Antimicrobial interactions between the phytoextracts of Callistemon citrinus and Eriobotrya japonica against Streptococcus mutans

Streptococcus mutans is a gram-positive bacterium in the oral cavity that is most implicated in the dental caries progression. The condition is very expensive to manage and the most commonly used products such as fluoride tooth pastes and alcohol-based mouth washes are associated with many side effects. The current study therefore focused on providing a scientific evidence to guide the use of a combination of Eriobotrya japonica (EJ) and Callistemon citrinus (CC) as actives in development of an effective and cheaper herbal formulation for management of dental caries. The objective of this study was to determine antimicrobial interactions of Eriobotrya japonica and Callistemon citrinus phytoextracts proportions against Streptococcus mutans bacteria. The leaves of both plants (EJ and CC) were shade-dried and pulverized into a coarse powder which were then cold macerated using ethanol (60 %) for 24 h. Phytochemical screening was conducted for the two dry extracts obtained after fan drying before they were mixed in to five different proportions (1:0, 3:1, 1:1, 1:3 and 0:1). Minimum inhibitory concentrations (MIC) and minimum bactericidal concentration (MBC) assays against Streptococcus mutans were done for all the proportions above with ciprofloxacin and 2.5 % Dimethyl sulfoxide (DMSO) as the positive and negative controls respectively. Antimicrobial interactions between the two extracts were also evaluated using Fractional Inhibitory and Bacterial Concentration Indices (FICI/FBCI). Results showed that EJ and CC had percentage yields of 20.05 % and 15.45 % respectively. All the extracts showed similar phytochemical profiles. They also demonstrated an inhibitory effect on Streptococcus mutans with MIC and MBC values ranging from 521 to 3333 g/ml and 1042 to 3667 g/ml respectively. However, CC: EJ (1:0) had the lowest MIC and MBC comparable to that of the standard drug at P <0.05. The FICI/FBCI were between 1.5 and 3.917. Therefore, CC: EJ (1:0) proportion markedly demonstrated better antimicrobial activity against the test organism and there are no beneficial antimicrobial interactions between the two plant extracts to inform their combination as actives for dental caries product formulation.