Additive antimicrobial activity of calcium hydroxide and chlorhexidine on common endodontic bacterial pathogens

It is an established procedure to use calcium hydroxide [Ca(OH)2] as a therapeutic component to achieve sterilization of infected root canals. Unfortunately, some bacterial species are relatively resistant to Ca(OH)2 and are therefore associated with treatment failures. The objective of this study was to identify combinations of substances that improve antimicrobial activity. Ca(OH)2 was used as a suspension or as a component of gutta-percha points. Efficacy of killing was compared for (a) Ca(OH)2 suspension alone; (b) zinc oxide (ZnO) points alone; (c) Ca(OH)2 suspension combined with ZnO points; (d) Ca(OH)2 suspension combined with ZnO/chlorhexidine points; and (e) Ca(OH)2 points combined with ZnO/chlorhexidine points. Common endodontopathogenic bacterial species (Enterococcus faecalis, Fusobacterium nucleatum, Peptostreptococcus micros, Porphyromonas gingivalis, Streptococcus intermedius) in pure cultures served as target organisms. In the in vitro assay, the antibacterial substances and approximately 10(7) colony forming units of test bacteria were co-incubated in diluted human serum under growth conditions appropriate for the single bacterial species. Samples were taken on days 0, 1, 2, 4, 7, and 14 of incubation and viable counts determined. Gram-negative bacteria were sufficiently killed by Ca(OH)2 alone. Combinations with other agents did not improve the killing kinetics. For the inactivation of the Gram-positive bacteria P. micros and S. intermedius, a combination of Ca(OH)2 and ZnO/chlorhexidine, killed the bacteria faster than Ca(OH)2 alone. For E. faecalis, combining Ca(OH)2 suspension with ZnO/chlorhexidine points lead to a faster decrease in the overall number of viable bacteria, but no test conditions lead to the complete loss of culture viability. The results of this study support the use of a combination of Ca(OH)2 and chlorhexidine for faster eradication of P. micros and S. intermedius from infected root canals.