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SENSITIVITY TESTING WITH TRIMETHOPRIM/SULPHAMETHOXAZOLE
Author(s) -
BUSHBY STANLEY R. M.
Publication year - 1973
Publication title -
medical journal of australia
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.904
H-Index - 131
eISSN - 1326-5377
pISSN - 0025-729X
DOI - 10.5694/j.1326-5377.1973.tb111179.x
Subject(s) - trimethoprim , drug resistance , biology , thymidine , in vitro , in vivo , antibiotics , microbiology and biotechnology , chemistry , biochemistry
In practice, TMP/SMX should be regarded as a single drug; its two components ultimately have the same effect on bacteria, that is, to deprive an organism of its folate co‐enzymes. Synergy occurs over a wide range of ratios, and attainment of optimum proportions assumes importance only when minimum concentrations are present. Sensitivity testing presents no problem provided a small inoculum is used, and the medium contains < 0.01 μg of thymidine/ml. Although thymidine interferes with the in‐vitro activity of TMP/SMX, it is not present in animals in sufficiently high concentrations to affect the in‐vivo activity. Sulphonamides enhance the sensitivity of sulphonamide‐resistant strains to TMP because, in spite of apparent resistance, the sulphonamides depress the store of folates in these organisms. Synergy can only be excluded by performing complicated two‐dimensional titration tests. In practice, inhibition zones of 14 mm indicate that the infection will respond to treatment with TMP/SMX. The effect of sulphonamides in delaying the emergence of TMP resistance is inversely proportional to the degree of sulphonamide resistance. The majority of TMP‐resistant variants, at least of the Enterobacteriaceas family, are probably avirulent. More significant is the detection of R factors conferring a high degree of TMP resistance; however, so far the transference rates are low, suggesting that the likelihood of the spread of these factors is not high.