Influence of pH and Eh upon Endodontic Irrigant Microbial Efficacy

OBJECTIVE: Within necrotic pulp of an infected root canal, there are a number of physical and biochemical environments that could effect the efficacy of endodontic irrigants; two important gradients are the Eh (oxdation/reduction potential) and pH. Here, we wanted to investigate the impact of pH and Eh upon efficacy four commonly used endodontic irrigants efficacy mimicking the microbial mixed-species presentation in necrotic pulp. It was our hypothesis that the marked changes would influence: 1) growth of selected organisms and 2) the recalcitrant synergy of their mixed population.

METHODS: Four clinical isolates (susceptible and vancomycin resistant Enterococcus faecalis, Streptococcus mutans and Candida albicans) recovered from infected root canals were used in four different combinations. Normal plate media pH was 7.2 with 5.0 and 9.0 evaluated as gradients. Eh was measured at +30 mv in an ambient environment compared to -180 mv in an anaerobic chamber. Various concentrations of MTAD, NaOCl, EDTA and chlorhexidine were dispensed on a sterile disc. Organisms were seeded as a lawn and a Zone of Inhibition (ZOI) measured following 48 hours of growth. ZOIs (mon) were plotted vs concentration, and linear regressor coefficient was calculated (y = mx + b).

RESULTS: Anaerobic conditions (-180 mv) increased growth of isolates (25%), while alkaline pH 9 reduced growth rate compared to pH 7.2 and 5.0 by 30%. Anaerobic conditions at pH 7.2 had the largest impact on the efficacy with MTAD and NaOCl maintaining the greatest activity, independent of strain combinations. At pH 5 or 9, MTAD maintained its higher efficacy compared to a marked reduced ZOI, in order, of EDTA, NaOCl and chlorohexidine.

CONCLUSION: Mimicking the environment of a necrotic pulp including pH, Eh and mixtures of organisms, unmasked reduced efficacy; MTAD was least susceptible to environmental changes investigated.