In vitro antimicrobial efficacy of two medical grade honey formulations against common high-risk meticillin-resistant staphylococci and Pseudomonas spp. pathogens

Background: Antimicrobial resistance is a problem in human and animal healthcare. Honey may be used for its wound healing properties and antimicrobial effects. Objective: To investigate the antimicrobial activity of two commercially available medical grade honeys (MGHs) against Staphylococcus spp. and Pseudomonas spp. isolates. Methods and materials: Two formulations, MGH1 (40% w/v honey) and MGH2 (80% w/v Manuka honey), were tested in vitro for minimal inhibitory concentrations (MIC) and minimal bactericidal concentrations (MBC) against 11 Staphylococcus and 11 Pseudomonas isolates at low [1.5 × 10⁴ colony forming units (cfu)/well] and high (1.5 × 10⁶ cfu/well) concentrations of inoculum, representing systemic and cutaneous bacterial loads during infection, respectively. Results: MGH2 showed a lower MIC against staphylococci than MGH1, although this was not statistically significant. MGH1 had stronger bactericidal effects against staphylococci than MGH2, although this effect was statistically significant only at the higher bacterial concentration (P < 0.01). For Pseudomonas spp., MGH1 had significantly higher antimicrobial activity (both MIC and MBC) than MGH2 against all isolates tested and at both bacterial concentrations (P < 0.05). Conclusions and clinical importance: Both MGHs were effective in vitro against common cutaneous pathogens including meticillin-resistant staphylococci and Pseudomonas species. The higher efficacy of the MGH1 formulation against Pseudomonas and its consistent effects against staphylococci, while containing only half of the amount of honey compared to MGH2, invites further investigation of the mechanisms and clinical applications of MGH1.