Streptobacillus moniliformis and Spirillum minus



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.




Cultivation


As previously mentioned, S. moniliformis requires the presence of blood, ascitic fluid, or serum for growth. Growth occurs on blood agar, incubated in a very moist environment with 5% to 10% carbon dioxide (CO2), usually after 48 hours of incubation at 37°C. Colonies are nonhemolytic. The addition of 10% to 30% ascitic fluid (available commercially from some media suppliers) or 20% horse serum should facilitate recovery of the organism. In broth cultures, the organism grows as “fluff balls” or “bread crumbs” near the bottom of the tube of broth or on the surface of the sedimented red blood cell layer in blood culture media. Colonies grown on brain-heart infusion agar supplemented with 20% horse serum are small, smooth, glistening, colorless or grayish and have irregular edges.


Colonies are embedded in the agar and may also have a “fried egg” appearance, with a dark center and a flattened, lacy edge. These colonies are also referred to as L-phase colonies because they have undergone spontaneous transformation to the L form. Staining of L-form colonies yields coccobacillary or bipolar-staining coccoid forms (usually a special stain, such as the Dienes stain performed by pathologists), is required. Acridine orange stain also reveals the bacteria when Gram stain fails because of lack of cell wall constituents.


As previously stated, Gram-stained organisms from standard colonies show extreme pleomorphism, with long, looped, filamentous forms, chains, and swollen cells. The club-shaped cells can be 2 to 5 times the diameter of the filament. Carbolfuchsin counterstain or Giemsa stain may be necessary for visualization (see Figure 39-1).

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Aug 25, 2016 | Posted by in MICROBIOLOGY | Comments Off on Streptobacillus moniliformis and Spirillum minus

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