Antimicrobial properties of chitosan from different developmental stages of the bioconverter insect Hermetia illucens

Abstract

Growing antimicrobial resistance has prompted researchers to identify new natural molecules with antimicrobial potential. In this perspective, attention has been focused on biopolymers that could also be functional in the medical field. Chitin is the second most abundant biopolymer on Earth and with its deacetylated derivative, chitosan, has several applications in biomedical and pharmaceutical fields. Currently, the main source of chitin is the crustacean exoskeleton, but the growing demand for these polymers on the market has led to search for alternative sources. Among these, insects, and in particular the bioconverter Hermetia illucens, is one of the most bred. Chitin can be extracted from larvae, pupal exuviae and dead adults of H. illucens, by applying chemical methods, and converted into chitosan. Fourier-transformed infrared spectroscopy confirmed the identity of the chitosan produced from H. illucens and its structural similarity to commercial polymer. Recently, studies showed that chitosan has intrinsic antimicrobial activity. This is the first research that investigated the antibacterial activity of chitosan produced from the three developmental stages of H. illucens through qualitative and quantitative analysis, agar diffusion tests and microdilution assays, respectively. Our results showed the antimicrobial capacity of chitosan of H. illucens, opening new perspectives for its use in the biological area.

Figure 1

Structure of chitosan with its active amino groups, after their protonation in acid conditions, responsible for the antimicrobial activity (image obtained with ChemDraw).

Figure 2

FTIR spectra of bleached (black line) and unbleached (red line) chitosan samples extracted from H. illucens larvae (a), pupal exuviae (b) and dead adults (c). Commercial chitosan (wine lines) derived from crustaceans are also reported.

Figure 3

Inhibition zones of bleached and unbleached chitosan samples produced from H. illucens larvae (a), pupal exuviae (b), dead adults (c). Chitosan samples from H. illucens (circle a), commercial chitosan (circle b), distilled water (circle c) and acetic acid (circle d) on E. coli and M. flavus resulting from the agar diffusion test are reported.

Figure 4

Results of microdilution assay for bleached and unbleached chitosan from larvae (a), pupal exuviae (b) and dead adults (c) of H. illucens, commercial chitosan and acetic acid at the four concentrations of 1.25, 0.6, 0.3, 0.15 mg/ml against E. coli and M. flavus. Bars indicate the absorbance of the bacterial culture (black bars) and that of the culture treated with H. illucens chitosan samples (gray bars), commercial chitosan (red bars) and acetic acid (yellow bars). Data are presented as mean ± standard error of three independent experimental biological replicates. Different letters indicate significant differences (p < 0.05) between absorbance values of the bacterial culture alone and that of bacteria treated with the different concentrations of each treatment. Asterisks indicate significant differences (p < 0.05) among treatments for the same concentration. Data are analyzed with two-way ANOVA and Bonferroni post-hoc test.