R Factor with a Mutation in the Tetracycline Resistance Marker

Abstract

THERE is evidence that resistance to tetracyclines (TCR) in Gram-negative bacteria that is mediated by R factors involves an inducible mechanism. Thus two levels of resistance to tetracyclines have been demonstrated in various strains of Escherichia coli harbouring R factors^1,2. R factor bearing cells cultured in the absence of tetracycline are about 50% resistant when challenged with 50 µg of tetracycline/ml., as can be seen from the effect of the drug on the level of protein synthesis in the cells². Cells cultured in the presence of a sub-inhibitory concentration of tetracycline, however, are not significantly inhibited by a subsequent challenge with 50 µg of tetracycline/ml.; the 50% level of inhibition is only reached at concentrations in excess of 150 µg/ml. This adaptation, which is apparently associated with a decrease in the absorption of tetracycline by the cells, can be suppressed with inhibitors of protein and RNA synthesis². The tetracycline resistance mechanism thus resembles an inducible system although the end products of the resistance gene(s) have not yet been identified.