细菌可持续发展有妙招

　　

美国圣母大学生命科学院的教授Shahriar Mobashery近日发表新文章，他们发现，细菌也有可持续发展意识，细菌会循环利用自身的细胞壁。在最新的文章中，科学家揭示这一循环利用过程中的相关机制。

 

细胞壁是维持细菌形态，维护细菌正常生理功能的重要器官。细胞壁是一个坚硬的实体器官，它包围在细菌周围，是细菌抵御外界环境的第一道防线，很多抗生素的设计就是以细菌细胞壁为靶位，破坏细胞壁以杀死侵入机体的细菌。

 

科学家们发现，细菌在自我增殖的过程中亲代的细胞壁可回收被子代利用。尽管这一机制是众所皆知的，但是，这一机制具体复杂的过程一直没有被成功的解读。

 

Mobashery的研究小组在实验室合成大肠杆菌（Escherichia coli）的细胞壁，通过合成的细胞壁来观察细胞壁在循环利用过程中发生的一系列反应。

 

研究小组发现，糖基转移酶家族的M1tB在细胞壁回收过程中起关键的作用。他们还调查了M1tB转化细胞壁的效率，结果，在细菌繁殖的一个世代里，每个M1tB酶分子可回收14,000个细胞壁。

 

产生M1tB只是循环之路的起点，当M1tB从细菌出来进入人体时，会导致人体感染细菌的促炎症反应。

 

原文摘要：Lytic Transglycosylase MltB of Escherichia coli and Its Role in Recycling of Peptidoglycan Strands of Bacterial Cell Wall

 

. Am. Chem. Soc., 130 (36), 11878–11879, 2008. 10.1021/ja805482b

 

【Abstract】

The cell wall is an indispensable structure for the survival of bacteria and a target for antibiotics. Peptidoglycan is the major constituent of the cell wall, which is comprised of backbone repeats of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). A peptide stem is appended to the NAM unit, which in turn experiences cross-linking with a peptide from another peptidoglycan in the final steps of cell wall assembly. In the normal course of bacterial growth, as much as 60% of the parental cell wall is recycled, a process that is not fully understood. A polymeric cell wall is fragmented by the family of lytic transglycosylases, and certain key fragments are transported to the cytoplasm for recycling. The genes for the six known lytic transglycosylases of Escherichia coli were cloned, and the enzymes were purified in this study. It is shown that MltB is the only lytic transglycosylase to turn over a synthetic peptidoglycan fragment of two NAG-NAM repeats; hence this enzyme is likely to be the lytic transglycosylase responsible for processing of shorter peptidoglycan strands. Lytic transglycosylases have been proposed to go through an oxocarbenium species that would trap the 6-hydroxyl moiety of the glucosamine residue of muramic acid to generate the so-called 1,6-anhydromuramyl moiety. It is documented herein by characterization of the products of turnover that this process takes place to the total exclusion of the entrapment of a water molecule by the reactive intermediary oxocarbenium species. Furthermore, turnover of the E. coli sacculus (whole cell wall) by MltB was characterized. It is documented that each MltB molecule is able to process the cell wall 14 000 times in the course of a single doubling time for E. coli.

 

（生物通 张欢）