Aspergillus ustus

Aspergillus ustus is a microfungus and member of the division Ascomycota. It is commonly found in indoor environments and soil.[1] Isolated cases of human infection resulting from A. ustus have been described; however the majority of these are nail infections.[2][3]

Aspergillus ustus
Aspergillus ustus group colony on Modified Leonian's agar
Scientific classification
Kingdom: Fungi
Division: Ascomycota
Class: Eurotiomycetes
Order: Eurotiales
Family: Trichocomaceae
Genus: Aspergillus
Species:
A. ustus
Binomial name
Aspergillus ustus
(Bainier) Thom & Church (1926)
Synonyms
  • Sterigmatocystis usta Bainier (1882)
  • Aspergillus minutus Abbott (1927)
  • Aspergillus ustus var. laevis Blochwitz (1934)

Identification and taxonomy

Morphology and physiology

Colonies of A. ustus appear dull brown sometimes with a purplish to grey brown or dark brown with a yellow to brown reverse side;[4][5][6] colonies are flat to furrowed often with a cental bump.[6] Microscopically, the fungus is characterized by elongated conidial heads with bent Hülle cells scattered throughout the pigmented mycelium.[6] The conidia are rough-walled and spherical, ranging in color from green to yellow-brown.[6][7] The vesicles range from 7–15 μm in diameter and are hemispherical to almost round.[6] Although A. ustus is able to grow at human body temperature, other species in this group, such as A. baeticus and A. pseudoustus, require relatively low temperature for growth and thus are unlikely to cause human infection.[7]

Hülle cell and conidia of A. ustus group.

Secondary metabolites

Despite that A. ustus has long been recognized as a common soil-associated fungus, knowledge of its biochemistry remained poor until recently. One of the first metabolites to be isolated and characterized from this fungus, ustic acid (C11H7O7), reacts with Iron (III) chloride to form a deep purple-colored compound. Ustic acid has since been shown to occur in other fungi, notably closely related Aspergilli such as A. granulosus and A. puniceus.[6] Aspergillus ustus produce a number of other metabolites including autocystins (and versicolourins), austalides, a sterigmatocystin-like chemical, and nidulol. Some metabolites of A. ustus have antibacterial properties.[4] Metabolite chemistry has been shown to be useful for the taxonomy and identification of this fungus.[6]

Phylogenetic position

Like other members of the genus Aspergillus, the A. ustus group is affiliated with the family Trichocomaceae. A phylogenetic study of Aspergillus section Usti using morphology, secondary metabolite chemistry and gene sequencing (beta-tubulin and calmodulin) revealed 21 distinct species and showed an affiliation of the section with two teleomorph genera, Emericella and Fennellia.[8]

Ecology

Aspergillus ustus has been found on the surfaces of walls of caves and in indoor air of buildings[8] including hospitals,[3][9] soils and bat dung.[6] A sugarcane farm in Egypt was found to host A. ustus and other species of Aspergillus section Usti.[10] Aspergillus ustus is thought to have a world-wide distribution.[6] Species in the A. usti group have been isolated from caves (e.g., Aspergillus baeticus was isolated from the floors of the Grotto of the Marvels cave[11]).

Human infection

The most common clinical presentations of A. ustus infection involve onychomycosis and otitis media.[3] It only rarely found to cause serious infection (e.g., endocarditis, pneumonia, disseminated disease), typically as an opportunist in severely immunocompromised people, often secondary to immunosuppressive chemotherapy following hematopoietic stem cells transplant.[3] Mortality is high (50%) and survival is generally short (e.g., eight days).[12] The apparent recent increase in serious infections coupled with the propensity of this species to resist antifungal therapy has raised concern about A. ustus as an emerging agent of opportunistic fungal infection. However, the true incidence of A. ustus infection is very difficult to estimate based on challenges and inconsistencies with identifying this agent in the clinical setting, and the patchy nature of reporting. The fungus is thought to spread through air and water, and be passed on surfaces (where spores settle).[3]

Management and treatment

This species shows elevated resistance to antifungal drugs; however, four classes of drugs (itraconazole (a triazole), voriconazole, caspofungin and amphotericin B) demonstrate fungicidal effectiveness that is most pronounced in combination.[3][12] One promising regimen combines voriconazole and caspofungin.[3] Although disseminated infection carries a high mortality rate, the response of skin to treatment varies depending on the degree of drug susceptibility exhibited by the particular strain.[12]

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References
  1. Samson, R.A.; Hoekstra, E.S.; Frisvad, J.C. (2004). Introduction to Food and Airborne Fungi. Utrecht: Schimmelcultures.
  2. Rath, P.-M.; Petermeier, K.; Verweij, P. E.; Ansorg, R. (1 June 2002). "Differentiation of Aspergillus ustus strains by Random Amplification of Polymorphic DNA". Journal of Clinical Microbiology. 40 (6): 2231–3. doi:10.1128/JCM.40.6.2231-2233.2002. PMC 130772. PMID 12037094.
  3. Panackal, Anil A.; Imhof, Alexander; Hanley, Edward W.; Marr, Kieren A. (2006). "Aspergillus ustus infections among transplant recipients". Emerging Infectious Diseases. 12 (3): 403–8. doi:10.3201/eid1203.050670. PMC 3291436. PMID 16704776.
  4. Raistrick, H; Stickings, C. E (Jan 1951). "Studies in the biochemistry of micro-organisms. 82. Ustic acid, a metabolic product of Aspergillus ustus (Bainier) Thom & Church". Biochemical Journal. 48 (1): 53–66. doi:10.1042/bj0480053. PMC 1275293. PMID 14820785.
  5. Kohno, J; Sakurai, M; Kameda, N; Nishio, M; Kawano, K; Kishi, N; Okuda, T; Komatsubara, S (October 1999). "Production, isolation and biological properties of TMC-120A, B and C, novel inhibitors of eosinophil survival from Aspergillus ustus TC 1118". The Journal of Antibiotics. 52 (10): 913–6. doi:10.7164/antibiotics.52.913. PMID 10604762.
  6. Houbraken, J.; Due, M.; Varga, J.; Meijer, M.; Frisvad, J.C.; Samson, R.A. (1 January 2007). "Polyphasic taxonomy of Aspergillus section Usti". Studies in Mycology. 59 (1): 107–128. doi:10.3114/sim.2007.59.12. PMC 2275196. PMID 18490949.
  7. Krimitzas, Antonios; Pyrri, Ioanna; Kouvelis, Vassili N.; Kapsanaki-Gotsi, Evangelia; Typas, Milton A. (1 January 2013). "A phylogenetic analysis of Greek isolates of Aspergillus species based on morphology and nuclear and mitochondrial gene sequences". BioMed Research International. 2013: 260395. doi:10.1155/2013/260395. PMC 3665174. PMID 23762830.
  8. Samson, R. A.; Varga, J.; Meijer, M.; Frisvad, J. C. (8 August 2011). "New taxa in Aspergillus section Usti". Studies in Mycology. 69 (1): 81–97. doi:10.3114/sim.2011.69.06. PMC 3161758. PMID 21892244.
  9. Kane, Julius (1997). Laboratory Handbook of Dermatophytes: A Clinical Guide and Laboratory Handbook of Dermatophytes and other Filamentous Fungi from Skin, Hair, and Nails. Belmont, CA: Star. ISBN 9780898631579.
  10. Jackson, MP; Wadolkowski, EA; Weinstein, DL; Holmes, RK; O'Brien, AD (February 1990). "Functional analysis of the Shiga toxin and Shiga-like toxin type II variant binding subunits by using site-directed mutagenesis". Journal of Bacteriology. 172 (2): 653–8. doi:10.1128/jb.172.2.653-658.1990. PMC 208490. PMID 2404947.
  11. Novakova, A.; Hubka, V.; Saiz-Jimenez, C.; Kolarik, M. (13 April 2012). "Aspergillus baeticus sp. nov. and Aspergillus thesauricus sp. nov., two species in section Usti from Spanish caves". International Journal of Systematic and Evolutionary Microbiology. 62 (Pt 11): 2778–85. doi:10.1099/ijs.0.041004-0. hdl:10261/61924. PMID 22505602.
  12. Verweij, PE; van den Bergh, MF; Rath, PM; de Pauw, BE; Voss, A; Meis, JF. (May 1999). "Invasive aspergillosis caused by Aspergillus ustus: case report and review". Journal of Clinical Microbiology. 37 (5): 1606–9. doi:10.1128/jcm.37.5.1606-1609.1999. PMC 84848. PMID 10203536.
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