Detection of galangin-induced cytoplasmic membrane damage in Staphylococcus aureus by measuring potassium loss

Galangin is one of the active antimicrobial principles of propolis or 'bee glue' and Helichrysum aureonitens, a perennial herb used by South African indigenes to treat infection. The effect of this compound and antibacterial agents with known mechanisms of action upon the cytoplasmic membrane integrity of Staphylococcus aureus was investigated by comparing potassium loss profiles from bacterial cell suspensions. Using an agar dilution assay, the minimum inhibitory concentrations (MICs) of the flavonol galangin, the bacteriostatic antibiotic novobiocin and the bactericidal antibiotic penicillin G against Staphylococcus aureus NCTC 6571 were determined as being 50 microg/mL, 62.5 ng/mL and 31.3 ng/mL, respectively. When 5x10(7)cfu/mL Staphylococcus aureus were suspended in 'potassium-free' media containing 50 microg/mL galangin, a 60-fold decrease in viability was observed after 12 h. Populations of 1x10(9) cfu/mL Staphylococcus aureus incubated for 12 h in 50 microg/mL galangin lost 21% more potassium than untreated control populations. Novobiocin had no effect on potassium loss, but populations incubated in 31.3 ng/mL penicillin G exhibited a 6% increase in potassium loss. This data clearly demonstrates that galangin causes a significant increase in potassium loss from Staphylococcus aureus cells, which may be attributed to either direct damage to the cytoplasmic membrane or indirect damage effected through autolysis/weakening of the cell wall and consequent osmotic lysis.