Infectious Disease and Microbiology – Eubacterium Species

Overview

Eubacterium species are anaerobic, pleomorphic Gram-positive bacilli that form part of the normal human microbiota. Because these organisms normally colonize the body, their recovery from a clinical specimen does not automatically establish that they are causing infection.

When pathogenic, Eubacterium species are commonly associated with polymicrobial infections, particularly abscesses, periodontal disease, and infections arising from sites containing anaerobic flora.

Important Species

Species described in this group include:

• Eubacterium lentum

• Eubacterium nodatum

• Eubacterium timidum

• Other Eubacterium species

Some organisms historically classified as Eubacterium have undergone taxonomic reclassification, so older names may still appear in infectious-disease literature.

Microbiologic Characteristics

Eubacterium species are:

• Anaerobic

• Gram-positive bacilli

• Pleomorphic in appearance

• Components of normal human flora

Their anaerobic nature means that specimens must be collected and transported appropriately if culture is required.

Epidemiology

Eubacterium species are members of the normal human microbiota.

They may inhabit sites such as the gastrointestinal tract and oral cavity.

Because colonization is common, isolation of an organism must always be interpreted in the context of the patient’s clinical syndrome and the specimen source.

Colonization vs. Infection

A particularly important principle is:

Positive culture ≠ necessarily active infection

Isolation from a normally colonized site may simply represent normal flora or contamination.

Evidence for true infection is stronger when the organism is recovered from a normally sterile site, particularly when accompanied by compatible clinical findings.

Clinical Infections

When Eubacterium species cause disease, infections may include:

• Abscesses

• Periodontal infections

• Septic arthritis

• Gynecologic infections such as endometritis

• Other anaerobic infections

These infections are almost always polymicrobial, meaning other bacterial species are frequently present.

Abscesses and Mixed Infections

Eubacterium species may be recovered from abscesses and infections at various anatomical sites.

A typical pattern is:

Disruption of mucosal barrier

→

Normal anaerobic flora enters deeper tissue

→

Polymicrobial abscess or invasive infection

This is characteristic of many infections caused by endogenous anaerobic flora.

Periodontal Disease

Certain Eubacterium species have been associated with periodontal disease.

The oral cavity contains a complex anaerobic microbiome, so periodontal infections are usually polymicrobial rather than caused by a single organism.

Eubacterium lentum

The source associates Eubacterium lentum with:

Septic arthritis in patients with colonic lesions

This association suggests that disruption of the gastrointestinal mucosal barrier may permit organisms from intestinal flora to enter the bloodstream and subsequently involve a joint.

Eubacterium nodatum

The source associates Eubacterium nodatum with:

Endometritis in women using an intrauterine device (IUD)

This represents another example of an anaerobic organism associated with infection following alteration or disruption of normal mucosal environments.

Diagnosis

Diagnosis is established by:

Culture using appropriate anaerobic media

Successful isolation requires appropriate specimen collection and maintenance of anaerobic conditions during transport and processing.

Interpretation of Culture

Because Eubacterium species are normal flora, laboratory isolation should be interpreted together with:

• Clinical manifestations

• Anatomical source of the specimen

• Presence of abscess or tissue inflammation

• Recovery from a normally sterile site

• Presence of additional organisms

The organism’s presence alone does not prove causation.

Treatment

The source lists:

Penicillin G

or

A cephamycin

as treatment options.

It also lists:

Metronidazole

for anaerobic coverage.

Additional Treatment

Other antimicrobial agents described in the source include:

• Imipenem

• Meropenem

• Clindamycin

• Ureidopenicillins

For clinically significant infection, antimicrobial selection should take into account the susceptibility pattern and other organisms present in a polymicrobial infection.

Source Control

Because these organisms frequently occur in abscesses and mixed anaerobic infections, antimicrobial therapy may need to be combined with appropriate source control.

This can include drainage of an abscess, debridement of infected tissue, or management of an underlying anatomical source.

High-Yield Clinical Pattern

Anaerobic Gram-positive bacillus

• ●

Normally part of human flora

• ●

Recovered from an abscess or other polymicrobial infection

→ Think Eubacterium species

Important Interpretation Pattern

Eubacterium isolated from a colonized site

→ May represent normal flora

Eubacterium isolated from a sterile site + compatible infection

→ More supportive of true invasive infection

Exam Essentials

Genus: Eubacterium

Type: Pleomorphic Gram-positive bacillus

Oxygen requirement: Anaerobic

Normal habitat: Part of normal human flora

Major infection pattern: Polymicrobial anaerobic infection

Common manifestations: Abscesses and periodontal disease

E. lentum: Associated in the source with septic arthritis in patients with colonic lesions

E. nodatum: Associated in the source with endometritis in women using an IUD

Diagnosis: Anaerobic culture

Important diagnostic issue: Isolation does not necessarily indicate infection

Treatment in source: Penicillin G, cephamycins, or metronidazole

Additional agents: Carbapenems, clindamycin, or ureidopenicillins

Management principle: Consider source control for abscesses

Key clinical pearl: Because Eubacterium species are part of the normal human flora, a positive culture does not by itself establish disease; true infections are typically anaerobic, polymicrobial infections, especially abscesses and infections arising after disruption of normal mucosal barriers.