Infectious Disease and Microbiology – Methylobacterium Species

Overview

Methylobacterium species are aerobic Gram-negative bacilli that are widely distributed in the environment but are very rare causes of human infection. They are distinctive because many species produce characteristic pink-pigmented colonies in culture.

Human disease is usually opportunistic and occurs particularly in immunocompromised patients or in association with indwelling medical devices. Important manifestations include bacteremia and peritoneal dialysis-associated peritonitis.

Classification

Genus: Methylobacterium

Important species:

• M. extorquens

• M. mesophilicum

• Other Methylobacterium species

Type: Aerobic Gram-negative bacillus

Historical Classification

Methylobacterium species were previously classified under the genus:

Protomonas

Therefore, older microbiology literature may refer to these organisms as:

Protomonas species

Microbiologic Characteristics

Methylobacterium organisms are:

• Aerobic

• Gram-negative bacilli

• Environmental organisms

• Generally slow-growing

• Characteristically associated with pink-pigmented colonies

The pink coloration is an important laboratory clue.

Pink-Pigmented Colonies

A particularly characteristic feature of Methylobacterium is the production of:

Pink to reddish-pink colonies

For this reason, these organisms are sometimes described as:

Pink-pigmented facultative methylotrophs

The unusual pigmentation can help distinguish them from many other nonfermenting environmental Gram-negative bacilli.

Incubation Period

The incubation period is:

Unknown

Because infections are usually opportunistic and often associated with medical devices or environmental exposure, a clearly defined incubation period is generally unavailable.

Epidemiology

Human infection is:

Very rare

However, recognition of Methylobacterium infections has increased with improved microbiologic identification techniques and greater awareness of opportunistic environmental organisms.

Environmental Reservoir

Methylobacterium species are widely distributed in:

• Water

• Soil

• Plants

• Moist environmental surfaces

Their ability to persist in water-associated environments is particularly relevant to healthcare-associated infection.

Risk Factors

Invasive infection occurs primarily in patients with:

• Immunosuppression

• Malignancy

• Indwelling vascular catheters

• Peritoneal dialysis catheters

• Other implanted medical devices

Thus, isolation of Methylobacterium from a normally sterile specimen in a vulnerable patient should not automatically be dismissed as contamination.

Bacteremia

One of the most important clinical manifestations is:

Bacteremia

This is particularly associated with immunocompromised individuals and patients with intravascular devices.

Catheter-Associated Infection

Because Methylobacterium is an environmental organism with an affinity for moist environments, infection can occur in association with:

Central venous catheters

Persistent bacteremia may indicate colonization or infection of an indwelling device.

High-Yield Bacteremia Pattern

Immunocompromised patient

• ●

Indwelling vascular catheter

• ●

Gram-negative bacillus

• ●

Pink-pigmented colonies

→ Think Methylobacterium

Peritoneal Dialysis-Associated Peritonitis

Methylobacterium species may cause:

Peritonitis

in patients undergoing:

Peritoneal dialysis

The dialysis catheter provides a potential route for introduction and persistence of the organism.

Clinical Features

Peritoneal dialysis-associated infection may present with:

• Abdominal pain

• Fever

• Cloudy dialysis fluid

• Peritoneal inflammatory findings

Culture of the peritoneal dialysis fluid can establish the microbiologic diagnosis.

Opportunistic Infection

Methylobacterium is considered a:

Low-virulence opportunistic pathogen

Most clinically significant infections occur when normal host defenses are impaired or when foreign material provides a surface for persistent infection.

Colonization vs. True Infection

Because Methylobacterium is an environmental organism, a positive culture should be interpreted according to the clinical setting.

Evidence supporting true infection includes:

• Repeated positive cultures

• Isolation from a normally sterile site

• Compatible clinical illness

• Immunosuppression

• Presence of an infected catheter or device

Diagnosis

The primary diagnostic method is:

Culture

Identification is based on growth characteristics and microbiologic testing.

Laboratory Clue

A particularly useful clue is:

Pink-pigmented colonies

When an unusual aerobic Gram-negative bacillus with pink pigmentation is recovered from blood or another sterile specimen, Methylobacterium should be considered.

Modern Identification

Because unusual environmental Gram-negative bacilli can be difficult to distinguish using conventional biochemical methods, specialized identification techniques may be helpful.

These can include:

• MALDI-TOF mass spectrometry

• Molecular identification methods

Accurate species identification may be important when determining clinical significance and antimicrobial susceptibility.

Treatment

The source lists:

Trimethoprim–sulfamethoxazole (TMP-SMX)

as treatment.

Because Methylobacterium infections are uncommon and susceptibility patterns can vary, treatment should ideally be guided by:

Antimicrobial susceptibility testing

Source Control

For infections associated with:

Central venous catheters

or

Peritoneal dialysis catheters

management may require consideration of:

Catheter removal

in addition to antimicrobial therapy, particularly when infection persists.

Treatment Principle

Clinically significant Methylobacterium infection

↓

Obtain cultures and susceptibility testing

↓

Administer an active antimicrobial

• ●

Evaluate infected catheter/device

↓

Remove or replace device when necessary

Prevention

Because these organisms are environmental and associated with water and moist surfaces, prevention in healthcare settings depends on:

• Appropriate catheter care

• Aseptic technique

• Proper handling of dialysis equipment

• Prevention of contamination of medical fluids and devices

• Appropriate infection-control practices

Methylobacterium vs. Pseudomonas

Methylobacterium

→ Aerobic Gram-negative bacillus

→ Environmental organism

→ Pink-pigmented colonies

→ Rare opportunistic infection

→ Catheter-associated bacteremia and peritonitis

Pseudomonas aeruginosa

→ Aerobic Gram-negative bacillus

→ Common opportunistic pathogen

→ Blue-green pigments may occur

→ Pneumonia, bacteremia, UTI, wound infection, and device-associated infection

The distinctive pink pigmentation is an important clue favoring Methylobacterium.

High-Yield Clinical Pattern

Immunocompromised patient

• ●

Central venous or peritoneal dialysis catheter

• ●

Bacteremia or peritonitis

• ●

Pink-pigmented aerobic Gram-negative bacillus

→ Think Methylobacterium species

Exam Essentials

Genus: Methylobacterium

Important species: M. extorquens, M. mesophilicum

Older name: Protomonas species

Type: Aerobic Gram-negative bacillus

Characteristic culture finding: Pink-pigmented colonies

Distribution: Environmental; associated with soil, plants, and water

Frequency of human infection: Very rare

Major risk group: Immunocompromised patients

Important device association: Vascular and peritoneal dialysis catheters

Major infections: Bacteremia and peritonitis

Diagnosis: Culture

Source treatment: Trimethoprim–sulfamethoxazole

ADDITIONAL TREATMENT • Ciprofloxacin • Aminoglycoside

Treatment principle: Susceptibility-guided antimicrobial therapy plus appropriate device/source control

Key clinical pearl: Methylobacterium is a rare opportunistic aerobic Gram-negative bacillus distinguished by its characteristic pink-pigmented colonies. Think of it when an immunocompromised patient with an indwelling catheter develops otherwise unexplained bacteremia or peritoneal dialysis-associated peritonitis.