Tolerance mechanisms of human-residential bifidobacteria against lysozyme

•Human-residential bifidobacteria (HRB) generally exhibited higher tolerance to lysozymes than non-HRB.•All bifidobacteria were resistant to the peptidoglycan-degrading property of lysozymes.•The varied susceptibility of bifidobacteria to lysozyme was due to their tolerance to the cationic cytotoxicity of lysozymes.

We previously reported that lysozyme present in breast milk is a selection factor for bifidobacterial colonization in infant human intestines. This study is aimed at examining their underlying mechanisms.Human-residential bifidobacteria (HRB) generally exhibited higher tolerance than non-HRB to lysozymes, except B. bifidum subspecies. To assess the involvement of enzymatic activity of lysozyme, peptidoglycan (PG) was isolated and the degree of O-acetylation (O-Ac) in 19 strains, including both HRB and non-HRB, was determined. Variety in the degree of O-Ac was observed among each of the Bifidobacterium species; however, all purified PGs were found to be tolerant to lysozyme, independent of their O-Ac degree. In addition, De-O-Ac of PGs affected the sensitivity to lysozyme of only B. longum-derived PG. To examine the non-enzymatic antibacterial activity of lysozyme on bifidobacteria, lysozyme was heat-denatured. The HRB and non-HRB strains exhibited similar patterns of susceptibility to intact lysozyme as they did to heat-denatured lysozyme. In addition, strains of B. bifidum (30 strains), which showed various tolerance of lysozyme, also exhibited similar patterns of susceptibility to intact lysozyme as they did to heat-denatured lysozyme.These results suggest that bifidobacteria are resistant to the peptidoglycan-degrading property of lysozyme, and the tolerance to lysozyme among some HRB strains is due to resistance to the non-enzymatic antibacterial activity of lysozyme.