Introduction: Extracellular material (ECM) surrounding Enterococcus faecalis may play a role in increasing resistance to environmental stresses. Our aim was to determine ECM levels in response to subminimal inhibitory concentrations of sodium hypochlorite (sub-MIC/NaOCl) or anaerobic growth and determine the impact on biofilm development.
Methods: From 37 E. faecalis clinical strains, 19 were selected according to their biofilm-producing ability by using a crystal violet biofilm assay: 10 strong, 4 intermediate, and 5 non-biofilm producers. Biofilm assays were subsequently performed on all strains when subjected to sub-MIC/NaOCl. All strains were evaluated for ECM production under aerobic and anaerobic conditions and with sub-MIC/NaOCl. ECM production was assessed by using scanning electron microscopy. Double-blinded independent assessors were used to score levels of ECM production. The esp gene was detected by using polymerase chain reaction. Gelatinase activity was determined by using Todd-Hewitt and gelatin agar.
Results: In aerobic conditions, ECM was expressed in all strains. In the presence of sub-MIC/NaOCl, of the 10 strong biofilm producers, 5 increased their ECM production, and 4 showed increased biofilm growth. Two strains had less ECM production and showed decreased biofilm growth. One isolate demonstrated no observable changes. Most non-biofilm producers demonstrated no observable differences in ECM production, although 1 strain increased biofilm growth. ECM production in anaerobic conditions was highly variable. The esp gene (n = 15) and gelatinase activity (n = 7) were evident among the isolates.
Conclusions: Clonal diversity among strains of E. faecalis suggests that some strong biofilm producers can upregulate ECM production and increase biofilm growth in response to sub-MIC/NaOCl.
Keywords: Biofilm; Enterococcus faecalis; clonal diversity; extracellular material; stress response.
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