Cellulose nanofibers (CNFs) are fibrous nanomaterials produced from plants. Since some nanomaterials are toxic, toxicity evaluation, including in vitro examinations using cultured cells, is essential for the effective use of CNFs. On the other hand, microorganisms in the environment can contaminate CNF suspensions. The contamination of CNF samples and the effects of contaminating microorganisms on in vitro examinations were investigated in this study. Microorganism contamination in CNF samples was examined, and microbial inactivation of CNF suspensions using gamma irradiation was evaluated. After gamma-ray irradiation at absorbed doses of 0.5, 1, 5, and 10 kGy, the cellular effects of CNF suspensions were examined using 6 types of cultured cell, HaCaT, A549, Caco-2, MeT-5A, THP-1, and NR8383 cells. CNF samples were contaminated with bacteria and CNF suspensions exhibited endotoxin activity. Gamma irradiation effectively inactivated the microorganisms contained in the CNF suspensions. When the absorbed dose was 10 kGy, the fiber length of CNF was shortened, but the effect on CNF was small at 1.0 kGy or less. CNF suspensions showed lipopolysaccharides (LPS)-like cellular responses and strongly induced interleukin-8, especially in macrophages. Absorbed doses of at least 10 kGy did not affect the LPS-like activity. In this study, it was shown that the CNF suspension may be contaminated with microorganisms. Gamma irradiation was effective for microbial inactivation of suspension for invitor toxicity evaluation of CNF. In vitro evaluation of CNFs requires attention to the effects of contaminants such as LPS.
Keywords: Cellulose nanofibers; gamma irradiation; lipopolysaccharide; microbial inactivation; nanomaterials.