Bacteroides fragilis

Definitions & Key takeaways

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.

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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!
Finally, the LPS initiates a unique immune response which promotes abscess formation in the peritoneal cavity. Ok, now there are some strains of Bacteroides fragilis called Enterotoxigenic Bacteroides fragilis which secrete a heat-labile zinc metalloprotease toxin.
The reason behind this is not well understood, but it’s possible that non-enterotoxigenic species that normally reside in the colon undergo mutations that make them more virulent.
This toxin stimulates IL-8 secretion, which is a proinflammatory cytokine that causes damage to the intestinal epithelium, so fluid and chloride are secreted into the intestinal lumen, causing inflammatory diarrhea.
Finally, even though it’s not part of the normal skin flora, Bacteroides fragilis is also one of the most common causes of foot infections in people with diabetes.
Now, symptoms vary widely depending on the site of infection. For example, intra-abdominal infections may cause a fever, abdominal pain, nausea, vomiting and a distended abdomen.
With skin infections, there’s pus, necrotic tissue, and a bad characteristic odor characteristic of anaerobic infections.
And with inflammatory diarrhea, there’s abdominal pain and, well, watery diarrhea. Diagnosis is made by isolating Bacteroides fragilis in cultures from biological samples, like peritoneal fluid, pus or stool.
Gas-liquid chromatography - a process that analyzes compounds that can be vaporized - can also be used to identify the specific compounds produced by Bacteroides fragilis, like acetic and succinic acid.
Gene sequencing can also be used to identify the bacteria. A computed tomography scan may show abscesses or the presence of gas at the infected site.
Treatment of Bacteroides fragilis infections consists of antibiotics and drainage of abscesses, or debridement of necrotic tissues.
However, Since Bacteroides fragilis produces β-lactamase, β-lactam antibiotics like penicillin can’t be used. So instead, metronidazole, or a combination of β-lactam and β-lactamase inhibitors, like Ampicillin and Sulbactam, are efficient Alright, as a quick recap, Bacteroides fragilis is a Gram-negative, pleomorphic rod-shaped bacterium that resides in the colon of humans.
It grows on blood agar, bile esculin agar, and it’s resistant to kanamycin, vancomycin and colistin. It’s the most common anaerobic bacteria found in peritoneal infections and foot infections in patients with diabetes mellitus.
Treatment consists of drainage of abscesses and debridement of necrotic tissue, and antibiotics like metronidazole.