Pasteurella multocida

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Pasteurella multocida


Introduction to bacteria

Bacterial structure and functions


Pasteurella multocida


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Pasteurella multocida

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USMLE® Step 1 style questions USMLE

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A 24-year-old man comes to the emergency department for evaluation of left leg pain and fever. The patient’s symptoms started 3 days ago when he was bitten by his friend’s dog. The dog is fully vaccinated. Temperature is 38.5°C (101.3°F), blood pressure is 120/80 mmHg, and pulse is 85/minute. Examination reveals bite marks along the left shin with surrounding swelling and erythema. Laboratory testing reveals leukocytosis. Imaging of the affected limb reveals osteomyelitis of the tibia and soft tissue infection. Gram stain reveals a Gram-negative pathogen.  Which of the following is the most likely cause of this patient’s presentation?  

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Cats, (disease vectors)

Pasteurella multocida p. , 147, 183

Cellulitis p. 491

Pasteurella multocida p. , 147

Pasteurella multocida

osteomyelitis p. 177

transmission p. 147, 183


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.


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.


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