Aspergillus Species

ENVIRONMENTAL

Aspergillus spp. are the second most isolated fungus after Candida spp. Aspergillus fumigatus is the species most commonly isolated; other pathogenic species include A. flavus, A. terreus, and A. niger. The conidia are readily found in the environment and can be recovered in air-settling plates. Many species of aspergilli have been implicated in human disease. Although some are easily distinguished from others, some aspergilli require extensive testing to determine a definitive identification at the species level.

SEPTATE OPPORTUNISTS

Hyphae of most opportunists molds contain cross-walls. Those of medical importance fall into the phylum Deuteromycota. The septate opportunistic molds may be divided into those that are dematiaceous (dark-colored hyphae and/or conidia), and those that are hyaline (light colored hyphae and conidia). Organisms with dark hyphae on tease mounts also have dark green to black colonies, especially on the colony reverse. The colonial color aids in the initial identification. Hyaline organisms exhibit light-colored colonial aerial hyphae, but they may be covered over with brightly colored comidia; thus, a tease mount is required. In the following descriptions, key identifying features are capitalized.

HYALINE OPPORTUNISTS

Opportunists with light-colored hyphae may cause hyalohyphomycosis (infection caused by hyaline fungi).

PATHOGENICITY

Mortality from infections caused by the aspergilli remains extremely high, frequently above 90% in the immunocompromised host. Neutropenia is the single most predictive factor for developing aspergillosis. Aspergillus spp. are the most frequent cause of disease in bone marrow transplant recipients in addition to other cancer and transplantation patients. Infection is initiated following inhalation of fungal conidia. In the lung air spaces, conidia germinate and invade the tissue. Within a few days, the patient develops a severe fever that fails to respond to antifungal therapy. Although not all patients suffer from chest pain, it is not uncommon for pneumonia-like symptoms to appear. The infection easily spreads hematogenously, and it is not uncommon to find multiorgan system involvement, including the brain, liver, heart, and bone. Allergic reactions to Aspergillus spp. are a problem to those with sensitivities to molds. Another frequent presentation is so-called fungus balls in the lungs of agricultural workers who routinely are in contact with fungal conidia from environmental sources. A. fumigatus was linked to at least one case following the use of injectable steroids. Most cases in this multistate outbreak were caused by Exserohilum rostratum, a common mold found in soil and on plants. A. fumigatus is the most common opportunistic pathogen of the genus. It causes disseminated aspergillosis, pulmonary disease, and allergic bronchopulmonary disease (farmer's lung), as well as keratomycosis, otomycosis, and infection of the nasal sinuses.

LABORATORY EVALUATION

Aspergilli may be uniseriate or biseriate. Uniseriate species are those whose phialides attach directly to the vesicle at the end of the conidiophore. Biseriate species possess a supporting structure called a metula. Metulae attach directly to the vesicle, and attached to each of the metula are phialides. Conidia are produced from the phialides. Other characteristics include an erect conidiophore arising from a foot cell within the vegetative hyphae. It is also important to note whether or not phialoconidia remain in long chains or are easily disturbed into individual phialoconidia. Chains of conidia can be aligned in very straight, parallel columns or in a radiating pattern around the vesicle, and the conidia can be rough or smooth. Aspergillus sp. must not be confused with the similar-appearing zygomycete, Syncephalastrum sp., which is aseptate, possesses no phialides, and exhibits chains of spores in tubes (merosporangia) off the vesicle. If possible, this genus should be speciated, especially A. fiumigatus.

CULTURE

On SABHI agar at room temperature, the rapid-growing colony velvety. Various colors are due to dense production of conidia: blue, green, yellow, is rugose and black, and white. The color of Aspergillus spp. colonies is derived from the conidia. Colors range from black to white and include yellow, brown, green, gray, pink, beige, and tan. Some species also form diffusible subsurface pigments on a variety of media. A granular texture is seen in species with abundant conidial formation. Most known pathogens in this group form green- to tan-colored colonies. A. fumigatus grows at 45°C, while most other Aspergillus species do not.

MICROSCOPIC

The mycelium is septate. Unbranched, rough or smooth conidiophores with a FOOT CELL at their base support a large VESICLE at their tip. The vesicle in turn supports short, flask-shaped PHIALIDES (old name; sterigmata) in a SINGLE or DOUBLE ROW, which produce CHAINS of smooth or rough PHIALOCONIDIA.