PASTEURELLA: CHARACTERISTICS, HABITAT, GROWING MEDIA, SPECIES - BIOLOGY - 2025

Pasteurella is a genus of gram negative bacteria found primarily in a wide variety of animals such as pigs, dogs, and cats. It was first described by the Italian botanist Vittorio Trevisan. Similarly, it is made up of a total of 21 species, the best known being Pasteurella multocida.

Likewise, these bacteria have certain characteristics that allow them to be identified at the laboratory level; besides being easily cultivable in blood agar and chocolate agar. In the hosts they parasitize, they are capable of triggering certain pathologies such as avian cholera.

Cats are common hosts for bacteria of the genus Pasteurella. Source: Pixabay

Taxonomy

The taxonomic classification of Pasteurella is as follows:

• Domain: Bacteria
• Kingdom: Monera
• Phylum: Proteobacteria
• Class: Gammaproteobacteria
• Order: Pasteurellales
• Family: Pasteurellaceae
• Genus: Pasteurella.

Morphology

Bacteria of the genus Pasteurella are pleomorphic, which means that they can occur in various forms. It is often rod or rod-shaped, as well as coccobacilli. They are approximately 0.3-1.0 microns in diameter by 1.0-2.0 microns long.

Likewise, some species such as Pasteurrella multocida have a capsule around their cells that is highly resistant and that helps protect them from desiccation and dehydration.

Similarly, scientists believe that the capsule is closely related to the virulence of these strains. Bacteria of the genus Pasteurella do not develop spores, which makes it difficult for them to survive in hostile environmental conditions.

Colonies of large size are observed in the cultures, with a greasy appearance that sometimes give off a characteristic musty odor. The colonies are shiny and round, smooth and convex. They are not transparent.

characteristics

It is gram negative

Bacteria of the genus Pasteurella acquire a fuchsia coloration when subjected to the gram staining process. This is because they are not able to retain particles of the dye in their cell wall.

Conditions required for cultivation

When grown in the laboratory, they require strict care measures. The ideal temperature at which they should be maintained is 35 ° C and rigorous antiseptic measures, as well as an anaerobic atmosphere. After 48 hours under these conditions, colonies begin to be seen in the culture medium.

Is aerobic or facultative anaerobic

Bacteria of this genus have different requirements when it comes to oxygen. There are species such as Pasteurella multocida that is clearly anaerobic. In contrast, other species of this genus (Pasteurella pneumotropica) can develop fully in the presence of oxygen.

It is oxidase positive

These bacteria are capable of synthesizing enzymes from the group of cytochrome c oxidases. Thanks to this they can use oxygen to obtain energy during cellular respiration in the electron transport chain.

It is catalase positive

Bacteria of this genus have the ability to synthesize the enzyme catalase. This enzyme allows it to carry out the splitting reaction of the hydrogen peroxide molecule (H ₂ O ₂), obtaining water and oxygen as final products.

Reduces nitrates to nitrites

The cellular machinery of these bacteria synthesizes the enzyme nitrate reductase that catalyzes the reactions in which nitrates are reduced to nitrites, with the consequent obtaining of water.

Ferment carbohydrates

Through their metabolism, Pasteurella bacteria can carry out the fermentation process, through which they are able to break down carbohydrates such as glucose, lactose, maltose and dextrose into important organic compounds.

It is pathogenic

These bacteria are responsible for some pathologies developed by some domestic animals. These include: hemorrhagic septicemia in cattle, fowl cholera and a so-called primary pasteurellosis.

In humans these bacteria can also be opportunistic pathogens. They are acquired mainly through a bite or scratch. The diseases that have been related to these bacteria are septicemia, otitis, meningitis, cellulitis and sinusitis, among many others.

Habitat

Bacteria of the genus Pasteurella are generally distributed throughout the world.

Regarding the habitat, they prefer humid environments such as the interior of the oral and genital cavities, as well as in the respiratory and gastrointestinal tracts. They are typical of certain animals, both wild and domestic; birds, pigs, cats and dogs are frequent hosts. Of course, each species has a certain predilection for a different animal species.

These bacteria are part of the bacterial flora of these animals, although they can sometimes be responsible for the development of certain pathologies. In humans, in very isolated cases, pathogens can be found.

Culture media

The most commonly recommended culture media for Pasteurella bacteria are blood agar and chocolate agar. However, this depends on what you want to achieve. For example, for the selective isolation of some species (such as Pasteurella haemolytica) MacConkey agar is recommended.

Blood agar

It is a culture medium widely used in the field of microbiology. It is ideal for the growth of a wide range of bacteria.

It is made up of enriched agar and 5% blood. This can be lamb, horse, rabbit, and even human blood. The type of blood that is frequently used for Pasteurella cultures is sheep's blood.

The other essential component of this culture medium is the base agar. To grow bacteria of the genus Pasteurella, the most commonly used type is trypticase soy agar. This is because it provides a medium rich in organic nitrogen through amino acids and long chain peptides, which allow the growth of fastidious bacteria such as Pasteurella.

Chocolate agar

It gets its name from the characteristic brown color it has. Like blood agar, it is also made up of blood, only that it previously undergoes a heating process by which the red blood cells are broken.

Likewise, the most frequently used base agar is trypticase soy agar, although Columbia agar can also be used. The type of blood that performs best with this type of culture medium is horse blood.

Chocolate agar is one of the most recommended culture media for Pasteurella. Source: CDC / Megan Mathias and J. Todd Parker

Similarly, for some species such as Pasteurella multocida, the chocolate agar culture medium is prepared, using Müeller Hinton agar as base agar.

Main species

The genus Pasteurella encompasses a total of 21 species. Some of them are important in the medical field because they generate pathologies in certain animals that can even transmit them to humans.

Pasteurella multocida

Like the rest of the bacteria of the genus Pasteurella, it is pleomorphic. It is facultative anaerobic and grows very well on blood agar, chocolate agar and Müeller Hinton agar. This bacteria is found in certain animals such as cats, dogs, pigs, cattle, and rabbits.

Its distinctive characteristics, which allow it to differentiate Pasteurella multocida from other species, are the absence of hemolysis, the decarboxylation of ornithine, a negative result in the urea reaction and the production of indole.

Because Pasteurella multocida is present in cats and dogs, it is common for humans to be infected by a scratch or bite. If this happens, it is possible to develop an infection known as cellulitis, in which the skin and underlying soft tissues are markedly affected.

Cellulite caused by Pasteurella multocida. Source: Cabalari

Likewise, this bacteria can enter the respiratory tract and cause infections such as pneumonia and bronchitis. If these clinical pictures are not resolved in a timely manner, the individual may develop bacteremia, which can even lead to endocarditis that can be life threatening.

Pasteurella pneumotropica

It is a gram negative coccobacillus. It develops in an environment with ample oxygen availability. In blood agar cultures it produces small colonies, whose color ranges from gray to yellow.

It has been isolated from a large number of mammals, the most representative being rats and mice. It is found primarily in the nasopharynx, cecum (large intestine), uterus, and conjunctiva.

Animals, in general, do not show symptoms of their infection. However, sometimes very inflamed lesions may appear that ooze a blackish substance.

Pasteurella canis

It is a coccobacillus that forms small gray colonies, circular in shape and smooth in texture. There is no hemolysis. It is also catalase and oxidase positive and is capable of fermenting glucose and sucrose. It is urease negative.

This bacterium is found in numerous animals such as dogs, cats, rabbits, horses, and sheep. Within these animals it is found mainly in the respiratory tract and in the oral cavity. Similarly, it is responsible for numerous infections in canines such as rhinitis, otitis, vertebral osteomyelitis and bronchopneumonia, among others.

Through an animal bite or scratch, Pasteurella canis can be transmitted to humans. In these cases, it can cause soft tissue infections, arthritis and conjunctivitis, as well as bacteremia.

References

1. Baron, S. (1996). Medical microbiology. University of Texas Medical Branch. 4th edition.
2. Brooks G., Carroll K., Butel J., Morse S., Mietzner T. Medical Microbiology. 25th edition. Mc Graw Hill Interamericana. 2010.
3. Carter, G. (1979). Diagnostic Procedures in veterinary bacteriology and mycology. Editorial Thomas. 3rd edition
4. Mogilner, L. and Katz, C. (2019) Pasteurella multocida. Pediatrics in review. 40 (2) 90-92.
5. Murray, P. (1995). Manual of Clinical Microbiology. American Society for Microbiology. 6 ^th
6. Wilkie, I., Harper, M., Boyce, J. and Adler, B. (2012) Pasteurella multocida: Diseases and Pathogenesis. Current topics in microbiology and immunology. 361. 1-22