Do salicylates and ascorbate increase the outer membrane permeability to hydrophobic antibiotics in Pseudomonas aeruginosa?

Acetylsalicylate and ascorbate have earlier been shown to increase the outer membrane (OM) permeability of Pseudomonas aeruginosa to a hydrophobic probe compound, nitrocefin. In order to elucidate whether these drugs increase the OM permeability to a wider set of hydrophobic compounds, the OM permeability to three other hydrophobic probes (rifampin, fusidic acid and sodium deoxycholate) was studied in the presence of salicylates or ascorbate. A high concentration (300 micrograms/ml, equal to 1.7 mM) of L-ascorbate decreased the minimum inhibitory concentration (MIC) of rifampin against P. aeruginosa by a factor of approximately 3. As a sharp contrast, the reference compound, polymyxin B nonapeptide (PMBN) which has a strong OM permeability-increasing action, decreased the MIC by a factor of approximately 100, at a concentration as low as 3 micrograms/ml (equal to 3 microM). If the assays were performed in a low ionic strength medium (L broth diluted 1/5 with water) instead of L broth, ascorbate was somewhat more effective. The MIC of fusidic acid was even less influenced by ascorbate. Additionally, ascorbate did not potentiate the bacteriolytic action of sodium deoxycholate, whereas the control compound hexametaphosphate had a marked effect. Furthermore, salicylate and acetylsalicylate sensitised, in all conditions tested, P. aeruginosa to none of the three probes. The results suggest that ascorbate and salicylates lack any significant OM permeability-increasing action.