Colonization of Enterococcus faecalis in a new SiO/SiO2-microtube in vitro model system as a function of tubule diameter

Abstract Objectives Endodontic pathogens can penetrate deep into dentinal tubules and therefore survive the chemo-mechanical disinfection procedures. Bacterial penetration has been mainly studies using sliced infected human teeth which, besides creating artifacts, can hinder the observation of the inner tubules due to the dense and opaque dentin structure. The aim of the present study was to develop a standardized dentin model by using artificial SiO/SiO 2 -microtubes of different diameters and lengths to test the penetration ability of Enterococcus faecalis . Methods E. faecalis was grown in Schaedler fluid media for 24 h and thereafter cell density was settled to 10 3  cells/ml by addition of fresh media. The bacterial solution was then incubated for 2, 3, 5 and 10 days with the SiO/SiO 2 -microtubes of different diameters (2–5.5 μm) and lengths (100–500 μm). The colonization of the tubes was evaluated by phase-contrast microscopy and the amount of colonization was determined by using a colonization index (CI; 0-none, 1-mild, 2-moderate, 3-heavy). Results The diameter of the tubes strongly influences the microbial colonization. After 2 days of cultivation the 5.5 μm tubes showed a moderate to heavy colonization (CI 2–3). In comparison, the 2 and 3 μm tubes were clearly less colonized at the same point in time. In detail: at day 3, only mild to moderate bacteria colonization (CI 1–2) were found in the 3 μm tubes and at day 10 penetration of the 2 μm tubes just started. The colonization of the 5.5 μm tubes was also influenced by their length. In case of the longer microtubes, though, a smaller share of heavily colonized tubes was observed. Significance Our results show that E. faecalis was able to penetrate and reproduce within the standardized SiO/SiO 2 -microtubes in a short time. To examine the mechanisms of bacterial adhesion and invasion into tubular structures the 2 μm tubes could serve as a model system because the diameters are similar to those of dentinal tubules.