Pasteurella multocida

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.

Alternatively, Pasteurella can enter the body through bites or scratches, causing a soft tissue infection called cellulitis at the site of inoculation.

In serious cases it can cause abscesses, necrotizing soft tissue infections and it can also spread locally to the bones, causing osteomyelitis, or to the joints, causing septic arthritis.

These complications are more likely following a cat bite, because cats have sharp, penetrative teeth that pierce deeper through the skin.Finally, in people who are immunocompromised the bacteria can disseminate from the primary infection site into the bloodstream and cause bacteremia.

The spleen also plays an important role in immunity against encapsulated bacteria, so bacteremia is also more likely in individuals with spleen issues, such as those who have undergone a splenectomy, which is when the spleen is removed, or in those with sickle cell disease, who have functional asplenia.

Eventually, bacteremia can cause serious problems, like sepsis which can lead to hypoperfusion of vital organs. As if that wasn’t enough, pasteurella multocida can spread from the bloodstream to the heart, causing endocarditis, to the peritoneum, causing peritonitis, and can also cross the blood-brain barrier, resulting in meningitis.With respiratory tract infections, symptoms may include fever, malaise, and dyspnea, or difficulty breathing.Symptoms of cellulitis, on the other hand, usually develop include swelling, erythema, tenderness and purulent drainage at the wound site.

With abscesses, there may be local redness, heat, swelling and pain, while in necrotizing soft tissue infections, additional symptoms include fever, formation of bullae, which are liquid-filled blisters, bleeding into the skin and skin necrosis.

With osteomyelitis, there’s bone pain, fever and weakness, and with septic arthritis, there’s joint pain. Next, with bacteremia and sepsis there may be fever, chills, hypotension and tachycardia.

As for the complications, endocarditis usually develops with fever and new heart murmurs, peritonitis develops with abdominal pain and diffuse abdominal rigidity and meningitis develops with headache, fever and neck stiffness.Diagnosing a Pasteurella multocida infection relies on identifying pasteurella multocida in cultures from pus, sputum, cerebrospinal or peritoneal fluid, or in blood cultures.

Alternatively, pcr can be done on these samples for a more specific result, and it identifies the exact infecting species and subspecies.

Pasteurella multocida infections are usually treated with penicillin, but antibiotics like tetracyclines, cephalosporins and quinolones can also be used.

Additionally, drainage or debridement may be needed for infected bite wounds. ##SummaryAlright, as a quick recap, Pasteurella multocida is a gram-negative cocobacillus, non-motile, non-spore forming, facultative anaerobic, oxidase, catalase and nitrate reduction positive.

It has virulence factors such as a capsule and lipopolysaccharide outer membrane which are involved in surface adhesion and escaping the immune system.

It’s transmitted by domestic animals such as cats or dogs through bites and scratches and produce mostly soft tissue and respiratory infections.

Diagnosis relies on cultures from biological samples like pus, cerebrospinal or peritoneal fluid, blood or respiratory tract samples and PCR.

Infection can be treated with penicillin,