Aspergillus lentulus: A newly identified Aspergillus species that is phenotypically similar to Aspergillus fumigatus but less susceptible to antifungal drugs
Among over 185 included in the genus,
Aspergillus fumigatus is the most commonly isolated species that leads to invasive aspergillosis in immunocompromised patients.
When grown on culture media,
A. fumigatus colonies are typically blue-green to gray with a white to tan reverse coloration.
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Under the microscope, A. fumigatus consists of septate hyphae, short (*lt;300 µm), smooth, colorless or greenish conidiophores, a round apical vesicle, uniseriate phialides that partially cover the surface of the vesicle only at the upper portion ("columnar" head), and round conidia (2-5 µm in diameter) forming radial chains over the phialides. As for several other moulds, the morphologic features are being utilized as the mainstay of identification.
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Microscopic appearance of A. fumigatus. www.doctorfungus.org
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Phylogenetic methods are now available and provide more accurate identification by comparisons of nucleotide sequences of multiple genes. These methods are particularly of use for discrimination of morphologically indistinguishable organisms and disclosure of cryptic species. Based on the data obtained by these phylogenetic methods, a novel sibling species of
A. fumigatus has been recently identified.
The story started with the isolation of 7
Aspergillus strains, which were morphologically similar to
A. fumigatus but sporulated poorly and generated relatively higher MICs/minimum effective concentrations (MECs) for various antifungal drugs (amphotericin B MIC:1-2 µg/ml; itraconazole MIC: 0.5-1 µg/ml; voriconazole MIC: 4µg/ml; and caspofungin MEC: 4->32 µg/ml).
These isolates were initially considered as variants of
A. fumigatus, based on their distinct mitochondrial cytochrome b gene sequences and unique randomly amplified polymorphic DNA PCR patterns. However, phylogenetic analysis of these isolates by multilocus sequence typing of five genes; the ß-tubulin gene, the rodlet A gene, the salt-responsive gene, the mitochondrial cytochrome b gene, and the internal transcribed spacer regions showed that 4 of these isolates clustered together as a monophyletic group in a new clade which was very distant from
A. fumigatus.
This new species was named as
Aspergillus lentulus, referring to the word "
lentulus" which means "somewhat slow" in Latin and was chosen to reflect the slow sporulating characteristic of the species on Czapek dox and malt extract agar.
More detailed analysis of the phenotypic features of this new species by differential interference contrast and scanning electron microscopy also showed that the vesicles of
A. lentulus are smaller than those of
A. fumigatus (15 vs. 22 µm) while the conidium characteristics are similar in two species. Unlike
A. fumigatus strains,
A. lentulus fails to grow at 48°C.
This recent discovery of the
Aspergillus spp.,
Aspergillus lentulus emphasizes the significance of phylogenetic analysis in discrimination of morphologically similar organisms. It now appears to be of both epidemiological and clinical significance to consider
A. lentulus for slowly sporulating isolates which are phenotypically similar to
A. fumigatus but are less susceptible to antifungal drugs.
Related reading
- Balajee SA, Gribskov JL, Hanley E, Nickle D, Marr KA. Aspergillus lentulus sp. nov., a new sibling species of A. fumigatus. Eukaryotic Cell 2005; 4: 625-632.
- Balajee SA, Weaver M, Imhof A, Gribskov J, Marr KA. Aspergillus fumigatus variant with decreased susceptibility to multiple antifungals. Antimicrob Agents Chemother 2004; 48:1197-1203.
