Efficacy of double antibiotics in hydroxypropyl methylcellulose for bactericidal activity against Enterococcus faecalis and Streptococcus gordonii in biofilm

•Ciprofloxacin & metronidazole in HPMC inhibit E. faecalis and S. gordonii growth.•Double antibiotics in HPMC gel decrease E. faecalis and S. gordonii cDNA amounts.•Double antibiotics in HPMC gel reduce mRNA expression of S. gordonii-specific gene.•By contrast, calcium hydroxide treatment induces mRNA expression of pbp5 gene.•Eradication of bacteria in the dentinal tubules by double antibiotics in HPMC gel. This study aimed to investigate the efficacy of double antibiotics, including ciprofloxacin and metronidazole, in a new vehicle, hydroxypropyl methylcellulose (HPMC), against Enterococcus faecalis and Streptococcus gordonii grown in biofilm. Human mandibular premolars were prepared and divided into four groups: (i) negative control, (ii) positive control, (iii) infected with E. faecalis and S. gordonii for 21 days and intracanally medicated with double antibiotics in HPMC, and (iv) infected with E. faecalis and S. gordonii for 21 days and intracanally medicated with calcium hydroxide (UltraCal™). The efficacy of medication for 14 or 28 days was determined by bacterial cultures and RT-qPCR for absolute quantities of E. faecalis and S. gordonii cDNA and for relative mRNA expressions of pbp5 and gtfG genes. There were significant decreases in the mean colony forming units and mean cDNA amounts of E. faecalis and S. gordonii in group (iii) on days 14 and 28 compared to those in group (ii) (p < 0.01). However, the mean cDNA amounts of E. faecalis and S. gordonii in group (iv) were found to be significantly increased on day 28 (p < 0.05). The mRNA expression of gtfG was significantly decreased in groups (iii) and (iv) on days 14 and 28, whereas that of pbp5 was significantly increased in group (iv) on days 14 and 28 (p < 0.01). Double antibiotics in HPMC gel showed an in vitro efficacy against E. faecalis and S. gordonii grown in biofilm, suggesting its clinical application as an intracanal medicament for both primary and persistent infections.