The Intestinal Spirochete Brachyspira pilosicoli Attaches to Cultured Caco-2 Cells and Induces Pathological Changes

BACKGROUND: Brachyspira pilosicoli is an anaerobic spirochete that has received relatively little study, partly due to its specialized culture requirements and slow growth. This bacterium colonizes the large intestine of various species, including humans; typically, a dense layer of spirochete cells may be found intimately attached by one cell end to the surface of colonic enterocytes. Colonized individuals may develop colitis, but the mechanisms involved are not understood. The current study aimed to develop an in vitro model to investigate this process. METHODOLOGY/PRINCIPAL FINDINGS: Four strains of B. pilosicoli were incubated at a high multiplicity of infection with monolayers of a human colonic adenocarcinoma cell line (Caco-2 cells). One strain isolated from a pig (95/1000) and one from a human (WesB) attached to the monolayers. Colonization increased with time, with the Caco-2 cell junctions being the initial targets of attachment. By electron microscopy, individual spirochete cells could be seen to have one cell end invaginated into the Caco-2 cell membranes, with the rest of the spirochete draped over the Caco-2 cell surface. After 6 h incubation, the monolayer was covered with a layer of spirochetes. Colonized monolayers demonstrated a time-dependent series of changes: staining with labelled phalloidin identified accumulation of actin at the cell junctions; ZO-1 staining revealed a loss of Caco-2 tight junction integrity; and Hoechst staining showed condensation and fragmentation of nuclear material consistent with apoptosis. Using quantitative reverse transcription PCR, the colonized monolayers demonstrated a significant up-regulation of interleukin-1β (IL-1β) and IL-8 expression. B. pilosicoli sonicates caused significant up-regulation of IL-1β, TNF-α, and IL-6, but culture supernatants and non-pathogenic Brachyspira innocens did not alter cytokine expression. CONCLUSIONS/SIGNIFICANCE: The changes induced in the Caco-2 cells provide evidence that B. pilosicoli has pathogenic potential, and give insights into the likely in vivo pathogenesis.