engleski

Gamma-irradiation for cultural heritage – reduction of fungal growth on paper materials

Background of the study. A number of cultural heritage (CH) objects e.g. paintings contain paper as base material which is vulnerable to biodeterioration due to fungal growth. Gamma-irradiation has been proposed as one of the physical methods for control of fungal contamination of CH but it can have unwanted side effects on natural and synthetic organic polymers in paper-based material when applied in high and repeated irradiation doses. Objectives. This study was aimed to assess gamma irradiation dose and dose rate necessary for reduction of fungal contamination to acceptable level on paper. Specific aims were: 1) to determine composition of mycobiota commonly occurring on paper ; 2) to assess antifungal effect of irradiation doses (2, 7, 20 and 50 kGy, at dose rates 0.1 and 8, 6 Gy/s) against commonly occurring mycobiota ; 3) as well as artificially inoculated secondary colonizer Cladosporium spaherospermum that can be found on cellulose materials. Methodology. Paper (Verge 120 g) was cut in squares 3.5 x 3.5 cm (0.15 g) and fungal composition of paper squares was analysed by plate count method. Briefly, to determine the concentration of naturally occurring mycobiota samples were homogenised and serially diluted (up to 10-4) in peptone water and plated onto Malt Extract Agar (MEA). First group of paper squares containing naturally occurring mycobiota were incubated at 25C and 70% of relative humidity (Rh) for 7 days. The second group of paper squares were sterilised by autoclave (15 min, 121C, 1.2 Bar), then inoculated with Cladosporium spaherospermum at concentration 100 CFU/g and incubated in the same manner. Upon 7 days of incubation naturally occurring mycobiota and Cladosporia concentration in positive control samples was determined as previously described. All other tubes with paper samples containing natural mycobiota or artificially inoculated Cladosporia were irradiated with 60-Co gamma source at RCDL to doses 2, 7, 20 and 50 kGy, and dose rates 0.1 and 8.6 Gy/s. Upon irradiation samples were incubated for 0, 7, 14, 28 and 56 days at 25C. After each incubation period samples were serially diluted (up to 104) and plated onto MEA to determine the number of viable fungi. Results. Alternaria spp., Aspergillus spp., Cladosporium spp.., Penicillium spp. white mycelia and yeasts comprised naturally occurring mycobiota, with initial concentrations of 200 CFU/g. These fungi were inhomogeneously dispersed on paper showing „hot- spot“ contamination. After 7 days of incubation at 25C and 70 of Rv the concentration of paper mycobiota and artificially inoculated Cladosporia was 10 and 100 folds higher, respectively, comparing to their initial level. Fungicidal effect of gamma radiation on naturally occurring mycobiota was dose and dose rate-dependent. Lower doses 2 and 7 kGy were not effective when applied at 0.1 Gy/s ; species of paper natural mycobiota recovered in concentrations 100-2000 CFU/g (Fig.1.). Reduced concentration of white mycelia (40 CFU/g) was recovered on 0th and 14th day upon exposure to 7 kGy at dose rate 8.6 Gy/s. Dose of 2 kGy (8.6 Gy/s) was less effective than 7 kGy but returned concentration of natural mycobiota to the initial level (200 CFU/g). High doses 20 and 50 kGy applied at both dose rates inhibited recovery of the majority of the fungi except yeasts and white mycelia ; yeasts were recovered on 14th day (200 CFU/g) and 90 CFU/g of white mycelia was viable on 0th day. Artificially inoculated Cladosporia were resistant to 2 kGy applied at both dose rates, while doses 7, 20 and 50 kGy were not effective when applied at lower dose rate ; upon irradiation Cladosporia recovered in concentration range 103-3x104 CFU/g. When these doses were applied at dose rate 8.6 Gy/s almost completely inhibited recovery of Cladosporia ; the only exception was obtained on 14th day after radiation with 20 kGy but concentration was 1000 fold reduced from the initial level (Fig. 2.). Conclusion. Taken together, fungicidal effect of gamma radiation was dose and dose rate-dependent. Lowest dose of 2 kGy at higher dose rate reduced majority of fungal growth (except Cladosporia) to the initial level that was recorded before incubation in humid conditions. Doses 7, 20, and 50 kGy were effective against mycobiota including Cladosporia, only when applied at higher dose rate. Species of Cladosporium, yeasts and white mycelia were the most resistant fungi to gamma- irradiation particularly when doses were applied at 0.1 Gy/s.

Gamma-irradiation, Cladosporium, radioresistant fungi, cultural heritage

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