Fling M E, Walton L, Elwell L P
Antimicrob Agents Chemother. 1982 Nov;22(5):882-8. doi: 10.1128/AAC.22.5.882.
Using-gene-specific radiolabeled probe DNAs, we analyzed 42 clinical bacterial isolates with high-level trimethoprim (Tp) resistance for the presence of a type I or a type II plasmid-specified dihydrofolate reductase (DHFR) gene. Plasmid DNA from 17 strains harbored a type I DHFR, whereas 11 isolates contained plasmids that harbored a type II DHFR structural gene. The plasmid DNAs from five strains appeared to hybridize with both type I and type II DHFR probe DNAs. In addition, eight isolates had type I resistance determinants integrated into the chromosomes, presumably on transposon 7 (Tn7). Among the strains analyzed in this survey, none of the chromosomally located, Tp-insensitive reductases were of the type II class. Both the plasmid and chromosomal DNAs of one isolate showed no homology with either the type I or type II DHFR probe DNA. The plasmid harbored by this strain encoded a "new" Tp-resistant enzyme that differed significantly, both in molecular weight and with respect to trimethoprim and methotrexate inhibition kinetics, from the previously characterized plasmid-associated dihydrofolate reductases.
我们使用基因特异性放射性标记探针DNA，分析了42株对甲氧苄啶（Tp）具有高水平抗性的临床细菌分离株，以检测I型或II型质粒指定的二氢叶酸还原酶（DHFR）基因的存在情况。17株菌株的质粒DNA含有I型DHFR，而11株分离株含有携带II型DHFR结构基因的质粒。5株菌株的质粒DNA似乎与I型和II型DHFR探针DNA都发生杂交。此外，8株分离株的I型抗性决定簇整合到了染色体上，推测是在转座子7（Tn7）上。在本次调查分析的菌株中，所有染色体定位的对Tp不敏感的还原酶均不属于II类。一株分离株的质粒和染色体DNA与I型或II型DHFR探针DNA均无同源性。该菌株携带的质粒编码一种“新型”Tp抗性酶，其分子量以及对甲氧苄啶和甲氨蝶呤的抑制动力学与先前鉴定的质粒相关二氢叶酸还原酶均有显著差异。
