Streptococcal tetracycline resistance mediated at the level of protein synthesis

The mechanism of tetracycline resistance was examined in strains containing each of the three previously identified resistance determinants in Streptococcus spp. Uptake of tetracycline was measured in tetracycline-sensitive cells as well as in cells containing each of the three resistance determinants. In cells containing tetL, uptake was not observed. However, in sensitive cells and cells containing either tetM or tetN, tetracycline was accumulated approximately 25-fold against a concentration gradient. Furthermore, there was no evidence for modification of intracellular tetracycline recovered from sensitive, tetM, or tetN cells. Protein synthesis in extracts derived from organisms containing tetM or tetN was resistant to tetracycline. In contrast, extracts of sensitive and tetL cells were sensitive to tetracycline.