الفهرس 
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Abstract
This study was conducted on eighty extracted maxillary central incisors human teeth to evaluate the cumulative effect of endodontic steps on Enterococcus faecalis which represents the largest percent of bacteria causing root canal therapy failure. The roots were cleaned and shaped with step back technique and, sterilized. The root apices were sealed with nail polish and inoculated with the 10 l of 0.5 McFarland E. faecalis in each root. The orifices were sealed and maintained in upright position at 37 (R@(BC for seven days. The excess broth was removed then S1 was taken. The roots were divided into eight groups, groups 14 were instrumented and irrigated with sterile saline solution then S2 was taken. In group 1 EDTA 17% , Calcium hydroxide paste were used. Group 2 EDTA 17%used. Group 3 Calcium hydroxide paste were used.. Group 4 sterile saline used alone.. Groups 58 prepared as groups 14 and 2.5% NaOCl was used instead of 0.9% sterile saline as an irrigant. Standard laboratory methods were followed to determine the number of CFU for the tested organism. Data were collected and analyzed. Selected specimens were examined and scanning electron microscope (SEM) to evaluate the bacterial contamination on the root canal walls initially and after the endodontic steps. The maximum bacterial reduction was achieved in group 5 followed by 7, 6, 8,1,3,2 and 4 in descending order. Scanning electron microscopic evaluation confirmed the final microbial results.