Streptobacillus moniliformis and Spirillum minus

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Streptobacillus moniliformis and Spirillum minus

Streptobacillus moniliformis is a gram-negative bacillus that requires media containing blood, serum, or ascites fluid as well as incubation under carbon dioxide (CO2) for isolation from clinical specimens. This organism causes rat-bite fever and Haverhill fever in humans. Spirillum minus has never been grown in culture but, because both are causative agents of rat-bite fever, these organisms are considered in this chapter.

Streptobacillus Moniliformis

General Characteristics

The genus Streptobacillus is a member of the Fusobacteriaceae family. The Streptobacillus genus has only one species, S. moniliformis, a facultative, nonmotile anaerobe that tends to be highly pleomorphic.

Epidemiology and Pathogenesis

The natural habitat of S. moniliformis is the upper respiratory tract (nasopharynx, larynx, upper trachea, and middle ear) of wild and laboratory rats (mice, gerbils, squirrels, ferrets, weasels); in addition, this organism occasionally has been isolated from other animals, such as cats and dogs that have fed on rodents. S. moniliformis is pathogenic for humans and is transmitted by two routes:

The incidence of S. moniliformis infections is unknown, but human infections appear to occur worldwide.

The pathogenic mechanisms of S. moniliformis are unknown. The organism is known to spontaneously develop L forms (bacteria without cell walls), which may allow its persistence in some sites.

Laboratory Diagnosis

Specimen Collection, Transport, and Processing

Unfortunately, the diagnosis of rat-bite fever caused by S. moniliformis is often delayed because of lack of exposure history, an atypical clinical presentation, and the unusual microbiologic characteristics of the organism. Organisms may be cultured from blood or aspirates from infected joints, lymph nodes, or lesions. No special requirements have been established for the collection, transport, and processing of these specimens except for blood. Because recovering S. moniliformis from blood cultures is impeded by concentrations of sodium polyanethol sulfonate (SPS) used in blood culture bottles, an alternative to most commercially available bottles must be used. After collection by routine procedures (described in Chapter 68), blood and joint fluids are mixed with equal volumes of 2.5% citrate to prevent clotting and are then inoculated to brain-heart infusion cysteine broth supplemented with heated horse serum and yeast extract, commercially available fastidious anaerobe broth without SPS, or thiol broth.

Direct Detection Methods

Pus or exudates should be smeared, stained with Gram or Giemsa stain, and examined microscopically (Figure 39-1). S. moniliformis is a pleomorphic, gram-negative rod. Cells may appear straight of variable size or as long tangled chains and filaments with bulbar swellings. The cells may also appear spiral shaped and resemble a string of pearls. Direct detection of the 16sRNA gene sequence for S. moniliformis using polymerase chain reaction (PCF) analysis has been described.