Enterococcus
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, 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.
Second, it may get in the bloodstream following gastrointestinal or genitourinary surgery. From the bloodstream, Enterococcus reaches the heart, where it sticks to the heart valves, forming bacterial vegetations.
Most often, this is possible when there’s underlying damage to the mitral or aortic valves. Next, in people with urinary catheters, Enterococcus causes urinary tract infections, or UTIs, like cystitis and pyelonephritis.
Finally, Enterococcus can cause other uncommon infections such as biliary tract infections, because of its ability to grow in bile, surgical wound infections such as cellulitis or skin abscesses, and intra-abdominal and pelvic abscesses.
Symptoms depend on the disease. With a bloodstream infection, there may be fever, chills, hypotension and tachycardia and with endocarditis, there may be fever, new heart murmurs and malaise.
With urinary tract infections in general, symptoms include dysuria, which is pain or a burning sensation during urination, urinary frequency, which means needing to urinate more often than usual and urinary urgency, which means strong need to urinate.
With cystitis, there’s also suprapubic pain and with pyelonephritis, there may be flank pain, and systemic symptoms like fever, chills, nausea or vomiting.
With skin infections, there may be swelling, erythema, or redness, tenderness or pain, and purulent drainage. Finally, with biliary tract infections, there may be fever, pain and jaundice and with intra-abdominal and pelvic abscesses there may be fever, abdominal pain, nausea and vomiting.
Diagnosis is based on isolating enterococcus in blood, urine or pus cultures, depending on the type of infection. With infective endocarditis, an echocardiogram may show bacterial vegetations on the heart valves.
With UTIs, the urinalysis shows an alkaline urine pH, above 7, pyuria which means white blood cells in the urine, and bacteriuria, which means bacteria in the urine.
An abdominal or pelvic ultrasound or CT scan can diagnose biliary tract infections as well as abdominal or pelvic abscesses.
Treatment for infections caused by Enterococcus faecalis is usually done with beta-lactams such as penicillin or ampicillin, or aminoglycosides such as gentamicin or streptomycin.
Resistant strains can be treated with vancomycin. Most strains of Enterococcus faecium, on the other hand, are highly resistant to beta-lactams and aminoglycosides.
So, in this case, vancomycin is used from the get-go. Sadly, both Enterococcus faecalis and Enterococcus faecium can become resistant even to vancomycin.
These are called vancomycin-resistant Enterococcus, or VRE for short, and require treatment with expensive antibiotics like linezolid, daptomycin or tigecycline.
Alright, as a quick recap, Enterococcus is a Gram-positive round bacteria, non-spore forming, facultative anaerobic and catalase negative.
It can survive in extreme conditions such as high sodium chloride concentration, high pH and extreme temperature. It can have alpha or gamma hemolysis on blood agar and it can grow on bile salts.
It’s an opportunistic pathogen which causes a wide variety of hospital-acquired infections like UTIs, endocarditis, bloodstream infections, biliary tract infections, wound infections and intra-abdominal infections.
For diagnosis, it can be isolated in cultures from blood, urine or pus depending on the type of infection. For treatment, E.
faecalis can be treated with beta-lactams, aminoglycosides and vancomycin, and E. faecium can be treated with vancomycin.
In case of VRE, it can be treated with linezolid, daptomycin or tigecycline.
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