Infectious Disease and Microbiology - Bacteroides Species

Basics

Bacteroides species are anaerobic Gram-negative bacilli that form an important component of the normal human microbiota, particularly within the gastrointestinal tract. Although they normally exist as harmless commensals, they can become important opportunistic pathogens when mucosal barriers are disrupted.

The Bacteroides fragilis group is particularly important clinically and is frequently involved in polymicrobial infections containing both aerobic and anaerobic organisms.

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Important Species

Clinically recognized Bacteroides species include:

• Bacteroides fragilis

• B. thetaiotaomicron

• B. ovatus

• B. vulgatus

• B. uniformis

• B. caccae

• B. eggerthii

• B. stercoris

• B. distasonis

• B. merdae

• B. capillosus

• B. putredinis

• B. tectum

• B. ureolyticus

• Other related species

Taxonomy within this group has changed over time, so some organisms historically classified as Bacteroides have subsequently been reassigned to other genera.

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Microbiologic Characteristics

Bacteroides species are:

• Gram-negative bacilli

• Obligate anaerobes

• Non-spore-forming organisms

Many species are normal inhabitants of the colon.

Despite representing only part of the normal intestinal microbiota, members of the B. fragilis group are disproportionately important causes of clinically significant anaerobic infections.

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Epidemiology

Bacteroides infections occur worldwide.

Because these organisms normally colonize the gastrointestinal tract, most infections are endogenous. Disease typically develops when disruption of a mucosal barrier allows organisms to enter normally sterile tissues.

Common predisposing events include:

• Gastrointestinal perforation

• Abdominal surgery

• Trauma

• Bowel ischemia

• Appendicitis

• Diverticulitis

• Pelvic infection

• Malignancy involving the gastrointestinal tract

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Polymicrobial Infection

A major characteristic of Bacteroides infection is its association with mixed aerobic-anaerobic infections.

Rather than occurring alone, Bacteroides may be isolated together with organisms such as enteric Gram-negative bacilli and other components of the normal gastrointestinal flora.

Therefore:

Disruption of bowel integrity + abscess formation + polymicrobial infection

→ strongly suggests involvement of anaerobic organisms such as Bacteroides.

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Infections

Intra-Abdominal Infection

Intra-abdominal disease is among the most important manifestations of Bacteroides infection.

Clinical syndromes include:

• Intra-abdominal abscess

• Peritonitis

• Appendiceal infection

• Diverticular infection

• Postoperative abdominal infection

• Infection following gastrointestinal perforation

B. fragilis is particularly important in intra-abdominal abscess formation.

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Pelvic Infection

Bacteroides species may participate in polymicrobial pelvic infections, including infections involving the female reproductive tract.

Possible manifestations include:

• Pelvic abscess

• Postoperative pelvic infection

• Other mixed anaerobic pelvic infections

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Pleuropulmonary Infection

Anaerobic pulmonary infection may occur following aspiration of organisms from the upper aerodigestive tract.

Possible manifestations include:

• Aspiration pneumonia

• Lung abscess

• Necrotizing pulmonary infection

• Empyema

These infections are frequently polymicrobial.

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Cervicofacial Infection

Bacteroides species may contribute to mixed anaerobic infections involving the oral cavity, head, and neck.

Possible sites include:

• Dental infections

• Deep neck infections

• Cervicofacial abscesses

These infections often contain several anaerobic and aerobic organisms simultaneously.

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Brain Abscess

Anaerobic bacteria, including Bacteroides, can occasionally contribute to brain abscesses.

Potential sources include:

• Dental infection

• Chronic sinusitis

• Otogenic infection

• Hematogenous dissemination

Brain abscess requires antimicrobial therapy with adequate CNS penetration together with drainage when appropriate.

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Bacteremia and Sepsis

Disruption of gastrointestinal or pelvic barriers may allow Bacteroides organisms to enter the bloodstream.

Patients may develop:

• Bacteremia

• Sepsis

• Septic shock in severe cases

Detection of Bacteroides bacteremia should prompt investigation for an underlying abdominal, pelvic, or gastrointestinal source.

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Skin and Soft-Tissue Infection

Bacteroides species can participate in polymicrobial infections involving:

• Surgical wounds

• Traumatic wounds

• Deep soft tissues

• Abscesses

• Necrotic tissue

Anaerobic infection should be considered particularly when there is tissue necrosis, abscess formation, or infection near gastrointestinal or genitourinary structures.

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Diagnosis

Anaerobic Culture

Diagnosis is established primarily through:

Anaerobic culture

Appropriate specimen collection is essential because exposure to oxygen may reduce recovery of anaerobic organisms.

High-quality specimens include:

• Abscess aspirates

• Deep tissue specimens

• Blood cultures

• Normally sterile body fluids

Superficial swabs are generally less useful because they may contain colonizing organisms and may not accurately represent the organisms causing deep infection.

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Treatment

Metronidazole

Metronidazole has traditionally been one of the most reliable agents against clinically important Bacteroides, particularly the B. fragilis group.

It provides strong anaerobic activity and is commonly incorporated into treatment regimens for intra-abdominal and pelvic infections.

However, antimicrobial susceptibility can vary, and resistance should be considered in refractory or severe infections.

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Carbapenems

Carbapenems provide broad coverage against anaerobic and aerobic organisms.

Options include:

• Imipenem

• Meropenem

• Ertapenem

These agents are particularly useful for complicated polymicrobial intra-abdominal infections when broad-spectrum treatment is required.

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Beta-Lactam/Beta-Lactamase Inhibitor Combinations

Important options include:

• Piperacillin-tazobactam

• Ampicillin-sulbactam

Historically, ticarcillin-clavulanate has also been used.

These combinations provide activity against many anaerobic organisms while simultaneously covering several aerobic pathogens involved in polymicrobial infections.

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Cephamycins

Agents with anaerobic activity include:

• Cefoxitin

• Cefotetan

However, resistance among Bacteroides species limits their reliability in some settings.

They should therefore not automatically be assumed to provide adequate treatment for every serious Bacteroides infection.

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Tigecycline

Tigecycline has activity against many anaerobic organisms, including members of the B. fragilis group.

Its use depends on the site and severity of infection as well as susceptibility patterns and patient-specific considerations.

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Antimicrobial Resistance

Antimicrobial resistance is increasingly important among Bacteroides species.

The B. fragilis group commonly produces beta-lactamases, making many unprotected beta-lactam antibiotics unreliable.

Resistance has also been reported to:

• Clindamycin

• Cephamycins

• Metronidazole, although resistance remains comparatively uncommon in many settings

Because susceptibility patterns vary geographically and over time, serious infections should be managed according to contemporary local susceptibility data whenever available.

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Clindamycin Resistance

Clindamycin was historically used extensively for anaerobic infections, but resistance among Bacteroides has become sufficiently common that it is no longer reliably active empirically in many settings.

Therefore:

Do not automatically assume that clindamycin covers the B. fragilis group.

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Source Control

Antibiotics alone may be insufficient for many Bacteroides infections.

Effective management frequently requires source control.

Examples include:

• Drainage of an intra-abdominal abscess

• Drainage of a pelvic abscess

• Removal of infected or necrotic tissue

• Surgical repair of gastrointestinal perforation

• Management of bowel ischemia

• Removal of infected foreign material when appropriate

This principle is particularly important in intra-abdominal infection.

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Prevention

Because many Bacteroides infections originate from the patient’s own intestinal flora, prevention focuses on reducing opportunities for organisms to enter sterile tissues.

Important measures include:

• Appropriate surgical technique

• Timely management of bowel perforation

• Prevention and treatment of intra-abdominal contamination

• Appropriate perioperative antimicrobial prophylaxis when indicated

• Proper wound care

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High-Yield Clinical Pattern

Bowel perforation

Polymicrobial intra-abdominal infection

Abscess formation

→ Think Bacteroides fragilis and other anaerobic organisms.

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High-Yield Treatment Pattern

Serious Bacteroides infection

→ Ensure anaerobic antimicrobial coverage

Common options:

→ Metronidazole

→ Piperacillin-tazobactam

→ Carbapenem

PLUS

→ Drain the abscess or otherwise control the source when necessary

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Exam Essentials

Organism:

→ Bacteroides species

Microbiology:

→ Anaerobic Gram-negative bacilli

Most clinically important group:

→ Bacteroides fragilis group

Normal habitat:

→ Gastrointestinal tract

Typical infections:

→ Intra-abdominal and pelvic infections

Characteristic infection pattern:

→ Polymicrobial infection with abscess formation

Other infections:

→ Pleuropulmonary infection, cervicofacial infection, brain abscess, bacteremia/sepsis, and skin or surgical-wound infection

Diagnosis:

→ Anaerobic culture

Classic antimicrobial:

→ Metronidazole

Broad-spectrum alternatives:

→ Carbapenems or beta-lactam/beta-lactamase inhibitor combinations

Important resistance issue:

→ Increasing resistance, particularly to clindamycin and some cephamycins

Critical management principle:

→ Source control

Key clinical pearl: When gastrointestinal contents escape into normally sterile tissue, particularly after bowel perforation or abdominal surgery, Bacteroides fragilis is one of the most important anaerobic pathogens to consider.