Antimicrobial Susceptibility of Common Pathogens

We purchased Escherichia coli (KCTC1682), Salmonella typhimurium (KCTC 1926), Staphylococcus aureus (KCTC 1621), Bacillus subtilis (KCTC 1021), and Propionibacterium acnes (KCTC 3314, KCTC 3220, KCTC 5527, and KCTC 5933) from the Korean Collection for Type Cultures (KCTC), Korea Research Institute of Bioscience & Biotechnology (Daejeon, Korea). S. aureus (CCARM 0027 and CCARM 3708) was obtained from the Culture Collection of Antimicrobial Resistant Microbes (Seoul, Korea). The MIC of phloretin against each of these bacteria was determined by broth microdilution assay [43 (link)]. We defined the MIC as the lowest concentration of phloretin that completely inhibited bacterial growth; this was calculated as the average of three independent measurements in the range of 0.5 to 512 µM. P. acnes was grown for 48 to 72 h at 37 °C on sheep blood agar under anaerobic conditions in a mixture of H₂ and CO₂ (95:5, v/v). Microbroth dilution using Muller–Hinton (MH) broth was used to determine the efficacy of phloretin against P. acnes. We diluted a bacterial suspension in MH broth to achieve turbidity equivalent to a McFarland standard of 0.5 (~1 × 10⁸ cells/mL), yielding 1 × 10⁴ cells/mL in each well after inoculation. Phloretin MICs were determined after incubation for 48 h at 37 °C.