USMLE® Step 1 style questions USMLE
A 10-year-old boy comes to the clinic because of a "bad cold" for the past two weeks. His mother says that two weeks ago, his symptoms began with a runny nose with slightly yellow snot, but then his symptoms progressed and he has been producing large amounts of dark yellow snot from his nose for the past 10 days. His medical history is noncontributory. His temperature is 38.3°C (101.5°F), pulse is 78/min, respirations are 18/min, and blood pressure is 122/84 mm Hg. Physical examination shows purulent nasal discharge. His lungs are clear to auscultation bilaterally. Sputum culture grows a gram-negative, aerobic, oxidase-positive diplococcus that is unable to metabolize maltose. Which of the following is the most likely cause of this condition?
Content Reviewers:Viviana Popa, MD
Moraxella catarrhalis is a Gram-negative diplococcus, which means it’s a spherical-shaped bacteria that usually hangs out in pairs of two.
Moraxella has had a bunch of names over the last century.
When it was first discovered it was named Micrococcus catarrhalis, then its name was changed into Neisseria catarrhalis.
Later it was moved to another genus, called Branhamella, and finally it was moved again to genus Moraxella, species Moraxella catarrhalis.
It normally colonizes the upper respiratory tract, and usually causes otitis media in children.
Now, Moraxella catarrhalis 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.
Moraxella catarrhalis is also non-motile, non-spore forming, aerobic, which means it needs oxygen to survive, and oxidase positive, which means it produces an enzyme called oxidase.
However, it’s maltose fermentation negative which means it cannot ferment maltose.
To check for this, a pure sample from the culture is transferred to a sterile tube containing a mix of phenol red and maltose, which is then incubated at 36 degree Celsius for 24 hours.
If acidic fermentation of maltose occurs, the resulting byproducts make the solution go yellow.
With Moraxella catarrhalis, the solution stays red.
Finally, it grows well on blood and chocolate agar and it doesn’t grow on modified Thayer-Martin agar, like other Gram-negative diplococci, such as Neisseria species.
So, on blood and chocolate agar it grows into round, gray-white colonies.
Finally, Moraxella catarrhalis colonies are positive for the hockey puck test, which means a loop is used to push the colonies across the plate.
Moraxella colonies are sturdy, and can be slid across the plate without breaking, which means a positive hockey puck test.
Now, Moraxella catarrhalis is an opportunistic pathogen, which means that it doesn’t usually cause disease, but it may take advantage of an opportunity like a weakened immune system to cause an infection.
This is more common in children, the elderly, people with chronic respiratory disease, or following a viral respiratory infection.
So, in these situations, it causes an infection by attaching to host cells using trimeric autotransporter adhesins, or TAAs, which are proteins found on the outer membrane.