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Center for Applied and Environmental Microbiology, Georgia State University, Atlanta, Georgia
D. Price
Interface Research and Development, LaGrange, Georgia
R.B. Simmons
A. Mayo
S.T. Zhang
S.A. Crow, Jr.
Center for Applied and Environmental Microbiology, Georgia State University, Atlanta, Georgia
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Microcycle conidiation and microniche colonization by aspergilli was observed in-situ on various indoor construction and finishing materials. Microcycle conidiation, direct conidiogenesis from a conidium or spore with minimal intervening hyphal development, for several decades has been considered a survival mechanism during stress for a variety of moulds. Adhesive transparent tape mounts and bulk materials from various indoor materials, including air filters from hospitals and healthcare institutions, were transported to the laboratory for light microscopic and scanning electron microscopic observations. Additional materials were held in moist chambers over nonsterile soils and examined periodically for fungal development. Microcycle conidiation was observed usually in areas of sparse fungal development, mostly in association with isolations of members of the Aspergillus flavus-, A. versicolor-, A. niger groups. Branched conidiophores and medusa heads, more often associated with colonization by Eurotium spp., were observed on some preserved woods. These conidiogenesis processes might be factors in the survival and blooms of indoor aspergilli.
Key words: Aspergillus spp., Eurotium spp., hospital air filters, indoor aspergilli, medusa-like conidiophore, microcycle conidiation
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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) or as the immediate recapitulation of sporogenesis after spore germination (Smith et al 1977). The phenomenon has been described for a variety of fungi involving genera such as Acremonium, Aspergillus, Cercospora, Neurospora, Paecilomyces, Penicillium and Trichoderma (Smith et al 1981, Allerman et al 1983, Hanlin 1994, Lapaire and Dunkle 2003.)
MC first was studied most intensely with strains of Aspergillus, particularly industrially important A. niger (Anderson and Smith 1971, 1972; Deans and Smith 1979; Smith et al 1981). Manipulation of nutrients and temperature permitted synchronous culture development in batch fermentations. Anderson and Smith (1972) found that conidia of A. niger incubated at 44 C in a defined medium failed to germinate but continued to swell and grow in an 18–24 h period. By this time cell walls had thickened and conidia size had increased two- to threefold (20~24 µm diam). After a step-down in temperature (30 C), conidiophore initials were observable at 2 h with development of mature conidiophores with normal size conidia at 8–24 h. The cycle was repeatable (Smith et al 1977). Such conidiophores of A. niger have been described as lacking metulae or sterigmata but directly forming phialides (Allerman et al 1983). MC by other species of Aspergillus, A. niveus, A. tamari and A. terreus has been observed without the need for temperature manipulations and stress culture conditions (Saxena et al 1992). These investigators demonstrated the proliferation of sterigmata with production of up to quaternary conidiophores from a parent conidium within 18 h of incubation on glucose-starch agar (DG-SS) under ambient atmospheric and temperature conditions. The MC cycles were observed through five successive transfers. MC by A. flavus grown on DG-SS appeared to be regulated by a low polyamine ratio, putrascine/spermadine, with putrascine essential for vegetative growth and spermidine involved in the development and maturity of the MC conidiophore (Khurana et al 1996).
MC has been proposed as a survival mechanism for fungal spores that encounter unfavorable growth conditions (Hanlin 1994, Lapaire and Dunkle 2003). The MC process has been manipulated in the laboratory to prepare dense concentrations of synchronous fungal inoculate under conditions that entail growth of certain species, but increase in conidial numbers with negligible increase in cell mass also may occur (Cascino et al 1990, Lapaire and Dunkle 2003).
In the course of our studies on fungal colonization of indoor construction and finishing materials we have observed microcycle conidiation involving species of the genus Aspergillus sporadically occurring on materials at sites where fungal growth was not evident with the unaided eye (see Ahearn et al 2004, Ahearn and Price 1995, Price et al 2005). This report notes the morphological variances in MC and conidiophore morphology on these materials and speculates on MC as a key element in the survival and proliferation of moulds in indoor environments and industrial products.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, Price et al 2002, Ahearn et al 2004, Price et al 2005). During these studies, but particularly in our comparative examinations of preserved and nonpreserved hospital filters (Price et al 2005) and preserved pressure-treated pine (Price et al 2002), we have observed MC. In these latter two studies, materials were examined both in the field and with bulk samples returned to the laboratory and held in closed moist containers (the wood in sealed plastic bags; the air filter sections suspended in humidity chambers over nonsterile soil). None of the samples were directly inoculated with fungi. Based on preliminary observations of apparent MC on preserved filters from hospitals, a series of preservative-treated, acrylic coated cardboard strips of similar composition to the air filter frames were placed in the chamber for indicated times and samples fixed in osmium tetroxide vapor for a minimum of 36 h and air dried. All samples were sputter coated with gold/palladium (Denton Desk II, Denton Vacuum Inc., Moorestown, New Jersey) before examination in the scanning electron microscope (LEO 1450vp, Carl Zeiss SMT, Thornwood, New York). Fungi were identified on the basis of characteristic morphologies with the keys of Domsch et al (1980) and Klich (2002). Representative isolates were characterized also with the Biolog system (Biolog Inc, Hayward, California). |
RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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MC by aspergilli in areas of sparse fungal development on pressure-treated lumber was associated with subsequent isolations compatible with the A. flavus-, and secondarily, A. versicolor- A. nidulans-groups (Price et al 2002). Single or catenulate clusters of thick-walled cells apparently derived from a single conidium and sometimes from several conidia germinated to conidiogenesis within an estimated 24 h (FIGS. 1–2). Philades not uncommonly elongated to produce secondary medusa-type spore heads (FIG. 3). Branched conidiophores and MC among isolates of Eurotium were observed among indoor isolations from a hospital in Florida some years earlier, so such a phenomenon was not of recent development (FIG. 4).
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). Morphologies at the base of the emerging conidiophores were suggestive of a MC origin for the microcolonies (FIGS. 7–8). Simultaneous germination of the chlamydospore-like swollen conidia was observed on wood in-situ and during incubation of wood sections in the moisture chambers (FIG. 9). Dense fungal development on several wood cabinet doors and on preservative-treated wallboard in-situ and on similar materials incubated in moisture chambers demonstrated MC among isolates of Eurotium herbariorum, A. nidulans, A. niger and A. terreus (FIG. 10). MC among isolates of E. herbariorum mostly was noted within or immediately adjacent to dense fungal development of mixed cultures. The MC was associated with and without in-situ presence of the sexual stage (FIG. 11). Medusa head conidiophores sometimes originating from a single cell were typical on several wood and wallboard samples (FIG. 12) and less common or lost when the fungi were isolated to pure cultures. Of particular note were two wood cabinet doors from separate buildings that were densely colonized by mixed species dominated by E. herbariorum (91% probability by the Biolog system). These doors were maintained in the laboratory within closed polyethylene bags at ambient conditions for about 2 y. These substrata continuously demonstrated medusa head conidiophores and the rare presence of MC on the wood and on subsequent enrichment culture with some isolates for 3–4 subcultures. In addition several isolates have maintained their capacity to produce commonly the medusa-type conidiophores through four transfers on malt-extract enrichment agar (FIGS. 13–14). In most isolates however medusa-head conidiophores and MC became less common with sequential transfers with alterations to branched conidiophores with penicillus-like morphologies (FIGS. 15–16). In subsequent transfers morphologies were more typical of the aspergilli, but apparent MC with branched conidiophores was observed.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Indoor point sources of Aspergillus spp., including A. fumigatus and other endemic agents of invasive aspergillosis (IA) associated with conidia-laden dust and construction in hospitals have been documented (Lentino et al 1982, Walsh and Dixon 1989, Arnow et al 1991). Still a correlation of airborne Aspergillus and construction with the incidence of nosocomial IA can be lacking (Hospenthal et al 1998, Anaissie et al 2003, Curtis et al 2005). Aspergilli have been documented in hospital water systems in association with IA where airborne aspergilli were negligible (Anaissie et al 2003). Curtis et al (2005) noted episodic increases of aspergilli in hospital air with water leaks and construction activities with overall densities of propagules somewhat greater than outdoor concentrations. These investigators concluded that the hospital contained numerous small to moderate sources of aspergilli. Price et al (2005) found aspergilli colonizing about 10% of the air filters from hospitals and office buildings. The preserved filters in this latter study that were colonized by fungi were one of the sources of our not uncommon observation of MC (at least one MC in every 3–4 tape preparations, ~4 cm2).
Rapid MC under stress laboratory conditions has been known for more than 20 y for most of the aspergilli noted in this report, and branched stipes and medusa-head-type conidiogenesis have been observed in mutant aspergilli (Busby et al 1996, Adams et al 1998). Our casual observations of MC and associated medusa heads in-situ have involved several decades and our directed observations at least 5 y. We have not determined definitive environmental conditions for consistent in-situ expression of these morphologies. We have demonstrated however that MC, which might provide mechanisms for survival, cryptic colonization and the sudden proliferation of aspergilli, can occur on indoor materials.
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FOOTNOTES |
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1 Corresponding author. E-mail: dgahearn{at}mindspring.com
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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