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Presently there is no method available that allows noninvasive and real-time monitoring of fungal susceptibility to antimicrobial compounds. The green fluorescent protein (GFP) of the jellyfish Aequoria victoria was tested as a potential reporter molecule for this purpose. Aureobasidium pullulans was transformed to express cytosolic GFP using the vector pTEFEGFP (A. J. Vanden Wymelenberg, D. Cullen, R. N. Spear, B. Schoenike, and J. H. Andrews, BioTechniques 23:686-690, 1997). The transformed strain Ap1 gfp showed bright fluorescence that was amenable to quantification using fluorescence spectrophotometry. Fluorescence levels in Ap1 gfp blastospore suspensions were directly proportional to the number of viable cells determined by CFU plate counts (r(2) > 0.99). The relationship between cell viability and GFP fluorescence was investigated by adding a range of concentrations of each of the biocides sodium hypochlorite and 2-n-octylisothiozolin-3-one (OIT) to suspensions of Ap1 gfp blastospores (pH 5 buffer). These biocides each caused a rapid (< 25-min) loss of fluorescence of greater than 90% when used at concentrations of 150 microg of available chlorine ml(-1) and 500 microg ml(-1), respectively. Further, loss of GFP fluorescence from A. pullulans cells was highly correlated with a decrease in the number of viable cells (r(2) > 0.92). Losses of GFP fluorescence and cell viability were highly dependent on external pH; maximum losses of fluorescence and viability occurred at pH 4, while reduction of GFP fluorescence was absent at pH 8.0 and was associated with a lower reduction in viability. When A. pullulans was attached to the surface of plasticized poly(vinylchloride) containing 500 ppm of OIT, fluorescence decreased more slowly than in cell suspensions, with > 95% loss of fluorescence after 27 h. This technique should have broad applications in testing the susceptibility of A. pullulans and other fungal species to antimicrobial compounds.

Significant substratum damage can occur when plasticized PVC (pPVC) is colonized by microorganisms. We investigated microbial colonization of pPVC in an in situ, longitudinal study. Pieces of pPVC containing the plasticizers dioctyl phthalate and dioctyl adipate (DOA) were exposed to the atmosphere for up to 2 years. Fungal and bacterial populations were quantified, and colonizing fungi were identified by rRNA gene sequencing and morphological characteristics. Aureobasidium pullulans was the principal colonizing fungus, establishing itself on the pPVC between 25 and 40 weeks of exposure. A group of yeasts and yeast-like fungi, including Rhodotorula aurantiaca and Kluyveromyces spp., established themselves on the pPVC much later (after 80 weeks of exposure). Numerically, these organisms dominated A. pullulans after 95 weeks, with a mean viable count +/- standard error of 1,000 +/- 200 yeast CFU cm(-2), compared to 390 +/- 50 A. pullulans CFU cm(-2). No bacterial colonization was observed. We also used in vitro tests to characterize the deteriogenic properties of fungi isolated from the pPVC. All strains of A. pullulans tested could grow with the intact pPVC formulation as the sole source of carbon, degrade the plasticizer DOA, produce extracellular esterase, and cause weight loss of the substratum during growth in vitro. In contrast, several yeast isolates could not grow on pPVC or degrade DOA. These results suggest that microbial succession may occur during the colonization of pPVC and that A. pullulans is critical to the establishment of a microbial community on pPVC.

Initial adhesion of fungi to plasticized polyvinyl chloride (pPVC) may determine subsequent colonization and biodeterioration processes. The deteriogenic fungus Aureobasidium pullulans was used to investigate the physicochemical nature of adhesion to both unplasticized PVC (uPVC) and pPVC containing the plasticizers dioctyl phthalate (DOP) and dioctyl adipate (DOA). A quantitative adhesion assay using image analysis identified fundamental differences in the mechanism of adhesion of A. pullulans blastospores to these substrata. Adhesion to pPVC was greater than that to uPVC by a maximum of 280% after a 4-h incubation with 10(8) blastospores ml(-1). That plasticizers enhance adhesion to PVC was confirmed by incorporating a dispersion of both DOA and DOP into the blastospore suspension. Adhesion to uPVC was increased by up to 308% in the presence of the dispersed plasticizers. Hydrophobic interactions were found to dominate adhesion to uPVC because (i) a strong positive correlation was observed between substratum hydrophobicity (measured by using a dynamic contact angle analyzer) and adhesion to a range of unplasticized polymers including uPVC, and (ii) neither the pH nor the electrolyte concentration of the suspension buffer, both of which influence electrostatic interactions, affected adhesion to uPVC. In contrast, adhesion to pPVC is principally controlled by electrostatic interactions. Enhanced adhesion to pPVC occurred despite a relative reduction of 13 degrees in the water contact angle of pPVC compared to that of uPVC. Furthermore, adhesion to pPVC was strongly dependent on both the pH and electrolyte concentration of the suspension medium, reaching maximum levels at pH 8 and with an electrolyte concentration of 10 mM NaCl. Plasticization with DOP and DOA therefore increases adhesion of A. pullulans blastospores to pPVC through an interaction mediated by electrostatic forces.

Sodium pyrithione and zinc pyrithione (NaPT and ZnPT, respectively) are widely used as cosmetic preservatives and general antimicrobial agents. They have been shown to be active against fungal cell walls, associated membranes and bacterial transport processes. Investigations were undertaken into the effect of these antimicrobial agents on substrate catabolism and intracellular ATP levels using an oxygen electrode and luciferin-laciferase technology, respectively. Results indicate that, while both compounds are poor inhibitors of substrate catabolism, sub-inhibitory concentrations of biocide greatly reduces intracellular ATP levels in both Escherichia coli NCIMB 10000 and Pseudomonas aeruginosa NCIMB 10548. This is thought to be due to the action of NaPT and ZnPT on the Gram-negative bacterial membrane.

A new technique for studying the properties of biofilms has been developed, based on the phenomenon of electrorotation. Biofilms of Klebsiella rubiacearum were formed on the surfaces of 6 microns diameter polystyrene beads, and the presence of such films was found to alter their electrorotation spectra. The effects of adding a biocide (polyhexanide) to the surrounding aqueous medium was also investigated. The dielectric properties of the beads with biofilms, before and after biocide treatment, were interpreted from the electrorotation spectra using modelling methods that have been well tested for other heterogeneous biological systems. The technique is of value in understanding the physico-chemical properties of biofilms and can be adapted for monitoring the presence of toxic chemicals and for testing the activity of biocides against biofilms.