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首页> 外文期刊>Applied Microbiology >Mycoplasma agalactiae Secretion of β-(1→6)-Glucan, a Rare Polysaccharide in Prokaryotes, Is Governed by High-Frequency Phase Variation
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Mycoplasma agalactiae Secretion of β-(1→6)-Glucan, a Rare Polysaccharide in Prokaryotes, Is Governed by High-Frequency Phase Variation

机译：β-（1→6）-葡聚糖（原核生物中的一种稀有多糖）的无乳支原体分泌受到高频相变的控制。

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Mycoplasmas are minimal, wall-less bacteria but have retained the ability to secrete complex carbohydrate polymers that constitute a glycocalyx. In members of the Mycoplasma mycoides cluster, which are important ruminant pathogens, the glycocalyx includes both cell-attached and cell-free polysaccharides. This report explores the potential secretion of polysaccharides by M. agalactiae , another ruminant pathogen that belongs to a distant phylogenetic group. Comparative genomic analyses showed that M. agalactiae possesses all the genes required for polysaccharide secretion. Notably, a putative synthase gene ( gsmA ) was identified, by in silico reconstruction of the biosynthetic pathway, that could be involved in both polymerization and export of the carbohydrate polymers. M. agalactiae polysaccharides were then purified in vitro and found to be mainly cell attached, with a linear β-(1→6)-glucopyranose structure [β-(1→6)-glucan]. Secretion of β-(1→6)-glucan was further shown to rely on the presence of a functional gsmA gene, whose expression is subjected to high-frequency phase variation. This event is governed by the spontaneous intraclonal variation in length of a poly(G) tract located in the gsmA coding sequence and was shown to occur in most of the M. agalactiae clinical isolates tested in this study. M. agalactiae susceptibility to serum-killing activity appeared to be dictated by ON/OFF switching of β-(1→6)-glucan secretion, suggesting a role of this phenomenon in survival of the pathogen when it invades the host bloodstream. Finally, β-(1→6)-glucan secretion was not restricted to M. agalactiae but was detected also in M. mycoides subsp. capri PG3~(T), another pathogen of small ruminants.IMPORTANCE Many if not all bacteria are able to secrete polysaccharides, either attached to the cell surface or exported unbound into the extracellular environment. Both types of polysaccharides can play a role in bacterium-host interactions. Mycoplasmas are no exception despite their poor overall metabolic capacity. We showed here that M. agalactiae secretes a capsular β-(1→6)-glucopyranose thanks to a specific glycosyltransferase with synthase activity. This secretion is governed by high-frequency ON/OFF phase variation that might be crucial in mycoplasma host dissemination, as cell-attached β-(1→6)-glucopyranose increases serum-killing susceptibility. Our results provide functional genetic data about mycoplasmal glycosyltransferases with dual functions, i.e., assembly and export of the sugar polymers across the cell membrane. Furthermore, we demonstrated that nonprotein epitopes can be subjected to surface antigenic variation in mycoplasmas. Finally, the present report contributes to unravel the role of secreted polysaccharides in the virulence and pathogenicity of these peculiar bacteria.
机译：支原体是最小的无壁细菌，但保留了分泌构成糖萼的复杂碳水化合物聚合物的能力。在作为重要反刍动物病原体的支原体支原体簇的成员中，糖萼包括细胞附着的和无细胞的多糖。该报告探讨了无乳链球菌（一种属于遥远的系统发生群的反刍动物病原体）可能分泌的多糖。比较基因组分析表明，无乳分枝杆菌拥有多糖分泌所需的所有基因。值得注意的是，通过计算机合成生物合成途径，鉴定出可能的合成酶基因（gsmA），其可能参与碳水化合物聚合物的聚合和输出。然后在体外纯化无乳链球菌多糖，发现其主要是细胞附着的，具有线性β-（1→6）-吡喃葡萄糖结构[β-（1→6）-葡聚糖]。进一步显示β-（1→6）-葡聚糖的分泌依赖于功能性gsmA基因的存在，该基因的表达受到高频相位变化的影响。此事件由位于gsmA编码序列中的poly（G）束长度的自发克隆内变异控制，并且在本研究中测试的大多数无乳分枝杆菌临床分离株中均显示出此事件。无乳分枝杆菌对血清杀伤活性的敏感性似乎是由β-（1→6）-葡聚糖分泌的ON / OFF切换决定的，表明该现象在病原体入侵宿主血流时在病原体存活中的作用。最后，β-（1→6）-葡聚糖的分泌不仅限于无乳分枝杆菌，而且还可以在蕈状支原体亚种中检测到。重要的是小反刍动物的另一种病原体capri PG3〜（T）。重要的是，即使不是所有细菌，许多细菌也都能分泌多糖，这些多糖附着在细胞表面或无结合地出口到细胞外环境中。两种类型的多糖均可在细菌-宿主相互作用中发挥作用。尽管支原体总体代谢能力差，但也不例外。我们在这里显示，无乳莫拉氏菌由于具有合酶活性的特定糖基转移酶而分泌荚膜β-（1→6）-吡喃葡萄糖。这种分泌受高频开/关相位变化的支配，这可能在支原体宿主传播中至关重要，因为细胞附着的β-（1→6）-吡喃葡萄糖增加了对血清的杀伤敏感性。我们的结果提供了关于具有双重功能的支原体糖基转移酶的功能遗传数据，即糖聚合物在细胞膜上的组装和输出。此外，我们证明了非蛋白抗原决定簇可以在支原体中发生表面抗原变异。最后，本报告有助于揭示分泌的多糖在这些特殊细菌的毒性和致病性中的作用。

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