USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 34-year-old man presents to his primary care physician for evaluation of sinus pressure. The patient has had a stuffy nose with purulent nasal discharge for the past two weeks. In addition, he has been experiencing headaches and ear fullness. His symptoms began improving around 11-12 days after onset but then took a turn for the worse. Past medical history is notable for type II diabetes mellitus, for which he takes metformin daily. Temperature is 38.9°C (102°F), pulse is 101/min, respirations are 19/min, and blood pressure is 132/71 mmHg. Physical exam demonstrates tenderness to palpation over the bilateral maxillary sinuses. Which of the following organisms is the most likely cause of this patient’s clinical presentation?
Contributors:Evan Debevec-McKenney, Jake Ryan, Evode Iradufasha, Elizabeth Nixon-Shapiro, MSMI, Robyn Hughes, MScBMC
Streptococcus pneumoniae or Strep. pneumoniae can be broken down into strepto- which means chain, -coccus, which means round shape, and pneumoniae, which refers to the fact that it causes pneumonia - no surprises there.
So, Strep pneumoniae are round bacteria that tend to grow in chains, usually in lancet-looking pairs called diplococci.
They’re the most common cause of community-acquired pneumonia - meaning pneumonia acquired somewhere other than the hospital.
Ok now, a little bit of microbe anatomy and physiology.
Strep pneumoniae has a thick peptidoglycan cell wall, which takes in purple dye when Gram stained - so this is a gram-positive bacteria.
They’re non-motile and don’t form spores, and also, they’re facultative anaerobes, meaning that they can survive in both aerobic and anaerobic environments.
Finally, they’re catalase negative - which means they don’t produce an enzyme called catalase.
Ok, now, when cultivated on a medium called blood agar, Strep pneumoniae colonies cause alpha hemolysis, also called green hemolysis, because they produce hydrogen peroxide, which partially oxidizes initially red hemoglobin in the blood agar to green methemoglobin.
Other Streptococcus species, like Strep viridans, are also alpha hemolytic.
So, an optochin test is done to distinguish Strep pneumoniae. That’s where a few drops of optochin are added to the culture.
Strep pneumoniae are optochin sensitive, meaning the bacteria dies after adding optochin, whereas Strep viridans are optochin resistant - meaning they survive.
Now, Strep pneumoniae 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.
So first, Strep pneumoniae is encapsulated, meaning it’s covered by a polysaccharide layer called a capsule.
The capsule has pili and fimbriae, which are hair-like extensions that help it attach to a host cell.
Once attached to a mucosal surface like in the nasopharynx or the middle ear, Strep pneumoniae can multiply and produce biofilms.
A biofilm is basically a layer of goop-like material made of exopolysaccharides or EPS, within which Strep pneumoniae bacteria live and reproduce.
Comparing a biofilm to strawberry jam, the seeds would be the bacteria and the rest of the jam would be the EPS.
The bacteria in the biofilm hide from the host's immune system and antibiotics, and even exchange resistance genes. Sneaky...
As if that wasn’t enough, Strep pneumoniae also produces toxins, and the most important ones are IgA protease and pneumolysin.
IgA protease destroys Immunoglobulin A or IgA, which normally binds invading bacteria, so neutrophils can destroy them. That’s like a computer virus first taking over the antivirus software!
And then there’s pneumolysin, which activates the host’s complement system – a set of plasma proteins involved in immunity.
This results in a local inflammation which not only destroys the bacteria, but also the host tissues - like alveolar capillaries and pneumocytes.
And here’s the most interesting part. Strep pneumoniae can actually peacefully colonise the nasal cavities and sinuses, where it doesn’t do any harm so long as the immune system keeps them in check, restricting their growth and preventing them from spreading somewhere else in the body.
Problems arise in individuals with weaker immune systems, like infants and the elderly.
Additionally, some toxic compounds in cigarette smoke can weaken the local respiratory defense mechanisms, making individuals more susceptible to Strep pneumoniae infections.
Strep pneumoniae can cause a number of infections.
One example is rhinosinusitis, which is when the mucous membrane lining the nose and the paranasal sinuses gets inflamed, resulting in fever, facial pain, and headaches.
Sometimes, when the walls of the paranasal sinuses are very thin, and Strep pneumoniae can get into the cranial cavity - resulting in meningitis.
With meningitis, people develop a fever, neck stiffness, and a headache.
Strep pneumoniae can also invade the Eustachian tube, and cause an infection of the middle ear - or otitis media, which causes pain and an earache.
Chronic otitis media can spread to the mastoid antrum behind the ear, where it causes mastoiditis.