Abstract
Chitosan and chitosan oligomers are receiving increasing attention due to their antimicrobial properties. In the present study, they were assayed as a preventive treatment against white-rot decay of Populus wood (very important in economic and environmental terms), caused by Trametes versicolor fungus. Their capacity to incorporate different chemical species into the polymer structure with a view to improving their anti-fungal activity was also assessed by mixing oligo-chitosan with propolis and silver nanoparticles. The minimum inhibitory concentration of medium-molecular weight chitosan (MMWC), chitosan oligomers (CO), propolis (P), nanosilver (nAg), and their binary and ternary composites against T. versicolor was determined in vitro. Although all products exhibited anti-fungal properties, composites showed an enhanced effect as compared to the individual products: 100% mycelial growth inhibition was attained for concentrations of 2.0 and 0.2 mg·mL−1 for the CO-P binary mixture, respectively; and 2 µg·mL−1 for nAg in the ternary mixture. Subsequently, MMWC, CO, CO-P and CO-P-nAg composites were tested on poplar wood blocks as surface protectors. Wood decay caused by the fungus was monitored by microscopy and vibrational spectroscopy, evidencing the limitations of the CO-based coatings in comparison with MMWC, which has a higher viscosity and better adhesion properties. The usage of MMWC holds promise for poplar wood protection, with potential industrial applications.
Highlights
In the field of wood protection, significant efforts have been devoted over the last decades to the assessment of natural products that can pose an alternative to other traditionally used chemical compounds, which have toxic effects on human beings and the environment [1]
medium-molecular weight chitosan (MMWC) presented a moderate activity against mycelial growth, with significant differences in comparison with the control treatment
Another work on Sphaeropsis sapinea and Trichoderma harzianum wood-degrading fungi suggested that the application of 1 mg·mL−1 of low-molecular weight chitosan reduced growth rate by a factor of 3 with respect to the control [47], in line with the results reported by Silva-Castro et al [40], who found that 1 mg·mL−1 of chitosan oligomers (CO) inhibited 86% of the mycelial growth of Heterobasidium annosum basidiomycete fungus
Summary
In the field of wood protection, significant efforts have been devoted over the last decades to the assessment of natural products that can pose an alternative to other traditionally used chemical compounds, which have toxic effects on human beings and the environment [1]. In this sense, renewable polymers have attracted intense industrial interest, and, amongst them, chitosan has promising application perspectives [2]. The presence of chitosan inside the cell can lead to intracellular responses such as inactivation or blocking of enzymes activities, and of DNA transcription and translation [7,8]
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