USMLE® Step 1 style questions USMLE
A 27-year-old medical student comes to the office because of redness in her left eye that began in the morning when she had difficulty opening the eye. She has noticed a yellowish mucous discharge expressed from that eye and edema of the eyelid. She has no pain with ocular movements. Culture of the discharge on agar grows anaerobic Gram negative rods that are catalase positive and oxidase negative with a red pigment. Which of the following species is the most likely infectious agent?
Serratia marcescens is a Gram-negative bacteria which belongs to a family of bacteria called the Enterobacteriaceae.
Now, the genus consists of at least 20 species, of which eight are known to have caused infections in humans, with Serratia marcescens being the main human pathogen.
Serratia marcescens is widely distributed in water, soil and plants and it causes a variety of hospital-acquired infections.
Now, Serratia marcescens 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.
And since it’s a Gram-negative bacillus, it looks like a little pink rod under the microscope.
Ok, now Serratia marcescens is motile and also facultative anaerobic which means it can survive in both aerobic and anaerobic environments.
It’s urease positive which means it can produce an enzyme called urease that dissociates urea into carbon dioxide and ammonia.
This can be tested by transferring a pure sample of bacteria from the culture to a sterile tube containing a mixture of “urea agar” broth and phenol red.
Then, the mixture is incubated. So, with Serratia marcescens, urease makes urea dissociate into carbon dioxide and ammonia.
Ammonia then makes the mixture change color from orange-yellow to bright pink.
Also, it’s catalase positive which means it can produce an enzyme called catalase.
To test for this, a few drops of hydrogen peroxide are added to the colony of the suspected bacteria.
So, if catalase is present, it makes the hydrogen peroxide dissociate into water and oxygen, causing the mixture to foam.
Furthermore, it produces another three enzymes, DNase, lipase, and gelatinase, which are unique to Serratia and this can help easily differentiate it from other Enterobacteriaceae.
Finally, Serratia marcescens grows well on MacConkey agar which is a medium that contains a pH sensitive dye and lactose.
This medium helps identify whether Gram-negative bacteria are lactose fermenters or not.
Some bacteria, like Klebsiella, Enterobacter and Escherichia coli can ferment lactose, which results in the production of acid, that makes the pH sensitive dye turn pink - so their colonies will be pink.
Now, Serratia marcescens and also Citrobacter are slow lactose fermenters, so they are a bit lazy, but eventually ferment lactose.
So at first, the colonies appear colorless but 48 hours later, they turn pink.
Finally, some strains of Serratia marcescens make a characteristic red pigment, called prodigiosin, that is typically found staining showers, toilet bowls, and around wetted tiles.
Production of this pigment can lead to the formation of red colonies on agar, if the plate is incubated at 25 degrees Celsius.
Ok, now Serratia marcescens has very few notable virulence factors such as fimbriae to attach to host cells and hemolysins which cause formation of tiny holes in the cell membrane, leading to cell damage.
However, it can form adherent biofilms on medical equipment such as respiratory equipment, intravenous and urinary catheters and contact lenses.
A biofilm is basically a layer of “slime” made of exopolysaccharides or EPS, within which Serratia 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.
Biofilms make it difficult for antibiotics to reach the bacteria, so the infection is more difficult to treat.