Classification
Gram-negative bacteria:
Most bacteria can be classified according to a lab procedure called Gram staining.
Bacteria with cell walls that have a thin layer of peptidoglycan do not retain the crystal violet stain utilized in Gram staining. These bacteria do, however, retain the safranin counterstain and thus appear as pinkish-red on the stain, making them gram negative. These bacteria can be further classified according to morphology (diplococci, curved rods, bacilli, and coccobacilli) and their ability to grow in the presence of oxygen (aerobic versus anaerobic). The bacteria can be more narrowly identified by growing them on specific media (triple sugar iron (TSI) agar) where their enzymes can be identified (urease, oxidase) and their ability to ferment lactose can be tested.
* Stains poorly on Gram stain
** Pleomorphic rod/coccobacillus
*** Require special transport media
General Characteristics
Basic features
- Gram-negative bacilli
- Pleomorphic: appear as either straight rods or coccobacilli
- 0.5–0.8 μm in diameter and 1.5–4.5 μm long
- Anaerobes, although tolerate exposure to oxygen
- Encapsulated
- Ferment carbohydrates:
- Including complex plant polysaccharides indigestible to the host
- Allows for commensal/symbiotic relationship between the bacteria and host
- Part of indigenous bacterial flora of mucosal surfaces, but opportunistic pathogens
Micrograph of Bacteroides
Photomicrograph depicts the gram-negative bacterium Bacteroides fragilis.
Virulence factors
- Adherence to host tissue:
- Fimbriae: Long, hair-like projections
- Lipopolysaccharide (LPS):
- Weakly endotoxic
- Primary role is adherence to mucosal surfaces
- Protection from host immune response:
- Superoxide dismutase and catalase:
- Neutralize oxygen radicals created by the immune system
- Key for pathogenesis in areas of the body not free from oxygen
- Capsule:
- Protects from complement-mediated phagocytosis
- Leads to abscess formation
- Polysaccharides on the capsule can be altered to evade the host immune response.
- Conjugative transposons: aid in the transfer of antibiotic resistance genes
- Superoxide dismutase and catalase:
- Destruction of host tissue:
- Hyaluronidase and chondroitin sulfatase:
- Destroy brush border enzymes in the intestine
- Make it harder for the host to absorb nutrients
- More nutrients available for bacteria
- Enterotoxin:
- Destroys tight junctions in the intestinal epithelium
- Leads to diarrhea
- Neuraminidase: cleaves polysaccharides, freeing up glucose for bacteria
- Hyaluronidase and chondroitin sulfatase:
Pathogenesis
Reservoir
- Large intestine of humans
Transmission and infection
Transmission and infection from the translocation of bacteria from mucosal surfaces to other tissues may be due to:
- Blunt trauma
- Intestinal disease
- Penetrating wounds
- Abdominal surgery
- Ruptured appendix
Pathogenesis of Bacteroides fragilis
The pathogen contains an antiphagocytic capsule, which adheres to the peritoneal surface, resulting in abscess formation. Surgery and trauma accentuate the problem because they disrupt the abscess, allowing for the endogenous spread of the organism into the bloodstream. When the pathogen spreads into the bloodstream, the pathogen releases degradative enzymes that destroy the host’s cells.
Diseases Caused by Bacteroides fragilis
Bacteroides fragilis (B. fragilis) is naturally found in the small intestines as part of the natural flora. B. fragilis is an opportunistic pathogen that rarely infects the gastrointestinal system, more often causing disease when bacteria escape the small intestine.
| Condition | Characteristics | Infection type |
|---|---|---|
| Intra-abdominal | Intra-abdominal infection |
|
| Perforated appendicitis |
| |
| Gastrointestinal | Inflammatory diarrhea |
|
| Gynecologic | Pelvic infections |
|
| Fallopian tube and ovarian abscesses |
| |
| Skin and soft tissue | Diabetic foot infection |
|
| Animal bite infections |
| |
| Brain | Meningitis and brain abscess |
|
| Disseminated | Bacteremia |
|
Diagnosis and Management
Diagnosis
- History:
- Predisposing factors:
- Recent abdominal surgery
- Trauma
- Malignancy
- Immunocompromised state
- Known history of Bacteroides infections
- Predisposing factors:
- Physical exam:
- Presence of pyogenic abscesses
- Foul smell is often associated with anaerobic infections.
- Laboratory testing:
- Appropriate tissue samples must be utilized to culture Bacteroides:
- Blood cultures and fine-needle aspirates of infectious material
- Requires an anaerobic environment to propagate
- Defining laboratory characteristics:
- Isolated as a single agent, but usually seen with other anaerobes
- Gram –
- Grows on blood agar and Bacteroides-bile-esculin (BBE) agar
- Kanamycin, vancomycin, and colistin-resistant
- Grows in 20% bile
- Catalase producing
- Variably indole positive
- Appropriate tissue samples must be utilized to culture Bacteroides:
Management
- Prevention:
- Planned prophylactic antibiotics for abdominal surgeries
- Antibiotic therapy if contamination from the colon is suspected
- Treatment:
- When present, the drainage of abscesses is the primary form of treatment
- Given the polymicrobial nature of most infections, antibiotic therapy should be directed to multiple anaerobes:
- Antibiotic of choice for all anaerobic infections is metronidazole.
- Effective antibiotics include: piperacillin/tazobactam, meropenem
- Resistant to penicillin due to the production of beta-lactamase
- Moxifloxacin previously utilized; now, resistance reported in up to 57% of cases
References
- Wexler, H.M. (2007). Bacteroides: The good, the bad, and the nitty-gritty. Clinical microbiology reviews, 20(4), 593–621. https://doi.org/10.1128/CMR.00008-07
- Moncrief, J.S., Obiso, R. Jr., et al. (1995). The enterotoxin of Bacteroides fragilis is a metalloprotease. Infect Immun. 63(1), 175–81. https://doi.org/10.1128/IAI.63.1.175-181.1995
- Franco, A.A. (2004). The Bacteroides fragilis pathogenicity island is contained in a putative novel conjugative transposon. J Bacteriol, 186(18), 6077–92. https://pubmed.ncbi.nlm.nih.gov/15342577/
- Takesue, Y., Kusachi, S., et al. (2018). Antimicrobial susceptibility of common pathogens isolated from postoperative intra-abdominal infections in Japan. J Infect Chemother, 24(5), 330–340. https://doi.org/10.1016/j.jiac.2018.02.011
- Snydman, D.R., Jacobus, N.V., et al. (2010). Lessons learned from the anaerobe survey: Historical perspective and review of the most recent data (2005–2007). Clin Infect Dis. 50 Suppl(1), S26–33. https://doi.org/10.1086/647940
