Phialophora species: General Characteristics, Pathogenesis, Clinical Find

Phialophora species: General Characteristics, Pathogenesis, Clinical Findings, Laboratory Diagnosis, Treatment, Prevention and Control

Phialophora is a dematiaceous fungus which inhabits the soil, plants, and decaying food; and widely distributed in nature. Phialophora spp. are the causative agents of some human infections like chromoblastomycosis, mycetoma, and phaeohyphomycosis.

Taxonomic classification of Phialophora

( Medlar in 1915)

Kingdom: Fungi

Phylum: Ascomycota

Class: Euascomycetes

Order: Chaetothyriales

Family: Herpotrichiellaceae

Genus: Phialophora

Species: P. verrucosa, P. americana, P. bubakii, P. europaea, P. parasitica, P. reptans, P. repens, P. richardsiae, and P. europaea.

Note: Morphological features, such as the shape of the collarettes, organization of the phialides, the existence of chlamydospores, and biochemical features, such as the assimilation of melibiose help in the differentiation of the species from each other

Habitats of Phialophora

Phialophora is a dematiaceous fungus that inhabits the soil, plants, and decaying food; and is widely distributed in nature. Phialophora spp. are the causative agents of some human infections like chromoblastomycosis, mycetoma, and phaeohyphomycosis.

General Characteristics of Phialophora

Phialophora contains more than 40 species, among them medically important are P. verrucosa, P. americana, P. bubakii, P. europaea and P. reptans. Both P. verrucosa and P. americana produce their conidia from phialides with conspicuous darkened collarettes, however, sequencing has demonstrated a close relatedness, suggesting that these species may be synonymous. P. verrucosa is primarily an agent of chromoblastomycosis since it is the second most common cause of chromoblastomycosis worldwide (after Fusarium pedrosoi), mycetoma, and phaeohyphomycosis although other reported infections include endocarditis, keratitis, and osteomyelitis.

Risk group: It comes in a risk group (RG)-2 organism.

Morphological Description of Phialophora verrucosa

Colonies on Sabouraud dextrose agar (SDA) are slow-growing, initially dome-shaped, later becoming flat, suede-like, and olivaceous to black in color.

Phialides are flask-shaped or elliptical with distinctive funnel-shaped, darkly pigmented collarettes.

Conidia are ellipsoidal, smooth-walled, hyaline, mostly 3.0-5.0 x 1.5-3.0 μm, and aggregate in slimy heads at the apices of the phialide.

Pathogenicity of Phialophora

Phialophora species are among the causes of chromoblastomycosis and phaeohyphomycosis. P. verrucosa is the principal causative agent of chromoblastomycosis in tropical and subtropical areas, particularly at Japan and South America. The clinical forms of phaeohyphomycosis may be diverse, including cutaneous infections, subcutaneous cysts, keratitis, endocarditis, arthritis, osteomyelitis, cerebral infection, fatal hemorrhage, and disseminated infection. P. europaea has been isolated from cutaneous and nail infections in North-western Europe.

Clinical Findings

It depends on the site of infection. e.g. in the case of chromoblastomycosis. It involves the foot or leg, but other exposed body parts may be infected, especially where the skin is broken. Initially small, itchy, enlarging papules may resemble dermatophytosis (ringworm). These papules extend to form dull red or violaceous, sharply demarcated patches with indurated bases. The later untreated case may cause Lymphatics obstruction and itching may persist, and secondary bacterial superinfections may develop, causing ulcerations and occasionally septicemia. Phaeohyphomycosis may show the following clinical manifestations-invasive sinusitis, subcutaneous nodules or abscesses, keratitis, lung masses, osteomyelitis, endocarditis, mycotic arthritis, brain abscess, and disseminated infection.

Laboratory Diagnosis of Phialophora

Specimen: It depends on the nature of the infection site e.g. in the case of chromoblastomycosis skin scrapping or tissue biopsy whereas in keratitis corneal scrapings (most frequent) or tissue biopsy and skin lesions (either cellulitis or metastatic lesions) and also nails clipping in case of nail infection.

KOH mount: Presence of fungal elements

Fungal culture: To obtain growth of fungi. Exhibits slow to moderate growth, usually maturing in about 7 to 12 Days. Colonies are woolly to velvety, dark grey, brown, or olivaceous black on the surface and reverse.

LPCB preparation: Observation of fungal structures from culture.

Dematiacious (melanin pigment) – Hyaline to brown, septate hyphae. Phialides are pale brown to brown, bottle or vase, or shaped with a darker collarette at the apical end. Phaialides are located laterally or terminally on the hyphae.
Conidia are unicellular, smooth and thin-walled, hyaline to brown and round or ovoid (1-3 X 2-4 µm) which accumulate at the apex of the collarette giving the appearance of a vase of flowers.

Histopathological examination: The organisms appear as dark round cells, 5 – 12 µm in diameter. Similar to the other fungi causing chromoblastomycosis, spherical or polyhedral, dark brown, thick-walled sclerotic bodies (muriform cells) are visualized in tissues infected with Phialophora species. The absence of muriform cells has been reported in cases that are immunosuppressed or debilitated. Phaeoid hyphae may also be observed.

Molecular test: Internal Transcribed Spacer (ITS) sequencing recommended (de Hoog et al. 1999).

Treatment of Phialophora

Useful antifungal drugs are-

Itraconazole
Voriconazole
Amphotericin B and
Posaconazole.

Key Notes on Phialophora

Some human pathogens with phialidic conidiogenesis previously assigned to Phialophora have been moved to other genera, namely, Phaeoacremonium and Pleurostomophora.
Chromoblastomycosis is a chronic fungal infection of the skin and the subcutaneous tissue caused by traumatic inoculation of a specific group of dematiaceous fungi (usually Fonsecaea pedrosoi, Phialophora verrucosa, Cladosporium carrionii, or Fonsecaea compacta) through the skin.
Phaeohyphomycosis is a chronic infectious condition caused by dematiaceous fungi which usually involve the skin and subcutaneous tissue.
ITS rRNA Gene-Based Phylogenetic Reconstruction Using Algorithms with Local and Global Sequence Alignment for Black Yeasts and Their Relatives
Organisms that are similar to Phialophora are Exophiala, Fusarium, Lecythophora, Phaeoacremonium, Phialemonium, and Wangiella.
Some differentiating features are as follows:

Phialophora differs from Exophiala by having phialides and from Wangiella by having phialides with collarettes.
P. richardside : Phialides with saucer-shaped or flared collarettes.
P. verrucosa: Collarettes vase-shaped, darkly pigmented; phialides flask-shaped.
Lecythophora hoffmannii: Colonies first yeast-like, flat, cream-colored, turning pink to salmon (old cultures turning black in the center); phialides not separated from hyphae by a septum; conidia one-celled, often curved.
Phialemonium species: Colonies yeast-like (some), flat, fast-growing, white, cream-colored, light gray, some with the green color or light vinaceous color; tapering phialides without a basal septum, branching phialides; conidia one-celled, obovate or curved in shape.
P. parasitica: Phialides cylindrical to obclavate, elongate, hyaline to pale brown
P. repens: Phialides cylindrical to lageniform, short, hyaline to pale brown.

Prevention and control of Phialophora

Store food properly to save it from contamination.
Broken skin parts are prone to infection and thus follow safety guidelines.
Maintain personal hygiene.
No vaccine is available.
To control the infection, some anti-fungal drugs are available.

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