Bacteroides and Prevotella - The Gut Bacterias

Introduction

The human intestines have a variety of bacteria, and the most commonly found bacteria are anaerobes. Among the anaerobes, the predominant species found are bacteroids. They have a symbiotic relationship with their host which is the human intestines. When present in any other parts of the body, bacteroides cause infection and abscesses. Prevotella is rod-shaped and non-motile. They are present in the mouth, vagina, and gut microbial colonies. They are also found in anaerobic infections of the respiratory tract.

What Are Bacteroids?

Bacteroids belong to the phylum Bacteroidetes and the class Bacteroidia. According to gram classification, Bacteroids are classified as gram-negative obligate anaerobic bacilli. They are non-spore forming and can be either motile or non-motile. The cell membranes of bacteroids contain sphingolipids. Their peptidoglycan cell layer contains an acid called meso-diaminopimelic acid. The common feature among bacteroids is that they all contain the same composition of 40-48 percent guanine-cytosine. The bacteroid fragilis group contains a total of 10 species of bacteria. They are as follows:

• B.fragilis.

• B.distasonis.

• B.vulgatus.

• B.ovatus.

• B.merdae.

• B.stercoris.

• B.eggerrthii.

• B.uniformis.

• B.thetaiotaomicron.

• B.caccae.

Bacteroides are the predominant species of bacteria present in the human intestine. They are involved in several metabolic reactions and protect our body from the invasion of pathogens. All ten species can be isolated from the intestine but infections can be caused by them in any organ of the body. It is difficult for laboratories to isolate and identify the bacteroid species. They usually identify B.fragilis and classify all other species into the B.fragilis group.

What Are the Clinical Manifestations of Bacteroids?

The infection caused by any species of bacteroides causes the hallmark feature of abscess formation (a painful collection of pus). They are the majority of bacteria isolated in a polymicrobial infection from typical sites like the abdomen and pelvis, skin and soft tissues, and perirectal and other solid organs. They are always isolated as a component of a multifactorial infection and rarely present as an isolated causative factor. Some infectious conditions where they are isolated are meningitis, endocarditis, osteomyelitis, and septic arthritis.

How Is Bacteroides Infection Diagnosed?

• Blood Cultures: Bacteroids are either isolated from the site of infection as a single agent or as a part of a polymicrobial mixed infection. Some identification tests from blood cultures are Gram staining and from growth on a liquid medium like blood agar Bacteroides-bile-esculin (BBE) agar.

• The bacteroid colony can be identified by the appearance of dark colonies with brown-black halos due to the hydrolysis of esculin in a BBE agar medium.

• Antibiotic resistance: Bacteroids are also identified by their resistance to antibiotics like kanamycin, vancomycin, and colistin by using a disk test. It will grow in 20 % bile, produce enzyme catalase, and is indole positive.

• Other Tests: Other tests include fermentation reaction and rapid identification kit test.

What Is the Pathogenesis of Bacteroides Infection?

For an organism to cause disease and become a pathogen, it should possess certain virulence factors.

• Polysaccharides: For a bacteroid, the virulence factors are polysaccharides present in the capsule which blocks opsonophagocytosis and promotes the formation of an abscess.

• Pili and Fimbriae: Both of the structures promote adhesion.

• Enzymes: Bacteroids produce several enzymes like hemolysin, peroxidase, collagenase, neuraminidase, hyaluronidase, heparinase, protease, catalase, and superoxide dismutase. These enzymes protect bacteroids by increasing aerotolerance by defending against oxygen radicals.

What Is Antibiotic Therapy Against Bacteroides?

Treatment involves a single drug in monotherapy or a combination of drugs.

Monotherapy with Carbapenem and combination drugs like Beta-lactam or Beta-lactamase inhibitor combinations like Ampicillin or Sulbactam and Piperacillin or Tazobactam and Metronidazole along with an Aminoglycoside or a third-generation Cephalosporin can be used actively. The second choice of drugs is Clindamycin, Piperacillin, Cefoxitin, Cefotetan, and Ceftizoxime.

What Is Prevotella?

Prevotella also belongs to the phylum Bacteroidetes and the class Bacteroidia. These rod shaped bacteria are gram-negative and non-motile. They occur as single cells and grow in strictly anaerobic conditions. Bacteroides melaninogenicus is recategorized as Prevotella intermedia and Prevotella melaninogenica. They are predominantly found in the mouth and vaginal microbial flora. There are many other strains of Prevotella which are as follows:

• Prevotella bivia.

• Prevotella copri.

• Prevotella dentalis.

• Prevotella denticola.

• Prevotella multiformis.

• Prevotella oralis.

• Prevotella salivae.

• Prevotella oris

What Are the Infections Caused by Prevotella Species?

The infections caused by prevotella are:

• Lung abscess.

• Pulmonary empyema.

• Chronic otitis media.

• Sinusitis.

• Aspiration pneumonia.

They are often isolated from infection sites of abscesses and burns around the mouth, paronychia (nail infection), urinary tract infection, ulcerative colitis, bites, brain abscesses, periodontitis, osteomyelitis, and bacteremia caused due to infections of the upper respiratory tract.

What Is the Pathogenesis of Prevotella Infection?

P. intermedia and P.nigrescens are known to cause inflammatory diseases of gums called acute periodontitis, acute necrotizing ulcerative gingivitis, and pregnancy gingivitis. They cause black pigmentation along with Porphyromonas gingivalis. The virulence factors present in Prevotella species are hemolysins, adhesins, lipopolysaccharide, protease, secretion system exopolysaccharides, and quorum-sensing substances.

What Is the Diagnosis and Treatment for Prevotella Infection?

• Diagnosis of Prevotella is done by phenotypic methods like the observation of growth and its colony morphology on various mediums, cell morphology, gram reactions, and catalase tests.

• They are bile-sensitive and resistant to vancomycin.

• The antibiotic therapy for treating prevotella infection includes Metronidazole, Carbapenems, Cephalosporins, Chloramphenicol, Amoxicillin/Clavulanate, Clindamycin, and Ureidopenicillin. But they are naturally resistant to antibiotics like Aminoglycosides, Trimethoprim, Sulfonamides, Fosfomycin, and Aztreonam.

Conclusion

The genome of circular chromosomes of many species and strains of bacteroides are studied. Research is going on to sequence their species to understand their pathogenesis better. It helps in providing better quality treatment to the patients affected by the infections. Currently, there are no vaccines available for bacteroides. Prevotella is found in abundance in the gut and rumen of cattle and sheep helping in the metabolism of proteins and carbohydrates. But in humans, they can exist as opportunistic pathogens. Prevotella has been an important causative agent in the oral diseases of gums and poses a problem to dentists. Two strains of prevotella namely P.intermedia and P.ruminicola have been completely genome sequenced which has helped to understand their pathogenicity better.