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Introduction to bacteria
Gram positive bacteria
Gram negative bacteria
Other bacteria



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USMLE® Step 1 style questions USMLE

2 questions

A 68-year-old man presents to the emergency department with a several week history of malaise, subjective fevers, and fatigue. He has a history of renal cell carcinoma diagnosed 5 months ago. The patient reports he had a recent stent placed in the left ureter approximately two months ago due to worsening metastatic disease burden. The hospital course was complicated by damage to the ureter, which was repaired surgically. Temperature is 37.7 °C (100.6 °F) , pulse is 102/min, respirations are 22/min and blood pressure is 130/62 mmHg. On physical examination the patient is noted to have conjunctival pallor. A new holosystolic murmur is appreciated at the cardiac apex. Which of the following is the most common organism implicated in this patient’s disease process?  

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Viviana Popa, MD

With Enterococcus, entero- refers to the intestines, while -coccus means round shape.

So Enterococcus is a genus of round bacteria that commonly colonizes the gut of humans and animals.

Enterococcus is also called Group D streptococcus in Lancefield classification developed by an American microbiologist Rebecca Lancefield.

There are two species that can cause infections in humans and these are Enterococcus faecalis, amounting for the majority of infections, and Enterococcus faecium, which causes disease more rarely.

Now, looking at an individual bacterium, Enterococcus has a thick peptidoglycan cell wall, which takes in purple dye when Gram stained - so this is a gram-positive bacteria.

When there’s more of them, Enterococci grow in short chains, usually in pairs.

They’re non-spore forming, facultative anaerobes, meaning that they can survive in both aerobic and anaerobic environments and catalase negative, which means they don’t produce an enzyme called catalase.

Enterococci also can tolerate extreme environmental conditions including high sodium chloride concentrations, high pH and even high temperatures.

They can survive at 60 degrees Celsius for up to 30 minutes!

Ok, now, enterococcus is pyrrolidonyl arylamidase positive, because it makes an enzyme called L-pyrrolidonyl arylamidase.

To test for this, a small sample is taken from a suspected bacterial colony, and then inoculated to a disk pad that’s embedded with pyrrolidonyl beta naphthylamide - another joy of a word.

With Enterococcus, pyrrolidonyl arylamidase hydrolyzes pyrrolidonyl beta-naphthylamide to produce beta-naphthylamide.

Try saying that 3 times fast! Finally, another reagent called N-methylamino-cinnamaldehyde is added to the disk, and it reacts with beta-naphthylamide, resulting in a bright red color that confirms Enterococcus is pyrrolidonyl arylamidase positive.

Now, most commonly, Enterococci are gamma hemolytic which means that when cultivated on blood agar they don’t induce hemolysis, so the agar under and around the colony remains unchanged.

But sometimes, they can induce alpha hemolysis, also called partial hemolysis, which means that the agar under the colony turns dark and greenish.

Finally, it can grow on bile salts, which is very useful to differentiate Enterococci from non-enterococcus group D streptococci.

This can be demonstrated with the Bile-Esculin test, that can be done on tubes or plates and uses a medium that contains peptone, beef extract, bile, esculin, ferric citrate and agar.

So, in the presence of bile, Enterococcus can hydrolyze esculin into glucose and esculetin.

Then, esculetin reacts with the ferric ions supplied by ferric citrate and form a black diffusible complex.

So, after 24 to 48 hours, Enterococci cause a diffuse blackening of more than half of the tube or black halos around colonies on plates.

Now, Enterococci are opportunistic pathogens which causes a wide variety of hospital-acquired infections, particularly in people with underlying cardiovascular conditions, or in people with immunosuppressive conditions, like an HIV infection.

They also cause disease in people who have been hospitalized for long periods of time and received multiple antibiotic treatments, or in people with indwelling medical devices such as central venous catheters or urinary catheters.

Under these circumstances, Enterococcus mainly causes infective endocarditis - in fact, Enterococcus is the second most common cause of infective endocarditis overall.

Enterococcus can get in the bloodstream one of two ways.

First, it can be inoculated directly into the blood through a blood vessel catheter.