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Introduction to bacteria

Bacterial structure and functions



USMLE® Step 1 questions

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High Yield Notes

17 pages


USMLE® Step 1 style questions USMLE

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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?  

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Enterobacter spp.

lactose fermentation p. 142

nosocomial infection p. 182

taxonomy p. 122


Content Reviewers

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.

It’s an opportunistic pathogen, which can be normally found in the intestinal flora and causes a wide variety of hospital-acquired infections, mainly respiratory and urinary infections.

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.

Alright, now Enterobacter is 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 Enterobacter, urease makes urea dissociate into carbon dioxide and ammonia.

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.


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.


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