USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 4-year-old girl is brought to the emergency department by her mother because of a 3-day history of right calf swelling, redness, and limited mobility of her right leg. According to the mother, about a week ago, the girl was attacked and bitten by a dog. However, she claims that the bite was thoroughly cleaned with mild soap and irrigated with tap water shortly after the incident. The physical exam shows a dry and dark scab with tooth markings, and increased local swelling and tenderness on her right calf. The dog owner, who is friends with the family claims that the dog has up-to-date vaccinations. Her temperature is 36.5°C (97.6°F), pulse is 100/min, respirations are 14/min, blood pressure is 100/70 mmHg. Which of the following is the most likely causal organism?
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.