Infectious Disease and Microbiology – Fusobacterium Species

Overview

Fusobacterium species are anaerobic Gram-negative bacilli that normally colonize the oral cavity and gastrointestinal tract. Most infections are endogenous, developing when organisms from the patient’s normal flora invade normally sterile tissues.

They commonly participate in polymicrobial abscesses and necrotizing infections. A particularly important species is Fusobacterium necrophorum, which is classically associated with Lemierre syndrome—postpharyngitis sepsis with internal jugular vein septic thrombophlebitis and metastatic infection.

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Important Species

Species traditionally included in this group include:

• Fusobacterium alocis

• Fusobacterium mortiferum

• Fusobacterium necrophorum

• Fusobacterium nucleatum

• Fusobacterium periodonticum

• Fusobacterium sulci

• Fusobacterium ulcerans

• Fusobacterium varium

• Other Fusobacterium species

F. necrophorum and F. nucleatum are particularly important in human infectious disease.

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Microbiologic Characteristics

Fusobacterium species are:

• Gram-negative bacilli

• Obligate anaerobes

• Common members of normal oral and gastrointestinal flora

• Important causes of endogenous anaerobic infection

The organisms are often described morphologically as slender or fusiform Gram-negative rods.

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Incubation and Source of Infection

A conventional incubation period is generally not applicable because most infections originate from the patient’s own microbial flora.

The typical sequence is:

Normal oral or bowel colonization

→

Mucosal disruption or local infection

→

Invasion of deeper tissue

→

Abscess, bacteremia, or metastatic infection

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Epidemiology

Fusobacterium species commonly colonize the:

• Oral cavity

• Oropharynx

• Gastrointestinal tract

Their presence as normal flora means that infection commonly develops when anatomic barriers are disrupted.

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Clinical Infections

Fusobacterium species can cause:

• Cervicofacial infections

• Pleuropulmonary infections

• Intra-abdominal infections

• Pelvic infections

• Soft-tissue infections

• Surgical wound infections

• Bite-wound infections

• Bacteremia and sepsis

• Endocarditis

Abscesses are common and are frequently polymicrobial.

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Cervicofacial Infection

Because Fusobacterium commonly inhabits the oral cavity, it can participate in cervicofacial and odontogenic infections.

These infections may include:

• Dental and periodontal infection

• Deep neck-space infection

• Peritonsillar infection

• Cervicofacial abscess formation

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Pleuropulmonary Infection

Aspiration of oral secretions containing anaerobic organisms may result in:

• Aspiration pneumonia

• Lung abscess

• Necrotizing pulmonary infection

• Empyema

These infections are frequently polymicrobial and may include other anaerobic components of the oral flora.

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Intra-Abdominal and Pelvic Infection

Because fusobacteria can form part of gastrointestinal flora, they may participate in:

• Intra-abdominal abscesses

• Peritonitis

• Pelvic abscesses

• Other polymicrobial abdominal infections

Source control is often important when an abscess is present.

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Soft-Tissue and Wound Infection

Fusobacterium species can cause soft-tissue and wound infections, including infections following:

• Surgery

• Human or animal bites

• Tissue trauma

These infections commonly involve multiple aerobic and anaerobic organisms.

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Lemierre Syndrome

The most important high-yield association is:

Fusobacterium necrophorum → Lemierre syndrome

This condition classically develops in an otherwise healthy adolescent or young adult following pharyngitis or tonsillitis.

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Pathogenesis of Lemierre Syndrome

The classic progression is:

Acute pharyngitis/tonsillitis

→

Spread into the lateral pharyngeal tissues

→

Internal jugular vein septic thrombophlebitis

→

Fusobacterial bacteremia

→

Septic emboli to distant organs

The lungs are particularly commonly involved.

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Clinical Features of Lemierre Syndrome

Patients may initially have:

• Severe sore throat

• Fever

• Tonsillitis or pharyngitis

They may subsequently develop:

• Persistent or recurrent high fever

• Sepsis

• Unilateral neck pain or swelling

• Tenderness along the internal jugular vein

• Respiratory symptoms from septic pulmonary emboli

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Septic Pulmonary Emboli

Infected thrombus within the internal jugular vein can release septic emboli into the bloodstream.

These frequently travel to the lungs and may produce:

• Multiple pulmonary nodules

• Cavitary lesions

• Pulmonary abscesses

• Pleural infection

• Respiratory distress

This pulmonary pattern is an important clue to Lemierre syndrome.

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Other Metastatic Abscesses

Hematogenous dissemination may also produce abscesses involving:

• Bones

• Joints

• Central nervous system

• Other distant organs

Thus, F. necrophorum can produce a severe metastatic septic illness after an initially localized throat infection.

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Endocarditis

Fusobacterium species can rarely cause infective endocarditis.

Persistent bacteremia or appropriate cardiac findings should prompt consideration of endovascular infection.

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Diagnosis

Diagnosis is established using:

Anaerobic culture

Appropriate specimens may include:

• Blood

• Abscess material

• Pleural fluid

• Deep tissue specimens

Proper anaerobic collection and transport are important for recovery of the organism.

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Diagnosis of Lemierre Syndrome

When Lemierre syndrome is suspected, evaluation typically aims to demonstrate:

Septic thrombosis of the internal jugular vein

along with evidence of infection.

Imaging of the neck can demonstrate the thrombosed vein, while chest imaging may identify septic pulmonary emboli or abscesses.

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Treatment

The source lists:

Metronidazole

and

Penicillin G

as principal antimicrobial options.

Therapy should provide adequate anaerobic coverage and be guided by the clinical syndrome and susceptibility information when available.

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Additional Treatment

Additional agents listed in the source include:

• Clindamycin

• Cefotetan

• Cefoxitin

• Imipenem

• Meropenem

• Chloramphenicol

For severe polymicrobial infection, antimicrobial therapy should also adequately cover other likely pathogens.

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Source Control

Abscess-forming infections frequently require source control in addition to antimicrobial therapy.

This may include:

• Drainage of abscesses

• Surgical debridement

• Management of infected wounds

• Treatment of the underlying dental or abdominal source

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High-Yield Clinical Pattern

Previously healthy adolescent or young adult

Recent pharyngitis

High fever/sepsis

Unilateral neck pain or swelling

Internal jugular vein thrombophlebitis

Multiple septic pulmonary emboli

→ Think Lemierre syndrome due to Fusobacterium necrophorum

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Exam Essentials

Genus: Fusobacterium

Type: Gram-negative bacillus

Oxygen requirement: Anaerobic

Morphology: Often slender/fusiform rods

Normal habitat: Oral and gastrointestinal flora

Usual source: Endogenous infection

Abscesses: Frequently polymicrobial

Major infections: Cervicofacial, pulmonary, abdominal, pelvic, soft-tissue and wound infections

Key species: F. necrophorum

Classic syndrome: Lemierre syndrome

Initial infection: Pharyngitis/tonsillitis

Major vascular complication: Internal jugular vein septic thrombophlebitis

Major metastatic site: Lungs → septic pulmonary emboli/abscesses

Other metastatic sites: Bone and CNS

Diagnosis: Anaerobic culture; imaging is important in Lemierre syndrome

Treatment in source: Metronidazole or penicillin G

Additional agents: Clindamycin, cephamycins, carbapenems

Management principle: Appropriate anaerobic therapy + source control

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Key clinical pearl: The classic association is Fusobacterium necrophorum → Lemierre syndrome: pharyngitis in a young patient followed by sepsis, internal jugular vein septic thrombophlebitis, and septic pulmonary emboli.