Infectious Disease and Microbiology - Anaerobiospirillum

Basics

Anaerobiospirillum is a genus of uncommon anaerobic gram-negative bacteria. The main species associated with human disease are Anaerobiospirillum succiniciproducens and Anaerobiospirillum thomasii.

These organisms are rarely encountered in clinical practice but can cause invasive disease, particularly in patients with significant underlying illness or impaired immunity.

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Microbiologic Characteristics

Anaerobiospirillum species are:

• Anaerobic

• Gram negative

• Spiral shaped

• Bacillary in form

Their curved or spiral morphology can resemble other spiral gram-negative organisms, but their anaerobic growth requirement helps distinguish them.

Because they are uncommon and may grow slowly or be overlooked, appropriate anaerobic culture techniques are important.

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Epidemiology

Human infection with Anaerobiospirillum is rare.

Most reported cases have occurred sporadically rather than as part of outbreaks.

Disease appears to be more likely in patients who are:

• Immunocompromised

• Chronically ill

• Debilitated

• Medically complex

The true incidence may be underestimated because these organisms are not routinely sought and can be difficult to identify.

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Bacteremia

One of the most important manifestations of Anaerobiospirillum infection is bacteremia.

Patients may present with:

• Fever

• Chills

• Malaise

• Hypotension

• Tachycardia

• Sepsis in severe cases

Bloodstream infection is especially concerning in immunocompromised or severely ill patients.

Because the organism is anaerobic, blood culture systems that adequately support anaerobic growth are necessary for recovery.

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Gastroenteritis

Anaerobiospirillum can also cause gastrointestinal infection.

Symptoms may include:

• Diarrhea

• Abdominal pain

• Cramping

• Nausea

• Fever

Gastroenteritis has been reported particularly in immunosuppressed individuals, although disease can occasionally occur in patients without obvious immune dysfunction.

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Immunocompromised Patients

Patients with weakened host defenses appear to be at increased risk of clinically significant infection.

In these patients, a gastrointestinal infection may have a greater tendency to become invasive or progress to bacteremia.

Therefore, recovery of Anaerobiospirillum from blood or another normally sterile site should be considered clinically significant.

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Diagnosis

Diagnosis depends primarily on:

Anaerobic culture.

Depending on the clinical syndrome, specimens may include:

• Blood

• Stool

• Other normally sterile body fluids

• Tissue specimens

Because the organism is uncommon, specialized identification methods may sometimes be necessary after culture growth.

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Laboratory Identification

The characteristic microscopic appearance is that of a spiral-shaped gram-negative bacillus.

However, morphology alone is not sufficient for definitive identification.

Important laboratory considerations include:

• Proper anaerobic transport of specimens

• Anaerobic incubation

• Biochemical or molecular identification when necessary

Failure to use anaerobic culture conditions can result in missed diagnosis.

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Differential Diagnosis

The differential diagnosis depends on the clinical syndrome.

For spiral or curved gram-negative organisms, considerations may include:

• Campylobacter

• Helicobacter

• Spirillum

• Other anaerobic gram-negative bacilli

For gastroenteritis, other bacterial, viral, and parasitic enteric pathogens must also be considered.

For bacteremia, the differential is broad and includes other gram-negative and anaerobic organisms.

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Treatment

There are limited clinical data defining the optimal antimicrobial regimen for Anaerobiospirillum infection.

Historically, agents with reported activity include:

• Chloramphenicol

• Metronidazole

• Doxycycline

Because susceptibility patterns may vary and evidence is limited, treatment should ideally be guided by antimicrobial susceptibility testing when available.

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Chloramphenicol

Chloramphenicol has historically been used for serious Anaerobiospirillum infections.

Its broad anaerobic activity and good tissue penetration make it a potential option.

However, its use is limited by important adverse effects, including:

• Bone marrow suppression

• Rare aplastic anemia

Therefore, it is generally reserved for situations in which safer alternatives are unsuitable or susceptibility data support its use.

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Metronidazole

Metronidazole has activity against many anaerobic organisms and has been used in Anaerobiospirillum infections.

It may be considered particularly when an anaerobic gastrointestinal source is suspected.

Clinical response and susceptibility data should guide therapy whenever possible.

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Doxycycline

Doxycycline is another possible treatment option.

Its role is less well established than that of more traditional anaerobic agents, but it may be useful when the isolate is susceptible and the clinical setting is appropriate.

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Supportive Management

In gastroenteritis, supportive care remains important.

Management may include:

• Oral or intravenous fluid replacement

• Correction of electrolyte abnormalities

• Monitoring for dehydration

• Management of sepsis when bacteremia is present

Patients with hypotension or systemic toxicity may require hospitalization and aggressive supportive care.

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Prognosis

The prognosis depends largely on:

• Host immune status

• Presence of bacteremia

• Severity of underlying disease

• Speed of diagnosis

• Appropriateness of antimicrobial therapy

Uncomplicated gastrointestinal infection may resolve with appropriate treatment, whereas bloodstream infection can be serious in immunocompromised patients.

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High-Yield Clinical Pattern

Immunocompromised patient

Diarrhea or gastrointestinal symptoms

Anaerobic spiral-shaped gram-negative bacillus

→ Consider Anaerobiospirillum.

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High-Yield Bacteremia Pattern

Fever and sepsis

Anaerobic blood culture positive

Spiral gram-negative bacillus

→ Think Anaerobiospirillum, particularly when gastrointestinal symptoms are also present.

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Exam Essentials

Genus:

→ Anaerobiospirillum

Important species:

→ A. succiniciproducens and A. thomasii

Morphology:

→ Spiral-shaped gram-negative bacillus

Oxygen requirement:

→ Anaerobic

Frequency:

→ Rare human pathogen

Major infections:

→ Bacteremia and gastroenteritis

Important risk group:

→ Immunocompromised patients

Diagnosis:

→ Anaerobic culture

Historically used treatments:

→ Chloramphenicol and metronidazole

Additional possible treatment:

→ Doxycycline

Key laboratory pearl:

→ Failure to use proper anaerobic culture conditions may cause the organism to be missed.