In this study, we have shown that Bald’s eyesalve, an Anglo-Saxon remedy for eye sty infections, displays growth inhibitory activity against S. aureus and P. aeruginosa, including a multi-drug resistant strain. This activity was more pronounced against S. aureus than P. aeruginosa, which is consistent with the fact that Gram-negative bacteria, particularly Pseudomonads, are notably more resistant to antibacterial agents than Gram-positive bacteria, including Staphylococcus sp. [12]. Our data indicate that the specific formulation composition had little impact on the inhibition of S. aureus. This is inconsistent with a previous report where the presence of an additional Allium species, onion or leek, was found to significantly contribute to the antibacterial activity of Bald’s eyesalve [6]; however, this previous study evaluated the antibacterial activity using a synthetic wound model in which S. aureus inoculum was grown as a biofilm for 24 hr prior to treatment and based upon colony-forming units [6]. Our conflicting results may be due to our assessment of the antibacterial activity of Bald’s eyesalve exclusively against planktonic S. aureus rather than S. aureus biofilms, and we did not investigate whether onion or leek are necessary for specific efficacy against bacterial biofilms. Previous studies have shown that quercetin, an antibacterial flavonoid found in onion [13], and its derivatives demonstrate anti-biofilm and anti-quorum sensing activity against S. aureus and P. aeruginosa [14, 15]. Although plant extracts have been shown to display growth inhibitory activity against P. aeruginosa [16, 17], to our knowledge our study represents the first report of the antibacterial efficacy of Bald’s eyesalve against P. aeruginosa.

Although all known Allium species contain organosulfur compounds, the chemical composition and resultant antibacterial activity of their extracts greatly varies [18]. Prior investigations have determined the main constituents of garlic essential oil to be diallyl disulfide (DADS), diallyl trisulfide (DATS), allyl methyl trisulfide, diallyl sulfide (DAS), and diallyl tetrasulfide (DATTS), while the main constituents of onion and leek essential oils were found to be dipropyl disulfide, dipropyl trisulfide, methyl propyl disulfide, methyl propyl trisulfide, and 1-propenyl propyl disulfide [18, 19]. Tsao et al. previously demonstrated that DAS, DADS, DATS, and DATTS exhibit MICs of 20, 4, 2, and 0.5 μg/ml against S. aureus, respectively, and 80, 64, 32, and 12 μg/ml against P. aeruginosa, respectively [20, 21].