Effect of the pH and osmolality of urine on the antibacterial activity of gentamicin.

The antibacterial activity of gentamicin against five strains each of Escherichia coli and Pseudomonas aeruginosa was followed by kinetic killing studies. Human urine having a variable osmolality and adjusted to pH 5.0 or 6.0 was used. Gentamicin was added at a final concentration of 10 or 50 mg/l. It was shown that human urine inhibited the action of gentamicin and that effect was enhanced in acid urine and depressed when the osmolality of the urine was lower. The pH and osmolality of urine variably affected the activity of gentamicin depending on the bacterial species. Lower osmolality rendered Ps. aeruginosa strains more easily susceptible to gentamicin as compared to E. coli. The data of the present study may be useful in clinical practice in the treatment of urinary infections; in cases of urine with low osmolality relatively low doses of gentamicin might be sufficient. In contrast, problems may be encountered in urinary infections in patients with renal insufficiency where reduced dosage of the drug is suggested. In those cases dilution and alkalinization of urine would be beneficial.