title: Fractional inhibitory concentration index of combinations of antibacterial agents against cariogenic organisms.
abstract: OBJECTIVE
  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 summation operator FIC index of the paired combinations of antibacterial agents against the six species of bacteria. Appropriate controls were included. The summation operator 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 summation operator FIC values for synergy as < or = 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.
authors.affiliation: Discipline of Oral Rehabilitation, The Prince Philip Dental Hospital, 34 Hospital Road, Hong Kong, SAR, People's Republic of China. botelho@hkusua.hku.hk
authors.forename: M G
authors.initials: MG
authors.lastname: Botelho
mesh_terms: D000190:Actinomyces / Q000187:drug effects; D000891:Anti-Infective Agents, Local / Q000008:administration & dosage* / Q000494:pharmacology; D001548:Benzalkonium Compounds / Q000008:administration & dosage / Q000494:pharmacology; D000077286:Cetrimonium; D002593:Cetrimonium Compounds / Q000008:administration & dosage / Q000494:pharmacology; D002594:Cetylpyridinium / Q000008:administration & dosage / Q000494:pharmacology; D002710:Chlorhexidine / Q000008:administration & dosage / Q000494:pharmacology; D003731:Dental Caries / Q000382:microbiology*; D004338:Drug Combinations; D004347:Drug Interactions; D007779:Lactobacillus acidophilus / Q000187:drug effects; D007780:Lacticaseibacillus casei / Q000187:drug effects; D008826:Microbial Sensitivity Tests; D013295:Streptococcus mutans / Q000187:drug effects; D013298:Streptococcus sanguis / Q000187:drug effects
chemical_list: D000891:Anti-Infective Agents, Local; D001548:Benzalkonium Compounds; D002593:Cetrimonium Compounds; D004338:Drug Combinations; D002594:Cetylpyridinium; D002710:Chlorhexidine; D000077286:Cetrimonium