Caldelari I, Loeliger B, Langen H, Glauser M P, Moreillon P
Division of Infectious Diseases, Department of Internal Medicine, Centre Hospitalier Universitaire Vaudois, 1011 Lausanne, Switzerland.
Antimicrob Agents Chemother. 2000 Oct;44(10):2802-10. doi: 10.1128/AAC.44.10.2802-2810.2000.
Penicillin tolerance is an incompletely understood phenomenon that allows bacteria to resist drug-induced killing. Tolerance was studied with independent Streptococcus gordonii mutants generated by cyclic exposure to 500 times the MIC of penicillin. Parent cultures lost 4 to 5 log(10) CFU/ml of viable counts/24 h. In contrast, each of four independent mutant cultures lost < or =2 log(10) CFU/ml/24 h. The mutants had unchanged penicillin-binding proteins but contained increased amounts of two proteins with respective masses of ca. 50 and 45 kDa. One mutant (Tol1) was further characterized. The two proteins showing increased levels were homologous to the arginine deiminase and ornithine carbamoyl transferase of other gram-positive bacteria and were encoded by an operon that was >80% similar to the arginine-deiminase (arc) operon of these organisms. Partial nucleotide sequencing and insertion inactivation of the S. gordonii arc locus indicated that tolerance was not a direct consequence of arc alteration. On the other hand, genetic transformation of tolerance by Tol1 DNA always conferred arc deregulation. In nontolerant recipients, arc was repressed during exponential growth and up-regulated during postexponential growth. In tolerant transformants, arc was constitutively expressed. Tol1 DNA transformed tolerance at the same rate as transformation of a point mutation (10(-2) to 10(-3)). The tolerance mutation mapped on a specific chromosomal fragment but was physically distant from arc. Importantly, arc deregulation was observed in most (6 of 10) of additional independent penicillin-tolerant mutants. Thus, although not exclusive, the association between arc deregulation and tolerance was not fortuitous. Since penicillin selection mimicked the antibiotic pressure operating in the clinical environment, arc deregulation might be an important correlate of naturally occurring tolerance and help in understanding the mechanism(s) underlying this clinically problematic phenotype.
青霉素耐受性是一种尚未完全理解的现象，它使细菌能够抵抗药物诱导的杀伤作用。通过将戈登链球菌独立突变体反复暴露于青霉素最低抑菌浓度（MIC）的500倍来研究耐受性。亲代培养物每24小时活菌数减少4至5个对数（10）CFU/ml。相比之下，四个独立突变体培养物中的每一个每24小时活菌数减少≤2个对数（10）CFU/ml。这些突变体的青霉素结合蛋白没有变化，但含有两种蛋白质的量增加，其各自的质量约为50 kDa和45 kDa。对其中一个突变体（Tol1）进行了进一步表征。显示水平增加的这两种蛋白质与其他革兰氏阳性细菌的精氨酸脱亚氨酶和鸟氨酸氨甲酰基转移酶同源，并且由一个与这些生物体的精氨酸脱亚氨酶（arc）操纵子相似度>80%的操纵子编码。戈登链球菌arc基因座的部分核苷酸测序和插入失活表明，耐受性不是arc改变的直接结果。另一方面，用Tol1 DNA进行耐受性的遗传转化总是导致arc去调控。在非耐受性受体中，arc在指数生长期受到抑制，在指数生长后期上调。在耐受性转化体中，arc组成型表达。Tol1 DNA转化耐受性的速率与点突变的转化速率相同（10^-2至10^-3）。耐受性突变定位在一个特定的染色体片段上，但在物理位置上与arc相距较远。重要的是，在大多数（10个中的6个）其他独立的青霉素耐受性突变体中也观察到了arc去调控。因此，虽然不是唯一的，但arc去调控与耐受性之间的关联并非偶然。由于青霉素选择模拟了临床环境中起作用的抗生素压力，arc去调控可能是自然发生的耐受性的一个重要关联因素，并有助于理解这种临床上有问题的表型的潜在机制。
