In Vitro Amyloid Formation by a Bacteriocin From <i>Bifidobacterium longum</i> subsp. <i>infantis</i>

Bifidobacterium longum subsp. infantis is a probiotic bacterium isolated from human milk-fed infants. This species secretes various metabolites that contribute to gut microbiome development and immune system maturation. In this study, we investigated bacteriocins, ribosomally synthesized peptides that typically exhibit antimicrobial activity. We produced Blon_0434, a B. infantis -derived bacteriocin belonging to the Lactococcin 972 family, by expressing it heterologously in Escherichia coli . Our results demonstrate that recombinant Blon_0434 is secreted via the Sec-dependent pathway but exhibited no detectable antimicrobial activity under the tested conditions. NMR structural analysis suggests that Blon_0434 is thermodynamically unstable, which may account for its inactivity. Unexpectedly, Blon_0434 formed amyloid-like fibrils in vitro, as demonstrated by thioflavin T fluorescence and transmission electron microscopy. The biological implications of Blon_0434 amyloid formation warrant further investigation, particularly regarding microbial interactions and host immune responses.