STREPTOBACILLI: CHARACTERISTICS, MORPHOLOGY, SPECIES, DISEASES - BIOLOGY - 2025

The estreptobacilos are a genus of bacteria which are characterized by rod - shaped and found associated forming chains. It was described for the first time in 1925 by the Romanian microbiologist Constantin Levaditi and is made up of 5 species. Of these the most studied is Streptobacillus moniliformis.

Some of the bacteria that make up the genus can be pathogenic for humans. Such is the case of the already mentioned Streptobacillus moniliformis and Streptobacillus notomytis.

Streptobacillu moniliformis seen under the microscope. Source: Iliana01117392

Taxonomy

The taxonomic classification of streptobacilli is as follows:

• Domain: Bacteria
• Kingdom: Monera
• Phylum: Fusobacteria
• Order: Fusobacteriales
• Family: Leptotrichiaceae
• Genus: Streptobacillus

Morphology

Bacteria of the genus Streptobacillus are rod-shaped, which can be found alone or in long, wavy filaments. They are approximately 0.1 to 0.7 microns wide and 1.0 to 5 microns long. Cells can have rounded or pointed ends.

It has been observed through the microscope that some cells have a bulge in the central area, so that sometimes, the long chains of bacterial cells look like "chains of beads", like a pearl necklace.

Likewise, bacterial cells do not present a protective capsule and neither do they produce spores to protect themselves when environmental conditions become hostile.

When grown in the laboratory, it develops colonies that are small, circular in shape and greyish in color. They also have a smooth and shiny appearance. Similarly, colonies have been observed that manifest the classic "fried egg" shape, whose dense center penetrates the agar.

Importantly, the appearance of colonies is also highly dependent on the culture medium. For example, on serum agar, they are approximately 1 to 2 millimeters long and develop within 3 days. While those that are cultured in serum broth present a white sediment at the bottom and on both sides of the tubes.

characteristics

They are gram negative

Bacteria of the genus Streptobacillus belong to the group of gram negative ones. When subjected to Gram staining, they adopt a fuchsia color, which means that in their cell wall they do not retain particles of the Gram stain.

Habitat

From a geographical point of view, the genus Streptobacillus is widely distributed throughout the planet.

Depending on the species they will be found in different habitats. For example Streptobacillus moniliformis is found in the oropharynx of some rodents, Streptobacillus hongongnensis is believed to be a member of the human pharyngeal microbiota, and Streptobacillus notomytis is also present in rodents such as rats.

Biochemistry

From a biochemical point of view, bacteria of this genus are:

-Catalase negative: meaning that they are not capable of splitting hydrogen peroxide molecules, because they do not synthesize the enzyme catalase.

-Indol negative: they cannot degrade the amino acid tryptophan to obtain indole, since they do not produce tryptophanase enzymes.

-Negative urease: these bacteria do not hydrolyze urea, due to their inability to synthesize the enzyme urease.

-It does not reduce nitrates to nitrites: this is because they do not synthesize the enzyme nitrate reductase.

They are pathogenic

Some of the species of this genus are considered pathogenic for humans. Of all of them, the one that has been most studied is Streptobacillus moniliformis. This is responsible for rat bite fever in humans. Also Streptobacillus notomytis is responsible for a small percentage of cases.

Main species

The genus Streptobacillus includes a total of 5 species, of which the best known and studied is Streptobacillus moniliformis.

Streptobacillus moniliformis

It is a gram negative bacterium that is found mainly as part of the pharynx microbiota of rodents such as rats. Measures approximately 0.5 microns wide ¡up to 5 microns long.

Likewise, they tend to form chains that look like a necklace. In addition, certain inflammation or lateral bumps that are characteristic of it can be observed frequently. Similarly, Streptobacillus moniliformis can present in two forms: the most frequent, which is bacillary; and in the form of L. The latter is considered non-pathogenic.

It develops adequately under average temperatures between 30 ° C and 37 ° C, taking an average of 3 days for the first colonies to appear. The ideal culture medium for this bacterium to grow is trypticase soy agar, which must be enriched with bovine serum (20%), ascites fluid (5%) and blood (15%).

It is a known human pathogen, which is acquired through the bite of rodents. It causes a disease in humans known as Haverhill fever or rat-bite fever.

Other species

The other species of this genus are not as well known and are also not of great importance from a medical point of view. These are:

-Streptobacillus felis: its characteristics are very similar to those of Streptobacillus moniliformis. It has been isolated from cats suffering from pneumonia.

-Streptobacillus hongkongensis: it owes its name to the fact that it was isolated for the first time in the city of Hong Kong. It has been isolated in patients with septic arthritis. Likewise, it has come to be considered as an inhabitant of the human oropharynx. However, it is very little known.

-Streptobacillus notomytis: bacteria frequently present in mice. It is responsible for a small percentage of rat or mouse bite fever in humans.

-Streptobacillus ratti: bacterium that has been isolated directly from specimens of black rats. It has also been little studied.

Diseases

The main disease caused by bacteria of the genus Streptobacillus is rat-bite fever or Haverhill fever.

-Rat bite fever

Two causative agents of this disease have been established: Streptobacillus moniliformis and Streptobacillus notomytis.

It is a disease caused by the transmission of some of these bacteria through direct contact with rodents. As its name indicates, it is caused by the bite of a rodent, although cases have also been described by contact with feces or saliva of the carrier animal.

People who work in research laboratories in which these types of animals are used constitute a risk group for this disease.

Streptobacillus moniliformis is found in the oropharynx of rodents. Source: Reg Mckenna

Symptoms

Bites generally tend to heal quickly. However, this is not synonymous with the fact that the bacteria have not entered the body. It is important to note that the disease has an incubation period of approximately 2 to 20 days. In these the affected will not present symptoms. Once this period is over, the symptoms that may appear are the following:

• High fever accompanied by chills
• Pain in muscles and joints
• Digestive disorders such as: vomiting and diarrhea
• Skin problems such as a rash on the arms and legs

As in any bacterial infection, if it is not treated in time, the bacteria can pass into the bloodstream, causing bacteremia that can even put the patient's life at risk, since it can affect organs of great importance such as the heart and brain.

Treatment

Since it is an infection whose causative agent is a bacterium, the ideal treatment is an antibiotic regimen with an average duration of between 7 and 10 days. It all depends on the doctor's judgment.

The most commonly used antibiotics are penicillin and amoxicillin. Erythromycin or doxycycline can be used in allergic patients.

References

1. Eisemberg, T., Nicklas, W., Mauder, N., Rau, J., Contzen, M., Semmler, T., Hofmann, N., Aledelbi, K. and Ewers, C. (2015). Phenotypic and Genotypic Characteristics of Members of the Genus Streptobacillus. Plos One 10 (8).
2. Elliot, S. (2007). Rat bite fever and streptobacillus moniliformis. Clinical microbiological reviews. 20 (1) 13-22
3. Fordham JN, McKay-Ferguson E, Davis A, Blyth T. (1992) Rat bite fever without the bite. Ann Rheum Dis.51: 411-2
4. Guzmán, L. (1997). Streptobacillus moniliformis (rat bite fever). Antimicrobe.
5. Jawetz, E., Melnick, L. and Adelberg, A. (1981) Medical Microbiology.
6. Martínez, M., Valenzuela, M. and Pietrantoni, D. (2011). Streptobacillus moniliformis. Chilean journal of infectology. 28 (1) 57-58.