Taxonomy

The filamentous fungi that comprise the genus Aspergillus reproduce mitotically through the production of spores borne on a specialized structure called the conidiophore. The morphology of this spore-bearing structure, with its distinctive foot cell, long stalk, swollen apex, and protuberant phialides, is the defining feature of the genus. It was first described in 1729 by Micheli, an Italian priest, who was reminded of a Roman Catholic device used to sprinkle holy water, the aspergillum.

All members of the genus Aspergillus produce an aspergillum-like conidiophore (Fig. 1). (The special vocabulary regularly encountered in Aspergillus taxonomy is summarized in Table 1.) The conidiophore wall is usually thicker and more hydrophobic than that of the other hy-phal cells. The stipe of the conidiophore attaches to the vegetative mycelium in a T- or L-shaped base that has been traditionally called a foot cell, even though it is not a sepa rate cell. The presence of the foot cell is an important diagnostic criterion of the genus. The apex of the stipe swells into the vesicle. Spore-producing phialides are borne directly (uniseriate) or indirectly on metulae (biseriate) on the surface of the vesicle, another important characteristic for distinguishing species within the genus. The phialides are sporogenous. Repeated mitotic divisions in the phialide nucleus lead to the formation of a chain of uninucleate or multinucleate conidiospores, also called conidia. The phial-ides also extrude primary wall layers and pigments that form part of the mature spore. Conidiospores are spherical or elliptical, smooth or echinulate, extremely hydrophobic, and easily airborne when mature.

Charles Thom (1872-1956), most famous for his work on penicillin and the other great mold genus, Penicillium, was also a major figure in the development of Aspergillus taxonomy. With Margaret Church in 1926 (1) and Kenneth Raper in 1945 (2) he produced the first two complete monographic treatments. Kenneth Raper went on to expand this work and with Dorothy Fennell published The Genus Aspergillus in 1965 (3), still considered by many to be the most authoritative source of Aspergillus taxonomy. Highly sensitive to substrate, temperature, moisture level, and other environmental variables, potential morphological plasticity is controlled in modern taxonomic schemes by the use of standard media and culture conditions. Pure cultures are grown on standardized agar media for specific lengths of time with defined temperature, light, and other environmental parameters. Species within Aspergillus are distinguished by cultural characters such as colony color, growth rate, and texture and by microscopic traits such as phialides and metulae, shape of the vesicle, spore size, and spore ornamentation. The ability to produce certain pigments, sclerotia, or cleistothecia is highly dependent on culture conditions, but can also be useful in distinguishing species. Modern taxonomists place increasing emphasis on secondary metabolite profiles, isoenzyme electrophoresis, and molecular genetic characters such as ribosomal DNA sequences to supplement the traditional characters.

After adequate characterization, identification of species depends on careful comparison with published descriptions. The last monograph of the entire genus was Raper and Fennell's The Genus Aspergillus (3), but several more recent books provide identification systems for the more common species (for example, see, Klich and Pitt [4]) Medically important species are described in Kwon-Chung and Bennett (5).

Some aspergilli possess a sexual stage. All the sexual stages result in the formation of ascospores inside of cleistothecia and are classified formally in the Ascomycete family Trichocomaceae. According to the rules of botanical nomenclature, which govern the naming of all fungi, the sexual (teleomorphic or perfect) state requires its own name. Because Aspergillus by definition refers only to the asexual (anamorphic or imperfect) state, many species within the genus have two names. For example, the asexual species Aspergillus tetrazonus produces a teleomorphic state called Emericella quadrilineata, and the teleomorph of the well-known genetic model Aspergillus nidulans is called Emericella nidulans. This dual nomenclature mystifies workers not trained in the rules of botanical nomen-

Aspergillus Tetrazonus

Table 1. Glossary of Aspergillus Taxonomy

Term

Definition

Anamorph

The nonsexual form of a fungus, loosely

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