manually during the course of incubation in order to rule out missed false-negative samples.

According to Szewc et al., their research into the "reported" fastidious nature of S. moniliformis, and its inhibition by the anticoagulant SPS showed, that when using a specific volume of blood for inoculum, it resulted in 100% recovery and successful growth of this organism and appeared to overcome the inhibitions and limitations that historically have been observed when using SPS for blood culturing and recovering S. moniliformis in a clinical setting.

[14] However, approximately 30% of patients diagnosed with rat bite fever do not recall being scratched or bitten by an infected animal.

[18] Infected individuals described symptoms including a sudden development of vomiting, severe headache, and cold sweats with a high fever.

[20] Lab personnel and pet store workers, who work closely with animals on a daily basis, also have an increased risk of infection.