近日，国际著名杂志PLoS Pathogen在线刊登了英国和日本研究人员共同合作的研究成果“Specialized Peptidoglycan Hydrolases Sculpt the Intra bacterial Niche of Predatory Bdellovibrio and Increase Population Fitness，”，文章中，研究者解析了掠夺性的食菌蛭弧菌可以利用特有的肽多糖水解酶来创造细菌自身的内环境并且增加细菌群体之间的适应性。
食菌蛭弧菌（Bdellovibrio bacteriovorus）是一种小的掠夺性细菌，可以侵入许多革兰氏阴性菌的细胞周质中，然后将被捕食细菌聚集成球状结构，这种球状结构被称为bdelloplasts，渗透压稳定，食菌蛭弧菌就利用这种方式来杀死被捕食细菌，并且在所形成的球状结构中进行稳定复制；在bdelloplasts这种结构中，食菌蛭弧菌可以获得足够的营养来繁殖，而且这种结构的形成和修饰过程中，被捕食细菌周质中的肽多糖发挥着必不可少的作用。
在细菌正常的生长过程中，青霉素结合蛋白（PBPs）可以对细胞膜中的肽多糖层进行合成并且修饰，肽多糖层可以给细菌提供一个稳定的渗透压环境以及对于细菌维持其细胞性状必不可少。细菌的PBPs分为三种类型，ClassA、B和C，Class A是肽多糖合酶，可以聚合肽聚糖单体成为新生链并且使得多肽交联生成，Class B仅仅具有肽交联活性。随着细菌的伸长和分裂，细胞壁也会生长，细菌细胞生长过程中，需要不断修饰肽多糖以便掺入更多的细胞壁结构单位，因此，细菌有多种多样的肽多糖水解酶，这些酶类可以清理自身的肽聚糖，因此PBPs在这个过程中扮演者多酶复合体的作用。
Class C的PBPs主要负责剪切或者重塑肽多糖的功能，它们包含着DD-羧肽酶类和DD-内肽酶类，前者主要从五肽中移去末端的D-丙氨酸残基，后者主要水解D-丙氨酸-meso-Dap的交联。研究者在这项研究中发现了Class C PBPs的一种新的用途，早在1978年，研究者在被食菌蛭弧菌入侵的的细菌混合物种就检测到了肽多糖的酶活性。
如今研究者运用转录组学分析了在食菌蛭弧菌侵入细菌过程中所诱导高表达的两个同源基因：bd0816和bd3459，这两个基因具有肽多糖修饰酶的活性。运用定点突变的方法，研究者表示，这两个基因编码的产物可以自行高度聚集起来，修饰被捕食细菌的肽多糖层，最终将被捕食细菌聚集成为bdellolast球状结构，从而使得食菌蛭弧菌更好的进行营养摄取和繁殖，研究者的研究发现为食菌蛭弧菌侵入革兰氏阴性菌的机制提供了新的视野。（生物谷：T.Shen编译）
doi:10.1371/journal.ppat.1002524
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Specialized Peptidoglycan Hydrolases Sculpt the Intra-bacterial Niche of Predatory Bdellovibrio and Increase Population Fitness
Thomas R. Lerner1#, Andrew L. Lovering2#, Nhat Khai Bui3, Kaoru Uchida4, Shin-Ichi Aizawa4, Waldemar Vollmer3, R. Elizabeth Sockett1*
Bdellovibrio are predatory bacteria that have evolved to invade virtually all Gram-negative bacteria, including many prominent pathogens. Upon invasion, prey bacteria become rounded up into an osmotically stable niche for the Bdellovibrio, preventing further superinfection and allowing Bdellovibrio to replicate inside without competition, killing the prey bacterium and degrading its contents. Historically, prey rounding was hypothesized to be associated with peptidoglycan (PG) metabolism; we found two Bdellovibrio genes, bd0816 and bd3459, expressed at prey entry and encoding proteins with limited homologies to conventional dacB/PBP4 DD-endo/carboxypeptidases (responsible for peptidoglycan maintenance during growth and division). We tested possible links between Bd0816/3459 activity and predation. Bd3459, but not an active site serine mutant protein, bound β-lactam, exhibited DD-endo/carboxypeptidase activity against purified peptidoglycan and, importantly, rounded up E. coli cells upon periplasmic expression. A ΔBd0816 ΔBd3459 double mutant invaded prey more slowly than the wild type (with negligible prey cell rounding) and double invasions of single prey by more than one Bdellovibrio became more frequent. We solved the crystal structure of Bd3459 to 1.45 Å and this revealed predation-associated domain differences to conventional PBP4 housekeeping enzymes (loss of the regulatory domain III, alteration of domain II and a more exposed active site). The Bd3459 active site (and by similarity the Bd0816 active site) can thus accommodate and remodel the various bacterial PGs that Bdellovibrio may encounter across its diverse prey range, compared to the more closed active site that “regular” PBP4s have for self cell wall maintenance. Therefore, during evolution, Bdellovibrio peptidoglycan endopeptidases have adapted into secreted predation-specific proteins, preventing wasteful double invasion, and allowing activity upon the diverse prey peptidoglycan structures to sculpt the prey cell into a stable intracellular niche for replication.
