USMLE® Step 1 style questions USMLE
A lab researcher is evaluating several different bacterial strains based on their agar growth patterns. In one experiment, a certain bacteria was grown on MacConkey agar which resulted in the formation of pink colonies. Which of the following is the most likely bacteria studied in this experiment?
Enterobacter is a genus of Gram-negative rod-shaped bacteria which belongs to a family of bacteria called the Enterobacteriaceae.
There are several species which cause infection in humans and the most important are Enterobacter cloacae and Enterobacter aerogenes.
Now, a little bit of microbe anatomy and physiology. First, Enterobacter 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.
Enterobacter is motile, non-spore forming, facultative anaerobic which means it can survive in both aerobic and anaerobic environments and oxidase negative which means it doesn’t produce an enzyme called oxidase.
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.
Ammonia then makes the mixture change color from orange-yellow to bright pink.
Finally, Enterobacter 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 Enterobacteriaceae, like Enterobacter, Klebsiella 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, while others, like Salmonella and Shigella, can’t ferment lactose so their colonies will be colorless.
Now, Enterobacter has very few notable virulence factors such as fimbriae, which help it attach to host cells, and they secrete hemolysins, which cause formation of tiny holes in the cell membrane, leading to cell damage.
So, an infection with Enterobacter can be acquired in two ways - endogenous and exogenous.
So, because of its ubiquitary nature, meaning it can be found in water, soil, plants and in a lot of medical equipment and products, it can be transmitted in an exogenous way through direct or indirect contact.
Also, it can be transmitted in an endogenous way in people who are seriously ill and take a lot of antibiotics.
Now, Enterobacter is an opportunistic pathogen which causes nosocomial infections in people with prolonged hospitalization, in people with underlying illness such as malignancy, burns or diabetes, in case of immunosuppression, like in HIV positive individuals, and in the presence of a foreign device such as central venous catheters, endotracheal tubes or urinary catheters.
In people who are mechanically ventilated, Enterobacter causes respiratory infections like tracheobronchitis, which is inflammation of the trachea and bronchi, pneumonia, lung abscesses and pleural empyema which is a collection of pus within the pleural cavity.
Enterobacter is a Gram-negative bacillus, facultative anaerobic, oxidase negative, and lactose fermenting bacteria that’s known to cause nosocomial infections affecting the urinary tract, lungs, and other parts of the body. It's usually treated with antibiotics. It’s diagnosed bia bacteria culture, of the blood, urine, or sputum, and treated with antibiotics.