SPIRILOS: CLASSIFICATION AND MORPHOLOGY - BIOLOGY - 2024

The espirilos, strictly speaking, are Gram negative bacteria in a spiral. The simplest can be found in aquatic environments, particularly in stagnant waters, although they can also be found in unhealthy places. As they require little oxygen to live, they are said to be microaerophilic bacteria.

In general, there are three main types of bacterial morphologies: that of bacilli, that of cocci and that of spiral bacteria. This does not mean, however, that all spiral bacteria are spirilli.

Spirillum. Wolframm Adlassnig's work. Wikimmedia commons.

In fact, this section will be dedicated to providing information on the little-known spirillae (strictly speaking) of this bacterial genus. We will also present others a little better known that belong to genres with a compound name.

From a classificatory point of view, although there are many bacteria with spiral morphology, the only spirillae are those belonging to the genus Spirillum or to other genera whose name uses the suffix -spirillum. This means that another spiral bacterium, but which is not a spiril, and will not be covered here, is Helicobacter pylori.

Spirilos should not be confused with spirochetes either. Although they may present morphological similarities, they belong to different types of bacteria. Spirochetes include human pathogenic bacteria, such as Treponema pallidum, the causative agent of syphilis.

Classification

Spirils are not a natural group that serves as the basis for a rational classification of bacteria. Rather, they allude to a form that has long fascinated microbiologists.

Many different bacteria of different phylogenetic clades are spiral in shape (loose sense). Here we will only talk a little about those that by nomenclature, and obviously by form, are called spirilla in the strict sense. They look like corkscrews, and other bacteria that look like them but are not the same, are helical.

Within the genus of the simplest spirilla, Spirillum, at least four species have been recognized: S. winogradskyi, S. volutans, S. pleomorphum and S. kriegii.

Other candidates, usually isolated from environmental samples, await confirmation. Some of them are believed to be used as plant growth promoters, and in detoxification of iron-contaminated soil.

Bacteria in the genus belong to the Spirillaceae family, and it is the only genus that makes up the family. The spirilli in this group are betaproteobacteria.

Other proteobacteria that include spirilli are those belonging to the Rhodospirillaceae family. In this family we find the non-sulfurous purple bacteria. This group of alphaproteobacteria includes the magnetobacteria of the genus Magnetospirillum. This group also includes nitrogen-fixing bacteria of the genus Azospirillum.

Finally, it must be remembered again that there are other bacteria with helical morphology - but that biologically speaking they are not spirils. Spirochetes, for example, even belong to a different phylum (Spirochaetes) from that of spirilli (Proteobacteria).

Although they are also proteobacteria (not alpha, nor beta), those of the Helicobacter genus are spiral bacteria of the Helicobacteraceae family.

Morphology

Spirils are among the largest known bacteria. They are elongated and show a spiral design due to their helical morphological structure.

Many of these bacteria also have a set of flagella at both ends. Thanks to them these bacteria can experience rotational movement, and move at high speed.

They can reach a length of 60 microns, and a variable diameter between 1.4 and 1.7 microns. Each helical turn, as if it were the movement of a screw, can include 1 to 5 turns at a time.

The spiral shape is genetically determined, and in many cases depends on the manifestation of a single gene. In the case of many spiral bacteria with a pathogenic lifestyle, the spiral shape is critical for virulence and pathogenicity.

For strict spirillae, and others with a similar shape, the loss of shape does not seem to affect the ability to survive and adapt.

Some spirils and environmental importance

Magnetospirilli, like those belonging to the genus Magnetospirillum, have a particularity that they share with a few other Gram-negative ones: they are magnetotactic.

This means that they can orient themselves in a magnetic field: they passively align and actively swim along the magnetic field. This orientation is achieved by the presence of an intracellular structure called a magnetosome.

This type of bacteria, and their magnetosomes, constitute an irreplaceable natural nanomaterial for the generation of multiple applications in industry, science and technology.

There are other spirils, for example of the Rhodospirillum and Azospirillum genera, that promote plant growth or intervene in the fixation of atmospheric nitrogen.

They are, undoubtedly, the biological key in the cycling of this fundamental element of the planet. Bacteria of this genus also confer tolerance or resistance to biotic or abiotic stress.

Are environmental spirilli medically important?

At least one species of Spirillum can infect humans through contact with physiological residues of rodents that carry the bacteria. It can cause a disease known as rat-bite fever. Treatment usually includes the use of beta-lactam antibiotics.

Other non-spiral bacteria, as already mentioned, are also important pathogens. Within rhodospirillae, however, we found that some of the genera of the Rhodospirillaceae family have been reported to include bacteria that are opportunistic pathogens in humans.

That is, they are not strict pathogens whose way of life requires parasitizing other living beings. In particular circumstances, however, they can do so and cause illness. Generally, these circumstances include depression of the immune system of the affected person.

References

1. Fukami, J., Cerezini, P., Hungria, M. / (2018) Azospirillum: benefits that go far beyond biological nitrogen fixation. AMB Express, 8:73.
2. Krieg, NR, Hylemon, PB (1971) The taxonomy of the chemoheterotrophic spirilla. Annual Review of Microbiology, 30: 303-325.
3. Lee, A. (1991) Spiral organisms: what are they? A microbiologic introduction to Helicobacter pylori. Scandinavian Journal of Gastroenterology Supplement, 187: 9-22.
4. Mathuriya, AS (2016) Magnetotactic bacteria: nanodrivers of the future. Critical Reviews in Biotechnology, 36: 788-802.
5. Ojukwu, IC, Christy, C. (2002) Rat-bite fever in children: case report and review. Scandinavian Journal of Infectious Diseases, 34: 474-477.
6. Vargas, G., Cypriano, J., Correa, T., Leão, P., Bazylinski, DA, Abreu, F. (2018) Applications of magnetotactic bacteria, magnetosomes and magnetosome crystals in biotechnology and nanotechnology: Mini-Review. Molecules, 23. doi: 10.3390 / molecules23102438.