微生物生物被膜的形成在病原微生物持续性感染中起着非常重要的作用，也是微生物在自然界的普遍存在方式之一。微生物生物被膜依赖于胞外多聚物质（EPS）维持其群体结构。多糖是生物被膜胞外多聚物质的重要组分之一，研究生物被膜胞外多糖有助于理解生物被膜的形成机制，从而有针对性地开发治疗手段，解决生物被膜相关的问题。
中国科学院微生物研究所微生物资源前期开发国家重点实验室马旅雁课题组以条件致病菌铜绿假单胞菌为模式菌来研究生物被膜。铜绿假单胞菌极易形成生物被膜的原因之一是它可以产生多种胞外多糖，包括脂多糖（LPS），名为Psl、Pel和Alginate的多糖，它们都与生物被膜的形成相关。Alginate的化学组份与褐藻中分离的多糖相似，被译为褐藻多糖。铜绿假单胞菌一旦在囊性纤维化（cystic fibrosis， CF）病人肺部定植后，常会突变成为黏液型菌株（Mucoid），即大量产生alginate多糖的菌。Alginate的大量合成一直以来被认为是铜绿假单胞菌难以从CF患者清除的原因之一。
近期，马旅雁课题组的研究表明，在非黏液型菌的生物被膜形成中起关键作用的Psl多糖也是黏液型铜绿假单胞菌形成生物被膜的关键多糖(FEMS Immunol. Med. Microbiol. DOI：10.1111/j.1574 -695X.2012. 00934.x)。这为治疗CF患者的铜绿假单胞菌感染提供了靶标。
此外，该课题组近期的研究还揭示了铜绿假单胞菌调控上述四种胞外多糖的一种机制。研究发现，同时具有磷酸甘露糖变位酶和磷酸葡萄糖变位酶活性的双功能酶AlgC是生物被膜形成相关的四种多糖LPS、Psl、Pel和Alginate的合成所必需的。进一步研究发现，AlgC是这些多糖合成途径中的关键节点酶，它通过控制这几种多糖前体物质的代谢流来控制这些多糖的合成。类似AlgC的酶在许多原核甚至真核生物的糖代谢中均有发现。该研究所揭示的调控机制很可能有普遍性。研究论文发表在Environ. Microbiol.( doi:10.1111/j.1462-2920.2012.02753.x)上。
相关工作得到了俄亥俄州立大学、华盛顿大学和圭尔夫大学部分合作者的支持。(生物谷：Bioon.com)
doi: 10.1111/j.1574-695X.2012.00934.x
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The roles of biofilm matrix polysaccharide Psl in mucoid Pseudomonas aeruginosa biofilms
Ma, Luyan; Wang, Shiwei; Wang, Di; Parsek, Matthew R.; Wozniak, Daniel J.
The opportunistic pathogen Pseudomonas aeruginosa causes life-threatening, persistent infections in patients with cystic fibrosis (CF). Persistence is attributed to the ability of these bacteria to form structured communities (biofilms). Biofilms rely on an extracellular polymeric substances matrix to maintain structure. Psl exopolysaccharide is a key matrix component of nonmucoid biofilms, yet the role of Psl in mucoid biofilms is unknown. In this report, using a variety of mutants in a mucoid P. aeruginosa background, we found that deletion of Psl-encoding genes dramatically decreased their biofilm formation ability, indicating that Psl is also a critical matrix component of mucoid biofilms. Our data also suggest that the overproduction of alginate leads to mucoid biofilms, which occupy more space, whereas Psl-dependent biofilms are densely packed. These data suggest that Psl polysaccharide may have significant contributions in biofilm persistence in patients with CF and may be helpful for designing therapies for P. aeruginosa CF infection.
doi: 10.1111/j.1462-2920.2012.02753.x
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Synthesis of multiple Pseudomonas aeruginosa biofilm matrix exopolysaccharides is post-transcriptionally regulated
Luyan Ma, Juan Wang, Shiwei Wang, Erin M. Anderson, Joseph S. Lam, Matthew R. Parsek, Daniel J. Wozniak
Exopolysaccharide is a critical biofilm matrix component, yet little is known about how the synthesis of multiple exopolysaccharides is regulated. Pseudomonas aeruginosa can produce several biofilm matrix exopolysaccharides that include alginate, Psl and Pel. Here we demonstrated that AlgC, a key enzyme that provides sugar precursors for the synthesis of alginate and lipopolysaccharides (LPS) is also required for both Psl and Pel production. We showed that forced-synthesis of Psl in alginate-producing mucoid bacteria reduced alginate production but this was not due to transcription of the alginate biosynthesis-operon. Likewise, when either alginate or Psl were overproduced, levels of B-band LPS decreased. Induction of Pel resulted in a reduction of Psl levels. Because the effects of reduced exopolysaccharide synthesis when another is overproduced didn't appear to be regulated at the transcriptional level, this suggests that the biosynthesis pathways of Psl, Pel, alginate, and LPS compete for common sugar precursors. As AlgC is the only enzyme that provides precursors for each of these exopolysaccharides, we propose that AlgC is a key checkpoint enzyme that coordinates the total amount of exopolysaccharide biosynthesis by controlling sugar precursor pool. Our data also provide a plausible strategy that P. aeruginosa utilizes to modulate its biofilm matrix exopolysaccharides.
