Advances in understanding bioactivity of chitosan and chitosan oligomers against selected wood-inhabiting fungi

Abstract

Nitrous acid deaminative depolymerisation was used to prepare three chitosan oligomer (CO) mixtures from high-molecular weight chitosan. These mixtures of chitosan oligosaccharides were analysed by electrospray ionisation mass spectroscopy, potentiometric titration and gel permeation chromatography. A method based on potentiometric titration of the amino groups of the oligomers gave an average degree of polymerisation (DP) for the three preparations of 5 (CODP5), 9 (CODP9) and 14 (CODP14). Chitosan acetate and the chitosan oligomer mixtures were assayed against Leptographium procerum, Sphaeropsis sapinea and Trichoderma harzianum on nutrient media. Leptographium procerum and S. sapinea growth was prevented by chitosan acetate and chitosan oligomers at concentrations of 0.3–0.4% (w/v), whereas T. harzianum was able to overcome the fungistatic action of these compounds. The oligomer preparation CODP14 exhibited superior specific activity to both CODP5 and chitosan acetate, suggesting an optimum molecular weight for bioactivity. All oligomer preparations were more effective at pH 4 than at pH 6. This result, in combination with the inactivity of N-acetylated CODP14, indicated that amino group protonation was an important factor for fungistatic activity. The CODP14 preparation was reduced with sodium borohydride and fractionated by alkali precipitation and ion exchange chromatography. Bioassays of these fractions pointed towards DP and degree of deacetylation (DD) as key factors in chito-oligosaccharide bioactivity. Conversely, the terminal aldehyde groups generated by depolymerisation did not contribute to the activity observed.

Growth of Phytopathogenic Fungi in the Presence of Partially Acetylated Chitooligosaccharides

“…For complete data, see ref. [20] D3A7 DP11 D6A5 D5A6 D4A7 DP12 D7A5 D6A6 •: major, : minor components optimum DP, to the cationic charge of the oligosaccharides [15]. Our studies indicate that reasons also can be important for the growth rates inhibition, i.e.…”

“…Thus, low-DP chitosans, in particular chitooligosaccharides with an average DP20, were more effective than high-DP chitosans in inhibiting mycelial growth of a variety of phytopathogenic and wood inhabiting fungi [4,13,14]. Oligosaccharides of DP9 and DP14 which had been prepared by nitrous acid depolymerization of chitosan were active against Leptographium procerum and Sphaeropsis sapinea, but not against Trichoderma harzianum, while oligomers of DP5 were inactive [15]. On the other hand, Fusarium solani was inhibited by chitooligosaccharides of DP C 7 [16] while oligomers of DP2-DP8 were not active against three Fusarium species [4].…”

Changes in hyphal morphology due to chitosan treatment in some fungal species

“…The exact mechanism by which the higher chitooligosaccharides and chitosans exert antimicrobial activity is unknown. Based on the observation that the fungistatic activity was higher at lower pH, it was assumed that the toxicity was correlated, besides to optimum DP, to the cationic charge of the oligosaccharides (Torr et al 2005). The present study indicated that reasons also could be important for the growth rates inhibition, i.e., enzymatic uptake of simple carbohydrates by the permeases could temporally be blocked by the presence of the large oligosaccharides .…”

Evaluation of in vitro inhibition of mycelial growth of Fusarium solani f. sp. piperis by different products in Brazil

“…These authors believed that such behavior may be a response to stress caused by the chitosan. Several studies have shown that the biological activity of chitosan is significantly dependent upon its molecular weight, acetylation degree (Alfredsen et al, 2004;Wu et al, 2004;Torr et al, 2005), pH of the medium (Devlieghere et al, 2004), and the microorganism membrane characteristics (Qi et al, 2004). In general, the lower the molecular weight and degree of acetylation of chitosan, the greater the efficacy atreducing the growth and multiplication of microorganisms (Goy et al, 2009).…”