AssessmentsYersinia pestis (Plague)
USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 33-year-old man comes to the emergency department because of weakness, headache, and chills. He has been camping outdoors in the southwest United States for the past month. His temperature is 38.9°C (102°F), pulse is 90/min, respirations are 15/min, and blood pressure is 115/85 mm Hg. His inguinal, axillary, and cervical lymph notes are swollen, smooth, and painful without fluctuation. Culture from lymph node pus aspiration with Wayson stain demonstrates a rod-shaped organism with bipolar staining. Which of the following is the most likely causative agent?
Yersinia pestis is a Gram-negative coccobacillus which belongs to a family of bacteria called the Enterobacteriaceae.
It causes a disease called plague, which is transmitted by rodents, mainly rats, as well as prairie dogs, and their fleas.
Plague is highly contagious, and there have been three major pandemics in human history - one of them, known as the Black Death, killed up to one-third of the European population. Yikes!
Now, Yersinia pestis 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.
On Wright-Giemsa and Wayson staining it has a bipolar staining which means that only the poles of the bacteria stain, and the rest of it remains unstained, so the bacteria look like safety pins under the microscope.
Alright, now Yersinia pestis is non-motile, non-spore forming, facultative anaerobic which means it can survive in both aerobic and anaerobic environments and facultative intracellular which means it can survive both inside and outside the cells.
It’s oxidase and urease negative which means it doesn’t produce these enzymes and catalase positive which means it produces an enzyme called catalase.
Also, it’s indole negative which means it doesn’t convert tryptophan into indole.
Yersinia pestis grows well on MacConkey agar, sheep blood agar, and chocolate agar.
MacConkey agar is used to identify if a bacteria ferments lactose or not, and it contains a pH sensitive dye and lactose.
So, if a bacteria ferments lactose, it means that it’s able to ferment lactose and produce acid which causes the pH sensitive dye to turn pink leading to formation of pink colonies.
Non-lactose fermenters, like Yersinia pestis, aren’t able to modify the PH sensitive dye, grow into colorless colonies.
Next, on sheep blood agar and chocolate agar, Yersinia pestis forms opaque, yellow colonies that look like fried eggs.
Finally, the triple sugar iron test, or TSI for short can be done to assess hydrogen sulfide production.
This medium contains three sugars - lactose, glucose and sucrose, as well as iron and a pH sensitive dye.
If the bacteria produces hydrogen sulfide, that reacts with the iron, and a black precipitate forms in the test tube.
Yersinia pestis doesn’t, so no precipitate forms.
Now, the pathogenesis of Yersinia pestis consists of two components - the transmission of bacteria from fleas and the host response to the bacteria.
First, fleas get infected with Yersinia pestis by feeding on a bacteremic host like rats, which then colonizes the flea midgut.
So, when the flea bites a human it regurgitates the bacteria into the bite wound.
So, once the bacteria gets inside the host, neutrophils and macrophages come to the wound site to kill the bacteria.
Now, the bacteria has a capsule which consists of a capsular antigen, called F1, with antiphagocytic properties.
But, inside the flea gut, it loses this capsule, so neutrophils can easily destroy the bacteria.
Sadly, macrophages are not as efficient in killing the bacteria so, if the bacteria is caught by macrophages it can survive inside them and get carried to lymph nodes where it replicates and cause lymphadenopathy.
Inside macrophages, it starts to use virulence factors to avoid destruction.
So, now it starts again to produce it’s capsular antigen F1.
Also, it has a type III secretion system, or T3SS for short, which is a collection of proteins that can dampen the immune response.
So, using T3SS it starts to inject inside the macrophage Yersinia outer proteins, or Yops for short, which block secretion of proinflammatory cytokines such as TNF-alpha and IL-8 and inactivate the macrophages leading to evasion of phagocytosis.