In vitro susceptibility of necrotic-pulp bacteria to newMix-MP and 3Mix-MP

Objectives: The aim of this study was to evaluate and compare the susceptibility of bacteria from human necrotic-pulp tissues and Enterococcus faecalis to the newMix-MP (Ciprofloxacin + Clindamycin + Metronidazole) and the 3Mix-MP (Ciprofloxacin + Minocycline + Metronidazole). Methods: The agar well diffusion test was used to evaluate the susceptibility of the test bacteria to the antibiotic mixtures due to their paste nature. Equal amounts of the antibiotic pastes were filled into the pre-cut small wells (2 mm in diameter) on BHI blood agar plates (25 ml per plate). The necrotic-pulp bacteria collected from thirty teeth and E. faecalis were diluted in RTF to give bacterial concentration of 3 x 10⁸ cells/ml, inoculated on the surface of agar plate, incubated for 48 h at 37^°C in 5% CO₂. Diameters (mm) of inhibition zones were measured and analyzed using a paired t-test to compare the susceptibility of the test bacteria to the two mixtures. Results: All test bacteria were susceptible to the antibiotic mixtures. The necrotic-pulp bacteria showed the inhibition zones of 40.55 ± 4.13 mm and 36.53 ± 3.21 mm for newMix-MP and 3Mix-MP, respectively. The inhibition zones observed from E. faecalis were 31.32 ± 1.48 mm for newMix-MP and 28.12 ± 1.67 mm for 3Mix-MP. The newMix-MP produced significant larger inhibition zones than 3Mix-MP (p<0.001) when tested against the necrotic pulp bacteria and E. faecalis. Conclusion: The newMix-MP exhibited the significant antibacterial activity against the necrotic-pulp bacteria and E. faecalis as shown by the higher susceptibility than the 3Mix-MP in agar well diffusion test. Thus, the newMix-MP may be used to treat the infected pulp as compared to the 3Mix-MP.