Role of Ionic Strength in Salt Antagonism of Aminoglycoside Action on Escherichia coli and Pseudomonas aeruginosa

Studies were designed to determine whether ionic strength (µ) is a significant factor in salt inhibition of aminoglycoside action against Escherichia coli and Pseudomonas aeruginosa. In both nutrient broth (a low µ medium) and Mueller-Hinton broth (a relatively high µ medium), protection of E. coli from dihydrostreptomycin or gentamicin action by MgCI2, NaCI, or Na2S04 was attributed to ionic strength alone. The percentage of protection increased with ionic strength and was independent of the particular salt used. Antagonism of aminoglycoside action against P. aeruginosa appeared to involve both a specific, divalent cation-dependent mechanism, revealed in Mueller-Hinton broth, and a nonspecific, ionic strength effect, elicited by sodium salts in nutrient broth. With media of relatively low salt content, variation in ionic strength itself over a range of µ of 0.02–0.14 significantly influences the effectiveness of aminoglycoside antibiotics against E. coli and P. aeruginosa.