Influence of composition of the culture media on the in vitro determination of the bacterial sensitivity to sulfonamides, trimethoprim and cotrimoxazole.

The difficulties concerning the interpretation of results of susceptibility tests to sulphonamides, trimethoprim, co-trimoxazole have been investigated. The zones of inhibition produced by discs containing these drugs are peculiar since frequently they contain a light bacterial growth and small colonies. Therefore the real sensitivity or resistance of the microorganism is questionable. Susceptibility tests performed on media of different composition, have shown that the phenomenon is chiefly due to the presence in the medium of thymidine and methionine. Both the substances are end-products of the metabolic pathway on which act sulphonamides, trimethoprim, co-trimoxazole. When they are in the medium, they can be available for the bacteria in spite of the inhibition caused by the mentioned drugs.