The Journal of Medical Microbiology, Vol 10, Issue 3 273-279, Copyright © 1977 by Society for General Microbiology
Sensitivity of Pseudomonas aeruginosa to sulphonamides and trimethoprim and the activity of the combination trimethoprim: sulphamethoxazole
D. Grey and J. M. Hamilton-Miller
The activities of three sulphonamides and trimethoprim against strains of
Pseudomonas aeruginosa have been studied. Sulphadiazine had most activity,
sulphadimidine had little, and the activity of sulphamethoxazole was
intermediate. According to their sensitivity to sulphamethoxazole, strains
were divided into two groups: "highly resistant" (16%, MIC greater than
1000 microgram per ml) and "moderately resistant" (84%, MIC less than or
equal to 1000 microgram per ml). The former were resistant on disk testing
to Sulphatriad 300 microgram. Sulphamethoxazole and trimethoprim did not
act in synergy against them. The moderately resistant strains were
sensitive to Sulphatriad; trimethoprim and sulphamethoxazole showed marked
synergy against them in agar-plate dilution tests. The concentrations of
trimethoprim and sulphamethoxazole necessary for synergy lay for each drug
within the range of concentrations at which they have been found in urine,
and the ratio of their MICs when acting in synergy was similar to the ratio
of their concentrations in urine. It is suggested that a disk containing
trimethoprim and sulphamethoxazole in a ratio of 1 : 2 rather than 1 : 20
would be more appropriate when testing strains from urine for their
sensitivity to co-trimoxazole.
