Bacteroides fragilis - Classification, Morphology, Culture, Virulence factor, Biochemical characteristics

Classification of Bacteroides fragilis

Domain: Bacteria

Phylum: Bacteroidota

Class: Bacteroidia

Order: Bacteroidales

Family: Bacteroidaceae

Genus: Bacteroides

Species: fragilis

Morphology of Bacteroides fragilis

Bacteroides fragilis are obligate, anaerobic, gram-negative rods in morphology. They are non-sporing, non-motile, normal flora of the human GI tract (typically isolated from mucosal surfaces of the oral cavity and GI tract).

In general, an infection caused by Bacteroides fragilis group occurs below the diaphragm.

Cultural characteristics of Bacteroides fragilis

Once cultured in the laboratory, Bacteroides fragilis forms non-hemolytic grey colonies of 1-3mm diameter in anaerobic BA.

In Bacteroides Bile Esculin medium (BBE), it gives glistering convex light to grey-colored colonies surrounded by a grey zone.

They can also be cultured in a liquid medium (Thioglycolate broth), and Robertson’s cooked meat medium.

Biochemical characteristics of Bacteroides fragilis

The biochemical characteristics of Bacteroides fragilis are:

• ferments sucrose

• grows in presence of 20% bile

• Indole: Negative

• Urease: Negative

• Catalase: Positive

• Oxidase: variable

• Esculin hydrolysis: Positive

• Arabinose: Negative

• Glucose: Positive

• Lactose: Positive

• Maltose: Positive

• Rhamnose: Negative

• Trehalose: Negative

• Salicin: Negative

Virulence factors of Bacteroides fragilis

The virulence factors of Bacteroides fragilis include:

1. Polysaccharide capsule: prevents phagocytosis and complement-mediated cell lysis

2. Fimbriae: attachment

3. Endotoxin: LPS (Lipo-Poly saccharide)

4. β- lactamase: Antibiotic resistance

5. Succinic acid: inhibit phagocytosis

6. Enzyme: mediates tissue damage