Infectious Disease and Microbiology - Aureobasidium Species

Basics

Aureobasidium is a genus of uncommon environmental fungi that can occasionally cause opportunistic infection in humans. The principal species associated with disease are Aureobasidium pullulans and Aureobasidium mansoni.

These organisms are especially important in immunocompromised patients and in individuals undergoing peritoneal dialysis.

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Microbiologic Characteristics

Aureobasidium species are dematiaceous filamentous fungi, meaning that they contain dark melanin pigment within their cell walls.

In human tissue they may appear in several different forms, including:

• Yeast-like cells

• Pseudohyphae

• Pigmented septate hyphae

In culture, particularly around 30°C, the organism grows predominantly in a hyphal form.

This variable morphology can make recognition more difficult unless a fungal infection is specifically considered.

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Epidemiology

Aureobasidium has a worldwide distribution, but human infection is rare.

The organisms are environmental molds and are generally considered opportunistic pathogens rather than common causes of disease.

Clinically significant infection is more likely in patients with:

• Immunosuppression

• Chronic debilitating illness

• Indwelling medical devices

• Peritoneal dialysis catheters

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Phaeohyphomycosis

Aureobasidium can cause phaeohyphomycosis, a broad category of infection produced by darkly pigmented fungi.

Disease occurs particularly in immunocompromised individuals.

Possible manifestations include:

• Cutaneous lesions

• Subcutaneous nodules

• Localized soft-tissue infection

• Occasionally deeper or disseminated disease

The appearance of pigmented fungal elements in tissue supports the diagnosis of a dematiaceous fungal infection.

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Peritoneal Dialysis-Associated Peritonitis

One of the more important clinical associations is peritonitis in patients receiving peritoneal dialysis.

Patients may develop:

• Abdominal pain

• Fever

• Cloudy dialysis fluid

• Nausea

• Peritoneal irritation

Fungal peritonitis should always be regarded as a serious complication because persistent infection can occur if the dialysis catheter remains in place.

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Diagnosis

Diagnosis is based primarily on:

• Fungal culture

• Histopathologic examination of tissue

In suspected peritonitis, peritoneal dialysis fluid should be submitted for fungal culture.

In localized tissue disease, biopsy may reveal pigmented fungal forms.

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Histopathology

Tissue specimens may show:

• Pigmented septate hyphae

• Pseudohyphae

• Yeast-like fungal elements

Special fungal stains can improve visualization.

Because Aureobasidium is an environmental organism, isolation from a nonsterile specimen must be interpreted carefully. Demonstration of fungal invasion in tissue or recovery from a normally sterile site strongly supports true infection.

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Differential Diagnosis

The differential diagnosis includes other dematiaceous fungi capable of producing phaeohyphomycosis, such as:

• Alternaria

• Cladophialophora

• Exophiala

• Curvularia

• Other pigmented molds

In peritoneal dialysis-associated peritonitis, other fungal pathogens such as Candida must also be considered.

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Treatment

Amphotericin B

There are limited clinical data defining the optimal treatment of Aureobasidium infection.

Amphotericin B has historically been considered a potentially effective therapy, particularly for invasive or serious disease.

Treatment should be individualized according to:

• Site of infection

• Severity

• Host immune status

• Culture and susceptibility results when available

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Peritoneal Dialysis Catheter Removal

For Aureobasidium peritonitis related to peritoneal dialysis, removal of the infected dialysis catheter is an important part of management.

Catheter removal helps eliminate the persistent source of fungal infection and generally improves the likelihood of successful treatment.

Antifungal therapy without source control may be inadequate.

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Azoles

Older data do not clearly establish the clinical effectiveness of azole antifungals against Aureobasidium.

Therefore, azoles should not automatically be assumed to be effective solely on the basis of the organism’s fungal classification.

If an azole is being considered, susceptibility testing and expert guidance are helpful.

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Source Control

When infection is associated with a foreign body or catheter, management should include evaluation for removal whenever feasible.

Potential source-control measures include:

• Removal of a peritoneal dialysis catheter

• Drainage of localized collections

• Debridement of infected tissue

• Reduction of immunosuppression when medically possible

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Prognosis

Localized disease may respond well when recognized early and managed with appropriate antifungal therapy and source control.

Prognosis is less favorable when infection is:

• Disseminated

• Deeply invasive

• Associated with severe immunosuppression

• Persistent because an infected device remains in place

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High-Yield Clinical Pattern

Immunocompromised patient

Pigmented fungal elements in tissue

Cutaneous or subcutaneous infection

→ Consider phaeohyphomycosis due to a dematiaceous fungus such as Aureobasidium.

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High-Yield Dialysis Pattern

Peritoneal dialysis patient

Abdominal pain

Cloudy dialysis fluid

Dematiaceous fungus in culture

→ Think fungal peritonitis, with Aureobasidium as a rare possibility.

Management often requires:

Antifungal therapy + dialysis catheter removal

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Exam Essentials

Genus:

→ Aureobasidium

Important species:

→ A. pullulans and A. mansoni

Organism type:

→ Dematiaceous filamentous fungus

Forms seen in tissue:

→ Yeasts, pseudohyphae, and pigmented septate hyphae

Distribution:

→ Worldwide

Frequency:

→ Rare human pathogen

Major infection:

→ Phaeohyphomycosis

Important risk group:

→ Immunocompromised patients

Important healthcare-associated infection:

→ Peritoneal dialysis-associated peritonitis

Diagnosis:

→ Culture and tissue biopsy

Historically useful antifungal:

→ Amphotericin B

Critical management step in dialysis-associated peritonitis:

→ Removal of the peritoneal dialysis catheter

Azoles:

→ Clinical efficacy not well established

Key clinical pearl: In fungal peritonitis related to peritoneal dialysis, source control is essential; removing the infected catheter can be as important as the antifungal therapy itself.