Fractional inhibitory concentration index of combinations of antibacterial agents against cariogenic organisms

Objectives: To examine the inhibitory effects between paired combinations of four antibacterial agents that have been investigated in dentistry against six bacteria associated with dentinal caries. Methods: The antibacterial agents tested were: chlorhexidine dihydrochloride, benzalkonium chloride, cetrimide and cetylpyridinium chloride. Two organisms each belonging to Streptococcus, Lactobacillus and Actinomyces species were evaluated against these agents. A checkerboard microtitration method was used to determine the minimum inhibitory concentration and fractionary inhibitory concentration (FIC) and ∑FIC index of the paired combinations of antibacterial agents against the six species of bacteria. Appropriate controls were included. The ∑FIC index was then used to determine if synergy, antagonism, or indifference occurred as a result of interactions between the antibacterial agents and the test organisms. Results: The data from a total of 295 test results were evaluated. Using ∑FIC values for synergy as ≤0.5 and antagonism as >4.0 as recommended by The American Society for Microbiology, no significant synergistic or antagonistic interactions were observed between the paired antibacterial agents when tested against selected organisms. However, a tendency for an additive antibacterial effect was observed with approximately a third of the test results. Nonetheless, no combination of antibacterial agents appeared to be superior to any other. Conclusions: According to these in vitro results, there appears to be no beneficial effect of combining the antibacterial agents against the individual cariogenic micro-organisms selected. However, it may be beneficial to use combined antibacterial agents that have a broader range of activity against an ecosystem of bacteria than using individual agents.