Investigation of the interactions between neutrophils and endodontic isolates of Enterococcus faecalis

Aim The primary aim of this study was to investigate the effect of phagocytosis by neutrophils on the antimicrobial sensitivity of Enterococcus faecalis strains. A secondary aim was to determine whether carriage of a plasmid encoding aggregation substance (AS), which has been reported to increase the survival of some strains inside neutrophils, affected the antimicrobial susceptibility of E. faecalis after phagocytosis by neutrophils. Methodology An assay was carried out to identify isolates of E. faecalis which demonstrated pheromone‐responsive clumping caused by the production of aggregation substance (AS). Four E. faecalis strains grown to both logarithmic and stationary phases were exposed to sodium hypochlorite (NaOCl) and chlorhexidine gluconate (CHX) to determine the minimum inhibitory concentrations of these two agents. The antimicrobial susceptibility tests were repeated with E. faecalis strains which survived phagocytosis by neutrophils for 18 h. Results As expected a laboratory strain of E. faecalis OG1RF which was AS negative became AS positive after introduction of the pheromone responsive plasmid pCF10 into the bacterium to give strain OG1RF(pCF10). These two strains and two endodontic isolates, E08‐584 which demonstrated pheromone‐responsive clumping and E08‐398 which did not, were selected for further study All the test E. faecalis strains were inhibited by low concentrations of sodium hypochlorite (MIC range 0.02–0.3%) and chlorhexidine gluconate (MIC range 0.0004–0.004%). Bacteria recovered from inside neutrophils after 18 h following phagocytosis were susceptible to both ¼MIC and MIC of CHX and NaOCl. Conclusions Aggregation substance did not appear to affect the antimicrobial susceptibility of any of the strains to CHX or NaOCl. All of the E. faecalis strains examined were capable of survival for 18 h inside the neutrophils following phagocytosis; regardless of their capacity to produce aggregation substance. In addition, all strains of E. faecalis had enhanced susceptibilities to the antimicrobial agents after residence inside neutrophils for 18 h.