原核生物毒素-抗毒素系统的研究进展

毒素-抗毒素系统(toxin-antitoxin system,TA)由具有杀菌或抑菌作用的毒素和能中和毒素毒性的抗毒素组成,在细菌和古菌的染色体和可移动遗传元件中分布广泛。TA系统具有结构和功能的多样性,目前分为八大类型(Ⅰ-Ⅷ型)。研究发现TA系统与细菌生物膜的形成、细菌毒力、耐药性细菌感染、质粒拷贝数的调控及原噬菌体切离后在细胞中的稳定维持等相关。文章综述了近年来TA系统的最新分类、TA系统的功能及抗毒素的其他调控功能等进展,并对TA领域的应用做了简要描述。     

原核生物Ⅶ型毒素-抗毒素系统研究进展北大核心

毒素-抗毒素(toxin-antitoxin,TA)系统是普遍存在于细菌、古细菌及原噬菌体中的遗传元件,通常由分别编码毒素和编码抗毒素的基因组成。毒素在细菌细胞中较为稳定,而抗毒素则容易被降解。大多数毒素为蛋白并具有酶的活性,通过影响蛋白质的翻译、DNA的复制等重要生命活动从而对细菌产生毒性,抑制细菌生长。抗毒素为蛋白质或非编码RNA,通过极其多样的方式,中和毒素的毒性。目前发现TA在调控质粒拷贝数、流产性感染、生物被膜的形成等过程中发挥着重要作用。随着研究的不断深入,新型TA不断被发现,极大地促进了我们对于TA的认识。目前TA已经扩展到I‒Ⅷ型,本文总结了近期发现的新型TA,并重点介绍了最新发现的Ⅶ型TA及其特殊的中和机制。由于TA与病原微生物的致病性密切相关,因此,深入研究这些TA可以为耐药微生物的治疗提供新的靶点。     

原核生物毒素-抗毒素系统的研究进展

毒素-抗毒素系统(toxin-antitoxin system,简称TA系统)广泛存在于原核生物(细菌和古菌)的基因组中，通常TA系统由毒素和抗毒素两部分组成，毒素发挥毒性抑制细菌生长，抗毒素可以解除抑制，它们通过体内的调控作用来对细菌或古菌的生长活动进行调节。研究发现，TA系统根据其性质及抗毒素中和毒素的方式不同可以分为8种类型Ⅰ～Ⅷ,不同类型的TA系统之间又存在着错综复杂的交互作用，而且此系统在细菌中发挥的作用也一直是近年来学者们研究的热点。现就TA系统的最新分类、TA系统的功能以及应用作一概述。     

毒素-抗毒素系统介导持留菌形成机制的研究进展

毒素-抗毒素(toxin-antitoxin,TA)系统是广泛存在细菌基因组上的由两个基因组成的操纵子,分别编码稳定的毒素蛋白和不稳定的抗毒素,其中毒素蛋白具有多种生物学功能。持留菌是指能够耐受高浓度抗生素或不利环境的一类细菌,它们同样具有TA系统。现就毒素-抗毒素系统介导持留菌形成机制的研究进展作一综述。     

细菌毒素-抗毒素系统的功能

毒素-抗毒素(Toxin-Antitoxin,TA)系统广泛存在于原核生物和古细菌的染色体和质粒中。此系统由2个共表达的基因组成,分别编码稳定的毒素蛋白和易降解的抗毒素,毒素通常发挥毒性作用抑制细菌生长,而抗毒素则可中和毒性,二者相互作用对细菌生长状态起精密调节作用。根据TA的组成和抗毒素的性质,目前已经发现有6型TA,这些TA系统在细菌中发挥的作用一直是近年来学者们研究的热点,文中对细菌TA的功能研究进展进行了综述。     

毒素-抗毒素系统的调控、生理功能及应用

毒素-抗毒素（toxin-antitoxin,TA）系统是由抗毒素及其同源毒素组成的小遗传元件,毒素可以抑制细胞生长或诱导细胞死亡,抗毒素则可以中和毒素的毒性。根据TA系统的组成和抗毒素的作用方式,TA系统可分为Ⅰ~Ⅷ型共八类,其中Ⅱ型TA系统存在最广泛,调控机制研究得最清楚。TA系统可以维持质粒等遗传元件的稳定性,同时在压力应激、促进生物膜形成、维持细菌致病力、抗噬菌体等方面都扮演着重要角色。研究TA系统的调控与生理功能能丰富人们对于生物多样性的认知,对于微生物资源的开发和利用具有重要的科学意义与应用价值。基于毒素和抗毒素的特点,TA系统被应用于生物医学领域和生物技术领域。本文综述了TA系统的分类、调控机制、生理功能和应用并简单描述了TA系统目前研究面临的问题和未来展望。     

细菌II型毒素-抗毒素系统活性的调控

细菌毒素-抗毒素系统(Toxin-antitoxin system,TA)由稳定的毒素和不稳定的抗毒素构成,几乎存在于所有细菌中。已证明染色体编码的II型TA系统作为胁迫反应因子,通过毒素作用于不同的细胞靶点来调控重要的细胞活动过程,使细菌适应不同的环境胁迫。因此,毒素活性的调控是II型TA系统介导细菌适应性胁迫反应的关键。本文总结了II型TA系统毒素活性调控机制的研究进展,并介绍了作者近年来对模式蓝藻Synechocystis sp.PCC6803中II型TA毒素活性调控的研究结果。     

毒素-抗毒素系统介导滞留菌形成机制

滞留菌是一类处于低代谢休眠状态的抗生素耐受细菌亚群，能够在致死性压力应激后存活下来，是抗生素治疗失败和复发性感染的主要原因之一。毒素-抗毒素系统(toxin-antitoxin system, TA)作为压力应激模块普遍存在于各种细菌中，由稳定的毒素和不稳定但可以中和毒素的同源抗毒素组成。压力情况下，第二信使(p)ppGpp激活Lon,随后大多数II型TA系统被激活，诱导滞留菌形成。同样在(p)ppGpp存在的情况下，Obg刺激hokB转录，使毒素积累，抑制细菌DNA复制、转录、翻译等重要的生理过程，驱动细菌形成滞留菌。SOS反应是激活TA系统的另一个主要途径，解除了对tisB转录的抑制，使其在细胞内积累并插入细胞膜，破坏质子动力势，降低胞内ATP水平，诱使休眠和滞留菌形成。讨论TA系统介导滞留菌形成的机制有助于提出新型抗菌策略。     

细菌毒素-抗毒素系统的研究进展

毒素-抗毒素系统(toxin-antitoxin system,TA)由两个共表达的基因组成,其中一个基因编码不稳定的抗毒素蛋白(antitoxin),另一个基因编码稳定的毒素蛋白(toxin).毒素-抗毒素系统最早发现于一些低拷贝的质粒,用来维持低拷贝质粒在菌群中的稳定存在.随后的研究表明,毒素-抗毒素系统广泛存在于细菌,包括一些致病菌的染色体上.在营养缺乏等不良生长条件下,由于基因表达的抑制和蛋白酶的降解作用,不稳定的抗毒素蛋白减少,从而产生游离的毒素蛋白,导致细菌的生长抑制和死亡.毒素-抗毒素系统的生理功能目前还存在争议,有学者认为细茼染色体上的毒素-抗毒素系统可以在不良生长状况下介导细菌的死亡,即细茼程序性细胞死亡(baeterial programmedcell death).但也有证据显示,毒素-抗毒素系统的功能更偏向于应激状态下的生理调节方面,即只起应激状态下的抑菌作用而不是杀菌作用.对细菌生长调控中毒素-抗毒素系统的作用机理进行综述,并探讨毒素-抗毒素系统研究的理论和应用价值.     

细菌的毒素-抗毒素系统:定时炸弹还是免死金牌

毒素-抗毒素系统(Toxin-antitoxin system,TA)在细菌和古菌的染色体和可移动遗传元件中广泛分布,目前分为六大类型(I型-VI型)。研究发现TA能够促进多重耐药菌群的形成,同时参与细菌的程序性死亡、调控生物被膜形成、介导细菌环境适应过程等多个重要的生命过程。TA的研究主要集中在肠道细菌和病原菌中,其中II型TA研究最为深入和广泛。本文综述了近年来新型TA的鉴定、毒素新型作用靶点、抗毒素的调控功能以及TA间的相互作用等进展,并对未来的TA领域的潜在发展趋势和应用前景也进行了评述。     

裂解或溶源-细菌遭遇噬菌体后的命运抉择

噬菌体是地球上数量最丰富的有机体,其在自然生态系统的塑造和细菌进化驱动中发挥着至关重要的作用。在与宿主的相互斗争中,噬菌体可以选择以下2种方式决定其与宿主的命运：（1）裂解：通过裂解宿主细胞最终大量释放噬菌体颗粒;（2）溶原：将其染色体整合到宿主细胞基因组中,与宿主建立一种潜在的互存关系。对于一些温和的噬菌体,这种倾向进一步受到感染多样性的调节,其中单一感染主要是裂解性的,而多重感染则多是溶原性的。溶原性的噬菌体不仅可以根据外界环境的理化因子,还可以通过细菌自身的群体感应系统来启动裂解-溶原开关,进而决定其宿主菌的命运。与此同时,宿主细菌在与噬菌体长时间的斗争中也进化出了针对噬菌体的手段。总而言之,噬菌体深刻影响着细菌的群落动态、基因组进化和生态系统等,而这一切都取决于噬菌体与宿主间的斗争模式（裂解/溶原性感染）。本文探讨了导致温和噬菌体对宿主菌进行裂解-溶原命运抉择的影响因素并系统性总结了细菌在面对噬菌体侵染时的应对策略的最新研究进展,以期能为噬菌体与宿主的研究提供建议和帮助。     

Early Abortive Lysis by Phage BF23 in Escherichia coli K-12 Carrying the Colicin Ib Factor

Growth of phage BF23 was restricted in Escherichia coli K-12 strains carrying a colicin I factor (ColIb); most infected cells lysed early without producing progeny phages. Either addition of chloramphenicol before phage infection or ultraviolet irradiation of phage prevented early abortive lysis, an indication that certain phage functions are required for this phenomenon. Very little or no phage-induced lysozyme was synthesized in the infected ColI(+) cells. This result suggests that early abortive lysis was not due to the lysozyme action. A small fraction (0.05) of BF23-infected ColI(+) cells showed normal phage growth. This "escaped growth" may reflect the physiological state of the host bacteria rather than the heterogeneity of the infecting phage. Host-controlled modification was not observed. A phage mutant, BF23hI, able to grow on ColI(+) cells, was isolated and was characterized to be recessive to the wild-type BF23 in its ability to undergo early abortive lysis. Among the T series phages, T5 induced early abortive lysis, and growth of T5 was restricted upon infection to ColI(+) cells. These results and the other observations, including the occurrence of phenotypic mixing between BF23 and T5, suggest that these two phages are related to each other even though the receptor sites for BF23 and T5 are apparently different.     

Characterization of lytic Pseudomonas aeruginosa bacteriophages via biological properties and genomic sequences

Pseudomonas aeruginosa is an important cause of infections, especially in patients with immunodeficiency or diabetes. Antibiotics are effective in preventing morbidity and mortality from Pseudomonas infection, but because of spreading multidrug-resistant bacterial strains, bacteriophages are being explored as an alternative therapy. Two newly purified broad host range Pseudomonas phages, named vB_Pae-Kakheti25 and vB_Pae-TbilisiM32, were characterized as candidates for use in phage therapy. Morphology, host range, growth properties, thermal stability, serology, genomic sequence, and virion composition are reported. When phages are used as bactericides, they are used in mixtures to overcome the development of resistance in the targeted bacterial population. These two phages are representative of diverse siphoviral and podoviral phage families, respectively, and hence have unrelated mechanisms of infection and no cross-antigenicity. Composing bactericidal phage mixtures with members of different phage families may decrease the incidence of developing resistance through a common mechanism.     

Genetic and physiological studies of abortive infections of hydroxymethyluracil-containing bacteriophages in lysogens of temperate Bacillus subtilis bacteriophage SPO2.

Wild-type bacteriophage phie and IS (interference-sensitive) mutants of the related phage SP82G did not productively infect strains of Bacillus subtilis that were lysogenic for temperate phage SPO2. In these abortive infections, the sensitive phages adsorbed to and penetrated the nonpermissive host, phage-directed macromolecular syntheses were initiated, but both viral and bacterial nucleic acid production abruptly stopped about 15 min after addition of the phages. The cessation of RNA and DNA synthesis was preceded or coincident with a reduction in oxygen utilization by the infected cultures. Genetic studies of both phie and SP82G suggest sensitivity to SPO2-mediated abortive infection was controlled by a single gene. A mutant of SPO2, SPO2ehp4-, lysogens of which no longer interfere with the development of SP82GIs, was also isolated. The discovery of this ehp- variant suggests the normal SPO2 prophage synthesized a substance that alters cell physiology in some manner detrimental to SP82GIs development. Since SPO2ehp4- grew on and lysogenized bacteria sensitive to wild-type SPO2, the product of the eph gene was apparently not an essential function of this temperate phage.Overall, these observations exhibit remarkable similarities to the inhibition of T4rII mutants by the product of the rex gene of phage lambda.     

Biological consequences of infection of Escherichia coli B by alkylated T7 bacteriophage.

Alkylation of T7 bacteriophage considerably delayed phage development and reduced the phage's killing action on host cells. Only a small fraction of infected cells produced phage. For these phages, the latent period was markedly prolonged but the burst was equivalent to or only slightly lower than that of untreated phage. In the progeny of alkylated phage, there was an increase in the fraction of defective particles as well as a change in their morphology. These data show that infection with alkylated T7 bacteriophage is to a large degree abortive; hence, biological consequences of this infection are very different from those characteristic of a normal virus infection.     

Within-host competition determines reproductive success of temperate bacteriophages

Within-host competition between parasites is frequently invoked as a major force for parasite evolution, yet quantitative studies on its extent in an organismal group are lacking. Temperate bacteriophages are diverse and abundant parasites of bacteria, distinguished by their ability to enter a facultative dormant state in their host. Bacteria can accumulate multiple phages that may eventually abandon dormancy in response to host stress. Host resources are then converted into phage particles, whose release requires cell death. To study within-host competition between phages, I used the bacterium Escherichia coli and 11 lambdoid phages to construct single and double lysogens. Lysogenic bacterial cultures were then induced and time to host cell lysis and productivity of phages was measured. In double lysogens, this revealed strong competitive interactions as in all cases productivity of at least one phage declined. The outcome of within-host competition was often asymmetrical, and phages were found to vary hierarchically in within-host competitive ability. In double infections, the phage with the shorter lysis time determined the timing of cell lysis, which was associated with a competitive advantage when time differences were large. The results emphasize that within-host competition greatly affects phage fitness and that multiple infections should be considered an integral part of bacteriophage ecology.     

Bacteriophage infection is targeted to cellular poles

The poles of bacteria exhibit several specialized functions related to the mobilization of DNA and certain proteins. To monitor the infection of Escherichia coli cells by light microscopy, we developed procedures for the tagging of mature bacteriophages with quantum dots. Surprisingly, most of the infecting phages were found attached to the bacterial poles. This was true for a number of temperate and virulent phages of E. coli that use widely different receptors and for phages infecting Yersinia pseudotuberculosis and Vibrio cholerae. The infecting phages colocalized with the polar protein marker IcsA-GFP. ManY, an E. coli protein that is required for phage lambda DNA injection, was found to localize to the bacterial poles as well. Furthermore, labelling of lambda DNA during infection revealed that it is injected and replicated at the polar region of infection. The evolutionary benefits that lead to this remarkable preference for polar infections may be related to lambda's developmental decision as well as to the function of poles in the ability of bacterial cells to communicate with their environment and in gene regulation.     

The minor capsid protein gp7 of bacteriophage SPP1 is required for efficient infection of Bacillus subtilis

Gp7 is a minor capsid protein of the Bacillus subtilis bacteriophage SPP1. Homologous proteins are found in numerous phages but their function remained unknown. Deletion of gene 7 from the SPP1 genome yielded a mutant phage (SPP1del7) with reduced burst-size. SPP1del7 infections led to normal assembly of virus particles whose morphology, DNA and protein composition was undistinguishable from wild-type virions. However, only approximately 25% of the viral particles that lack gp7 were infectious. SPP1del7 particles caused a reduced depolarization of the B. subtilis membrane in infection assays suggesting a defect in virus genome traffic to the host cell. A higher number of SPP1del7 DNA ejection events led to abortive release of DNA to the culture medium when compared with wild-type infections. DNA ejection in vitro showed that no detectable gp7 is co-ejected with the SPP1 genome and that its presence in the virion correlated with anchoring of released DNA to the phage particle. The release of DNA from wild-type phages was slower than that from SPP1del7 suggesting that gp7 controls DNA exit from the virion. This feature is proposed to play a central role in supporting correct routing of the phage genome from the virion to the cell cytoplasm.     

Large variabilities in host strain susceptibility and phage host range govern interactions between lytic marine phages and their Flavobacterium hosts

Phages are a main mortality factor for marine bacterioplankton and are thought to regulate bacterial community composition through host-specific infection and lysis. In the present study we demonstrate for a marine phage-host assemblage that interactions are complex and that specificity and efficiency of infection and lysis are highly variable among phages infectious to strains of the same bacterial species. Twenty-three Bacteroidetes strains and 46 phages from Swedish and Danish coastal waters were analyzed. Based on genotypic and phenotypic analyses, 21 of the isolates could be considered strains of Cellulophaga baltica (Flavobacteriaceae). Nevertheless, all bacterial strains showed unique phage susceptibility patterns and differed by up to 6 orders of magnitude in sensitivity to the same titer of phage. The isolated phages showed pronounced variations in genome size (8 to >242 kb) and host range (infecting 1 to 20 bacterial strains). Our data indicate that marine bacterioplankton are susceptible to multiple co-occurring phages and that sensitivity towards phage infection is strain specific and exists as a continuum between highly sensitive and resistant, implying an extremely complex web of phage-host interactions. Hence, effects of phages on bacterioplankton community composition and dynamics may go undetected in studies where strain identity is not resolvable, i.e., in studies based on the phylogenetic resolution provided by 16S rRNA gene or internal transcribed spacer sequences.     

Alteration of Host Specificity to Lytic Bacteriophages in Streptococcus cremoris

A mutant of Streptococcus cremoris strain ML1 was isolated based on its resistance to acriflavine. The mutant strain showed resistance to the growth of virulent bacteriophages to which the parental strain was sensitive whereas it became sensitive to a number of other virulent phages to which the parental strain was resistant. At the same time, infection of the mutant strain by another bacteriophage sc607 resulted in killing of cells without production of progeny phages. The phage adsorption appeared normal, suggesting that the killing was a postadsorption event. Such killing of bacterial cells was prevented by chloramphenicol treatment, indicating that involvement of some protein either synthesized by phage or phage-induced cellular protein. Synthesis of ribonucleic acid was abruptly terminated after infection of the mutant strain by phage sc607 but not of the parental strain. The alteration of host specificity in the mutant to different lytic bacteriophages and especially abortive infection by phage sc607 resembles the prophage-mediated interference observed in other bacteria.