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Pasteurella multocida





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Pasteurella multocida


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Pasteurella multocida

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Pasteurella multocida is a gram-negative coccobacillus which is involved in a zoonotic infection.

Most often, it causes soft tissue and respiratory infections.

There are 3 subspecies that cause disease in humans: Pasteurella multocida subspecia multocida, Pasteurella multocida subspecia septica and Pasteurella multocida subspecia gallicida.

All of these subspecies are encapsulated, meaning they have a polysaccharide layer called a capsule.

And depending on the capsular antigens found on the capsule, they can be grouped in 5 different serogroups: A, B, D, E and F.

Each of them is involved in a different type of disease and only serogroups A and D cause disease in humans.

So, serotype A causes respiratory infections and serotype D causes soft tissue infections like cellulitis.

Now, Pasteurella multocida has a thin peptidoglycan layer, so it doesn’t retain the crystal violet dye during gram staining.

Instead, like any other gram-negative bacteria, it stains pink with safranin dye.

But, sometimes, it can have a bipolar staining which means that only the poles of the bacteria stain pink, and the rest of it remains unstained, so the bacteria look like safety pins under the microscope.

Alright, now Pasteurella multocida is non-motile, non-spore forming, and facultative anaerobic which means it can survive in both aerobic and anaerobic environments.

It’s oxidase and catalase positive, which means it can produce both these enzymes, and it’s also nitrate reduction positive which means it can produce an enzyme that hydrolyzes nitrate into nitrite.

Finally, Pasteurella multocida grows well at 37ºc on sheep blood agar, which is the preferred culture medium, but it can also grow on chocolate agar, Mueller-Hinton agar or brain heart infusion agar.

The colonies are opaque or grey, and small - they’re only about 1-2 millimeters in diameter each.

Now, Pasteurella multocida has a number of virulence factors, that are like assault weaponry that help it attack and destroy the host cells, and evade the immune system.

First, on its capsule there are fimbriae, which are hair-like extensions that help the bacteria attach to host cells.

Additionally, the capsule acts as a shield and prevents the bacteria from being eaten up by phagocytes.

Underneath the capsule, there’s an outer membrane which consists of lipopolysaccharide, or LPS for short, which inhibits the complement system.

The complement system is a cascade of proteins that get activated one after another to cause bacterial lysis - so when it’s inhibited, the bacteria can avoid immune destruction.

Additionally, Pasteurella multocida makes a dermonecrotic toxin called Pasteurella Multocida Toxin, or PMT for short, which directly damages the endothelial cells, causing edema, redness and swelling.

Finally, the bacteria needs iron to survive, so it has iron acquisition proteins which are able to acquire iron from host proteins.

Now, Pasteurella multocida is transmitted by domestic animals such as dogs or cats, so it can be a commensal organism in people who have close contact with these animals and peacefully colonize the respiratory tract.

However, in people with underlying chronic pulmonary disease it can cause respiratory infections like sinusitis, otitis media, epiglottitis, pneumonia and empyema.


Pasteurella multocida is a gram-negative, facultative anaerobic coccobacillus inhabiting the upper respiratory tract of animals, particularly in cats and dogs. It is known to cause zoonotic infections from animals to humans, including cellulitis, osteomyelitis, endocarditis, or even meningitis.

Symptoms vary depending on the type of infection, and the diagnosis relies on cultures from biological samples like pus, cerebrospinal, blood, or respiratory tract samples, and PCR. Pasteurella multocida infections are treated with antibiotics such as penicillin, tetracyclines, cephalosporins, and quinolones.