Antibacterial activity of gentamicin in urine and tryptose phosphate broth

Antibacterial activity of gentamicin against 50 strains of Gram-negative bacteria was tested by recording the total number of viable cells in time course studies in tryptose phosphate broth (TPB) and urine derived from normal subjects. The antibacterial activity of gentamicin against a variety of Gram-negative bacteria including Enterobacteriaceae and Pseudomonas aeruginosa increases proportionally with pH when TPB is used. The acidic reaction of TPB (pH5) results in an impaired activity of gentamicin against Enterobacteriaceae but not Pseudomonas aeruginosa, when the drug was added at a final concentration of 50 micrograms/ml. When this broth was buffered to pH 6 the bacterial activity of gentamicin was expressed even when the drug was added at lower concentrations (10 micrograms/ml). The same phenomenon as described above was observed when normal urine instead of TBP was used for the determination of the killing rate of bacteria. However in that case killing time was longer. When gentamicin was added in acidic urine (pH5) at a concentration of 50 micrograms/ml no activity against Pseudomonas aeruginosa was noted. Lower concentrations of gentamicin (10 micrograms/ml) were inactive when the antibacterial activity was studied by using normal urine adjusted to various pHs (5, 5.5 and 6).