USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 48-year-old woman comes to the clinic because of a fever and fatigue that began a week ago. She was treated for an illness five months ago, but does not remember the name of the disease or the medication. She felt better at the time, and did not return for follow-up because she lives on a farm far away. Her temperature is 38.3°C (101°F), her pulse is 95/minute, respirations are 18/minute, and blood pressure is 110/75 mm Hg. During physical examination, she appears lethargic and there is hepatosplenomegaly. She has lower back pain with paravertebral tenderness that began recently. Which of the following microbes is the most likely cause of her illness?
Content Reviewers:Rishi Desai, MD, MPH
All cause a systemic disease called brucellosis, but each of them has a different host, and causes a different form of the disease.
Now, Brucella it’s a non-motile bacteria that doesn’t form spores.
It’s a strict aerobe, meaning that it needs oxygen to survive, and also, it’s facultative intracellular which means it can survive both inside and outside the cell.
Finally, it’s urease and catalase positive which means it produces both these enzymes.
Brucella is usually isolated on blood cultures, with biphasic methods like the Ruiz-Castaneda methods.
Biphasic means that the blood culture bottle has both a solid phase, and a liquid phase.
Now, Brucella is a very slow growing bacteria, so colonies usually grow in the solid medium after 6 to 8 weeks of incubation.
The colonies are raised, convex with smooth, shiny corners.
On the bright side, there are now some modern automated blood culture systems called the Bactec systems, which are more effective and can isolate Brucella after only 1 week. Neat!
Now, Brucella can enter the body one of two ways.
First, there may be direct contact with infected animals - and the host is different for each Brucella species.
So, B. abortus is transmitted by cattle, B. melitensis is transmitted by small ruminants such as goats and sheep, B. canis is transmitted by dogs and B. suis is transmitted by swine and rodents.
In this case the bacteria enters through skin lesions, mucous membranes and inhalation.
The second way is ingestion of contaminated animal products such as unpasteurized milk, cheese and undercooked meat.
So, once the bacteria is inside the bloodstream, it’s ingested by phagocytes like macrophages and neutrophils.
Normally, phagocytes destroy invading bacteria by wrapping them up in vesicles called phagosomes, which will merge with lysosomes to form a phagolysosome.
Lysosomes are round vesicles that contain hydrolytic enzymes, which are released inside the phagolysosome to destroy the invading bacteria.
However, Brucella has a few virulence factors that it uses to avoid destruction.
First, it has the ability to escape the immune recognition by using type IV secretion system, or T4SS for short, which is a collection of proteins that can dampen the immune response.
However, some of them are not able to escape and are caught by macrophages and ingested.
Inside the macrophage, Brucella uses another virulence factor on its outer membrane, called non-endotoxic lipopolysaccharide, or LPS for short.
LPS inhibits the fusion between the phagosome and the lysosome, and allows Brucella to avoid intracellular death.