Heterogeneity of tetracycline resistance determinants in Streptococcus

Abstract

Naturally occurring tetracycline resistance in streptococci [S. Faecalis, S. agalactiae and S. pneumoniae] is encoded by more than 1 genetic determinant. Two of these distinct determinants were cloned, and the regions that are necessary and sufficient for expression of tetracycline resistance were defined by deletion analysis. These cloned determinants were further characterized by DNA-DNA hybridization experiments which also identified a 3rd genetically unrelated tetracycline resistance determinant. Some of these genetic differences appear to represent mechanistic differences. The tetL determinant was associated with small nonconjugative plasmids and mediated resistance to tetracycline. The tetM determinant was most often nonplasmid associated and mediated resistance to minocycline as well as tetracycline. The tetN determinant was represented on a large conjugative plasmid and was genetically distinct from tetL and tetM, although phenotypically it resembled tetM.