The term streptobacilli describes a specific arrangement of bacteria, not a single type of organism. It refers to rod-shaped bacteria, known as bacilli, which link together to form chains. This configuration results from their division process, where newly formed cells remain attached end-to-end. This structure is one of several fundamental shapes used to classify and identify different kinds of bacteria in microbiology.
The Streptobacilli Arrangement
The name “streptobacilli” provides a direct clue to its structure, derived from Greek and Latin roots. The prefix “strepto-” comes from the Greek word streptos, which means “twisted chain.” The term “bacilli” originates from the Latin word bacillus, meaning “little rod.” When combined, streptobacilli literally translates to “rods in a chain.”
This chain-like arrangement is just one of several common bacterial morphologies. To better understand it, a comparison to other forms is useful. For instance, some bacilli only form pairs, known as diplobacilli. In contrast, spherical bacteria, or cocci, can create different structures. Streptococci are spherical cells that form chains, similar to streptobacilli but with a different cell shape, while staphylococci divide on multiple planes to form grape-like clusters.
The Genus Streptobacillus
While “streptobacilli” describes a physical arrangement, “Streptobacillus” is the formal scientific name for a specific genus of bacteria. This distinction is important, as many types of bacteria can grow in a streptobacilli formation, but only those belonging to the genus Streptobacillus carry this official designation. These bacteria are characterized as Gram-negative, meaning they do not retain the violet stain used in the Gram staining method of bacterial differentiation. This property helps distinguish them from other bacteria, such as Gram-positive bacilli.
The most medically relevant species within this genus is Streptobacillus moniliformis. This bacterium is a facultative anaerobe, capable of growing in environments with or without oxygen. It is naturally found as a commensal organism in the upper respiratory tracts of rodents, especially rats, meaning it lives in its host without causing disease. S. moniliformis is often pleomorphic, meaning it can exhibit variability in its shape, appearing as both individual rods and long, tangled filaments.
Illnesses Caused by Streptobacilli
The primary human illness associated with the Streptobacillus genus is rat-bite fever. As the name suggests, this infection is most commonly transmitted to humans through the bite or scratch of an infected rodent. The bacteria can also spread through the oral or nasal secretions of rodents.
A person can also become infected by consuming food, milk, or water contaminated with the feces or urine of an infected animal. When rat-bite fever is acquired through ingestion, it is often referred to as Haverhill fever. The symptoms of streptobacillary rat-bite fever appear within 3 to 10 days and include an abrupt onset of fever, chills, headache, muscle pain, and vomiting. A characteristic maculopapular rash often develops on the extremities a few days after the fever begins, and about half of patients experience joint pain, or polyarthritis.
Diagnosis and Treatment
Because the initial symptoms are non-specific, a healthcare provider will rely on a patient’s history of rodent exposure combined with laboratory tests. The most definitive diagnostic method is a blood culture, where a sample of the patient’s blood is placed in a special medium to encourage the bacteria to grow. Fluid from an affected joint may also be cultured.
Once identified, rat-bite fever is effectively treated with antibiotics. Penicillin is the standard treatment, often administered intravenously for several days before transitioning to an oral form. For patients who are allergic to penicillin, other antibiotics such as doxycycline or streptomycin are effective alternatives. With prompt diagnosis and appropriate antibiotic treatment, the prognosis for recovery from rat-bite fever is excellent.