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Bacteroides fragilis





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Bacteroides fragilis


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High Yield Notes
17 pages

Bacteroides fragilis

4 flashcards

USMLE® Step 1 style questions USMLE

1 questions

A researcher is performing different tests to describe the properties of an organism. In one such test, the organism is placed in a test tube containing varying degrees of oxygen, with the top of the tube containing high amounts of oxygen and the bottom containing no oxygen. After three days, colonies of this organism only formed at the bottom of the tube. Which of the following is the most likely organism?  

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Content Reviewers:

Viviana Popa, MD

With Bacteroides fragilis, ‘bacter-’ means rod, ‘-oides’ means shape and ‘fragilis’ means fragile.

So, Bacteroides fragilis is a Gram-negative, rod-shaped bacterium.

Although it's generally considered a rod-shaped, its can range from a sphere to a rod shape, so it’s considered a pleomorphic bacterium.

Bacteroides fragilis inhabits the human colon, and from all the Bacteroides species, it’s responsible for most human infections.

Now, a little bit more about this microbe. For starters, Bacteroides fragilis is Gram-negative, which means that it has a thin peptidoglycan cell wall that can't readily retain purple dye when Gram stained.

Additionally, it's a non-spore-forming and non-motile bacteria.

It’s also an obligate anaerobe, meaning that it can only live without oxygen.

Another fact is that it’s bile resistant and seems to like bile, which makes sense since it lives in the colon.

So, not only can it grow anaerobically in blood agar, but it can also readily grow in bile esculin agar, also known as Bacteroides Bile Esculin.

After 48 hours of incubation at 35°C Bacteroides fragilis forms dark colonies with brown-black halos due to the hydrolysis of esculin.

Finally, a disk test can be done, which identifies Bacteroides fragilis by its resistance to antibiotics like kanamycin, vancomycin, and colistin.

Ok, so normally, Bacteroides fragilis colonizes the human colon peacefully, without causing any trouble.

However, trauma or surgery may damage the intestinal wall, which allows Bacteroides fragilis to slip into the bloodstream.

From there, it can travel to virtually any organ in the body.

Alternatively, it can slip into the sterile peritoneal cavity accompanied by aerobic bacteria like E. coli, which are also part of the normal gut flora - so peritoneal infections are usually considered “polymicrobial” infection, to reflect that there’s more than one culprit.

Now, in the peritoneal cavity, Bacteroides fragilis can use its virulence factors and cause disease.

First, it has a lipopolysaccharide capsule, or LPS, for short, which it uses to attach to the surface of the peritoneal cavity.

At the same time, the aerobic bacteria that tagged along remove the oxygen from the peritoneal cavity, allowing Bacteroides fragilis, the obligate anaerobe, to proliferate.

The bacterial presence in the peritoneal cavity arouses a stream of neutrophils to the site of infection.

However, the anaerobic metabolism of Bacteroides fragilis generates succinic acid, which inhibits the function of neutrophils, so they can’t neutralize Bacteroides fragilis.

What is more, the LPS capsule of Bacteroides fragilis is also slippery and slimy, so it prevents phagocytosis - the bacteria literally “slip” from the immune cells’ grasp!


Bacteroides fragilis is a Gram-negative, anaerobic rod-shaped bacterium, commonly found in the human gastrointestinal tract living as normal flora. However, it can cause illness when there is a disruption of the mucosal surface, either by trauma or surgery, causing the spread of Bacteroides fragilis to the bloodstream or surrounding tissues, causing infections such as sepsis, abscesses, and peritonitis. Symptoms vary depending on the site of the infection but can include fever, distended and tender abdomen, and abscesses.