近日,，國際著名雜志PLoS Pathogen在線刊登了英國和日本研究人員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,，研究者解析了掠奪性的食菌蛭弧菌可以利用特有的肽多糖水解酶來創(chuàng)造細菌自身的內(nèi)環(huán)境并且增加細菌群體之間的適應(yīng)性,。
食菌蛭弧菌（Bdellovibrio bacteriovorus）是一種小的掠奪性細菌,，可以侵入許多革蘭氏陰性菌的細胞周質(zhì)中，然后將被捕食細菌聚集成球狀結(jié)構(gòu),，這種球狀結(jié)構(gòu)被稱為bdelloplasts,，滲透壓穩(wěn)定，食菌蛭弧菌就利用這種方式來殺死被捕食細菌,，并且在所形成的球狀結(jié)構(gòu)中進行穩(wěn)定復制,；在bdelloplasts這種結(jié)構(gòu)中，食菌蛭弧菌可以獲得足夠的營養(yǎng)來繁殖,，而且這種結(jié)構(gòu)的形成和修飾過程中,，被捕食細菌周質(zhì)中的肽多糖發(fā)揮著必不可少的作用。
在細菌正常的生長過程中,，青霉素結(jié)合蛋白（PBPs）可以對細胞膜中的肽多糖層進行合成并且修飾,，肽多糖層可以給細菌提供一個穩(wěn)定的滲透壓環(huán)境以及對于細菌維持其細胞性狀必不可少。細菌的PBPs分為三種類型,，ClassA、B和C,，Class A是肽多糖合酶,，可以聚合肽聚糖單體成為新生鏈并且使得多肽交聯(lián)生成，Class B僅僅具有肽交聯(lián)活性,。隨著細菌的伸長和分裂,，細胞壁也會生長，細菌細胞生長過程中,，需要不斷修飾肽多糖以便摻入更多的細胞壁結(jié)構(gòu)單位,，因此，細菌有多種多樣的肽多糖水解酶,，這些酶類可以清理自身的肽聚糖,，因此PBPs在這個過程中扮演者多酶復合體的作用。
Class C的PBPs主要負責剪切或者重塑肽多糖的功能,，它們包含著DD-羧肽酶類和DD-內(nèi)肽酶類,，前者主要從五肽中移去末端的D-丙氨酸殘基，后者主要水解D-丙氨酸-meso-Dap的交聯(lián),。研究者在這項研究中發(fā)現(xiàn)了Class C PBPs的一種新的用途,，早在1978年，研究者在被食菌蛭弧菌入侵的的細菌混合物種就檢測到了肽多糖的酶活性,。
如今研究者運用轉(zhuǎn)錄組學分析了在食菌蛭弧菌侵入細菌過程中所誘導高表達的兩個同源基因：bd0816和bd3459,，這兩個基因具有肽多糖修飾酶的活性。運用定點突變的方法,，研究者表示,，這兩個基因編碼的產(chǎn)物可以自行高度聚集起來,，修飾被捕食細菌的肽多糖層，最終將被捕食細菌聚集成為bdellolast球狀結(jié)構(gòu),，從而使得食菌蛭弧菌更好的進行營養(yǎng)攝取和繁殖,，研究者的研究發(fā)現(xiàn)為食菌蛭弧菌侵入革蘭氏陰性菌的機制提供了新的視野。（生物谷：T.Shen編譯）
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.
