Spirochaetes: General Characteristics, Morphology, Systematic

Spirochaetes is a phylum of bacteria characterized by being gram-negative and possessing a unique cellular ultrastructure. They have internal motility organelles called periplasmic flagella, which allow them to flex, rotate on their longitudinal axis, and move in liquid and semi-solid media.

Spirochaetes is one of the few bacterial phyla whose phenotypic characteristics account for its phylogenetic relationships based on 16S rRNA analysis.

General characteristics 

Some free-living pleomorphic Spirochaetes, such as Spirochaeta coccoides , do not have the ultrastructural and ethological characteristics of the phylum, but sequence analysis of the 16S rRNA gene places them within the Spirochaetaceae family.

They are chemoorganotrophic, they can use carbohydrates , amino acids, long chain fatty acids or long chain fatty alcohols as sources of carbon and energy.

Depending on the species, they can grow under anaerobic, microaerophilic, facultatively anaerobic, or aerobic conditions. Some are free-living and others have a specific association with the host, which can be arthropods, mollusks, and mammals, including humans. Some species are known to be pathogenic.

These bacteria make up a phylogenetically ancient and well-differentiated group, more related to the phylum Bacteoides and Acidobacteria, than with other groups.

It is a phylum formed solely by the class Spirochaetia and the order Spirochaetales, which includes four families that are well delineated: Spirochaetaceae, Brachyspiraceae, Brevinemataceae and Leptospiraceae.

They are elongated and helically wound (corkscrew-shaped), with a size ranging from 0.1 to 3 microns in diameter and 4 to 250 microns in length. They have an outer membrane made up of multiple layers called the cell envelope or outer sheath that completely surrounds the protoplasmic cylinder.

Cells have internal motility organelles called periplasmic flagella. These insert internally at each end of the protoplasmic cylinder and extend throughout most of the cell, overlapping in the central region.

In the case of the Leptospiraceae family, periplasmic flagella do not overlap in cells. The protoplasmic cylinder and flagella are enclosed by an outer sheath that has some characteristics analogous to the outer membrane of gram-negative staining bacteria.

On the other hand, Spirochaeta plicatilis , is a species of large bacteria that has 18 to 20 periplasmic flagella inserted near each end of the protoplasmic cylinder. 


The phylogeny of the phylum Spirochaetes is the result of recent analysis of the sequences of the 16S rRNA gene. Only one class, Spirochaetia, and only one order, Spirochaetales, is recognized on this edge.

The Spirochaetales order comprises four families that are well delineated: Spirochaetaceae, Brachyspiraceae, Brevinemataceae, and Leptospiraceae.


Bacteria in this family are helical cells, 0.1 to 3.0 microns in diameter and 3.5 to 250 microns in length. The cells do not have hooked ends like members of the Leptospiraceae family.

The periplasmic flagella insert internally at each end of the cell and extend for most of the length of the overlapping cell in the central region.

The diamino acid present in peptidoglycan is L-ornithine. They are anaerobic, facultatively anaerobic, or microaerophilic. They are chemo-organotrophic.

They use carbohydrates and / or amino acids as sources of carbon and energy, but they do not use fatty acids or long-chain fatty alcohols.

They are free-living or in association with animals, insects and humans. Some species are pathogenic. The species examined by 16S rRNA sequence analysis are distinct from members of the Brachyspiraceae, Brevinemataceae, and Leptospiraceae families.


This family contains only one genus, Brachyspira . They are helical-shaped bacteria with regular winding patterns. Cells measure between 2 and 11 microns by 0.2 to 0.4 microns.

They are unicellular, but occasional pairs and chains of three or more cells can be seen in growing cultures. Under unfavorable growing conditions, spherical or round bodies are formed.

They have gram-negative staining. They are compulsory anaerobic or aerotolerant. The ends of the cells can be blunt or pointed.

The cells have a typical spirochete cell structure, consisting of an outer sheath, a helical protoplasmic cylinder, and inner flagella in the space between the protoplasmic cylinder and the outer sheath.

Cells of the genus Brachyspira have 8 to 30 flagella, depending on the species. The number of flagella generally correlates with cell size, such that smaller cell species have fewer flagella.

The flagella unite internally, in equal numbers at each end of the cell, wrap around the protoplasmic cylinder, and their free ends overlap in the center of the cells.

It grows between 36 and 42 ° C, with an optimal temperature of 37 to 39 ° C. They are chemoorganotrophic, using various carbohydrates for growth. It has oxidase to reduce molecular oxygen.


This family contains only one genus, Brevinema . The cells are helical in shape and have a diameter of 0.2 to 0.3 microns by a length of 4 to 5 microns, showing one or two helical turns with irregular wavelengths ranging from 2 to 3 microns.

They have sheathed periplasmic flagella that give cells mobility by flexion, rotation and translation. They do not have cytoplasmic tubules. They are microaerophilic, associated with the host.


They are right-handed helical cells, which can measure 0.1 to 0.3 microns in diameter and 3.5 to 20 microns in length. Nonmotile cells have hooked tips, while actively mobile cells have a spiral leading end and a hook at the back end of the cell.

They have a periplasmic flagellum that inserts internally at each end of the cell, but rarely overlaps in the center of the cell. The periplasmic flagella are found along the helical axis.

The diamino acid present in peptidoglycan is an e-diaminopimelic acid. They are obligate or microaerophilic aerobic organisms. They are chemoorganotrophic.

They use long-chain fatty acids and fatty alcohols as carbon and energy sources. They are free-living or in association with animal and human hosts. Some species are pathogenic.


Most species of phylum Spirochaetes are not pathogenic, however some well-known species stand out for their pathogenesis.

Treponema pallidum  (Spirochaetaceae)

It is the organism that causes syphilis. It is a mobile bacterium that is generally acquired through close sexual contact and that penetrates the host’s tissue through the squamous or columnar epithelium. 

The disease is characterized by an area of ​​ulceration and primary inflammation in the genital areas, manifesting in a primary infection. Later stages of this infection are characterized by maculopapular eruptions and a possible granulomatous response involving the central nervous system . 

Other bacteria of the genus can cause non-venereal diseases, such as pinta (also known as blue disease, carate, insteps, lota, pinto disease and tina) produced by  Treponema carateum  or yaws (or buba, yaw, yaws, yaws tropica, polipapilloma tropicum or thymosis) produced by  Treponema pallidum  ssp. belong .

Borrelia burgdorferi  (Spirochaetaceae)

Causes Lyme disease. This species has a unique nucleus that contains a linear chromosome and linear plasmids. The different Borrelia species are transmitted by particular species of ticks of the genus Ornithodoros (Argasidae) in different parts of the world.

These ticks are found in dry savanna and scrub areas, particularly near rodent burrows, caves, woodpiles and dead trees, or in cracks in walls or ceilings and under wooden floors, anywhere inhabited by small rodents. .

The reservoir species are vertebrates such as rats, mice, squirrels, dogs, and birds. Ticks ingest Borrelia sp. by sucking the blood of infected animals or humans.

They feed at night, for at least 30 minutes before returning to their shelters. Infection occurs through the bite, through infected saliva, or through contamination of the mucous membranes with infected coxal fluid.

These bacteria are not excreted in the tick feces. Ticks remain infected for life, even as long as they lack blood for 7 years. They can be transmitted horizontally between males and females; or vertically, by females to their progeny.

In early stages, Lyme disease presents as a distinctive skin lesion called erythema migrans, also called erythema migrans chronicle. The early lesion is characterized by an expansive area of ​​red rash, often with a pale center (bull’s eye) at the site of the tick bite.

If left untreated, erosive arthritis similar to rheumatoid arthritis can occur and eventually chronic progressive encephalitis and encephalomyelitis. Other bacteria in the genus, such as  B. duttoniiB. hermsii,  and B. dugesi , can cause relapsing endemic fever.

Leptospira  spp. (Leptospiraceae)

The causative agent of leptospirosis, a febrile illness that can complicate into aseptic meningitis if left untreated. Symptoms of infection include fever, chills, and headache, with occasional jaundice.

The organisms can spread through animals, water, or soil contaminated by the urine of dogs, rats, or cattle. Animals can remain asymptomatic vectors for years, and organisms can remain viable after shedding for weeks or months.

Acquiring diseases is more common after heavy rains or floods. The incubation period can be up to 1 month.


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