Toxin-antitoxin loci and bacterial survival in the external environment

生物膜代表了细菌生命的主要形式。当出现有利于生长的表面或界面和条件时，细菌几乎总是会聚集成一组细胞，这些细胞被包裹在自我产生的保护性细胞外基质中。生物膜允许细菌在恶劣环境中生存，如紫外线照射、脱水、盐度和几种消毒剂和杀菌剂。微生物生物膜非常复杂。最令人惊奇的事实是，即使在基因相同的细胞组成的单物种生物膜中，也存在表型异质性。细胞在生物膜中的分化可能取决于细胞周围的局部环境条件。细菌的一个基本特性是它们在面对波动的环境时调节细胞生长的能力。大多数细菌染色体含有许多有毒基因，这些基因可诱导细胞亚群的可逆休眠状态。通过进入休眠状态，这些持久性细胞能够在压力条件下生存。当不利条件消退时，这些休眠的持久性生物复苏并重新填充生物膜。休眠表型使得无根细菌极难被杀死并从生物或非生物表面移除。染色体毒素-抗毒素（TA）模块已被提出作为细胞生长的调节剂，以应对环境应激。TA模块由一对编码两种成分的基因组成，一种是稳定的毒素，另一种是同源的不稳定的抗毒素。毒素通常是蛋白质，而抗毒素要么是RNA，要么是蛋白质。在正常情况下，毒素蛋白被抗毒素抵消。在正常生长条件下，TA对在细胞中形成稳定的复合物，防止毒性。然而，当毒素和抗毒素之间的平衡被扰乱时，通常是在细胞损伤或应激条件下，毒素从TA复合物中释放出来，导致细胞生长停滞。TA系统几乎存在于所有的细菌染色体中，这证明了TA系统在细菌生理中的重要性。使用口腔生物膜有机体变形链球菌作为模型，因为它与生物膜生活方式相关，我的总体目标是回答以下问题：微生物生长调节剂（如TA模块）是否参与细胞生长停滞过程，使整个生物膜种群受益，从而使物种在突然不利的环境变化中存活下来？众所周知，微生物生物膜很难根除。我的实验室正试图通过研究TA模块作为开发成功的防污策略的潜在目标来克服这一障碍。利用最先进的光学和基因组工具来阐明生物膜的生理和基因活动，我的研究有可能有助于开发具有成本效益的生物杀灭策略来控制和根除微生物生物膜。

Biofilms represent the predominant form of bacterial life. When presented with a surface or an interface and conditions advantageous for growth, bacteria almost always will assemble as a group of cells encapsulated within a self-produced protective extracellular matrix. Biofilms allow survival of bacteria in hostile environments, such as UV exposure, dehydration, salinity, and several disinfectants and biocidal agents. Microbial biofilms are very complex. The most amazing fact is that even in a mono-species biofilm of genetically identical cells, phenotypic heterogeneity exists. Cell differentiation in biofilms may depend on the local environmental conditions surrounding the cells. A fundamental property of bacteria is their ability to regulate cell growth when faced with a fluctuating environment. Most bacterial chromosomes contain a number of toxic genes that induce a reversible state of dormancy of a subpopulation of cells. By entering into a dormant state, these persister cells are able to survive stress conditions. When the unfavourable conditions subside, these dormant persisters resuscitate and repopulate the biofilms. The dormancy phenotype makes sessile bacteria extremely difficult to kill and remove from biotic or abiotic surfaces. Chromosomal toxin-antitoxin (TA) modules have been proposed to function as regulators of cell growth in response to environmental stress. TA modules consist of a pair of genes that encode two components, a stable toxin and its cognate labile antitoxin. While toxins are always proteins, antitoxins are either RNA or proteins. Under normal circumstances, the toxin protein is counteracted by the antitoxin. TA pairs form a stabilized complex in the cell preventing toxicity under normal growth conditions. However, when the balance between the toxin and antitoxin is perturbed, usually following cellular damage or stressful conditions, the toxin is released from the TA complex leading to cell growth arrest. TA systems are found in nearly all bacterial chromosomes, which attest to their importance in bacterial physiology. Using the oral biofilm organism Streptococcus mutans as a model – chosen because of its relevance of a biofilm lifestyle, my overall goal is to answer the following question: Do microbial growth modulators such as TA modules participate in cell growth arrest processes that benefit the whole biofilm population and thereby enable survival of the species in the event of a sudden unfavourable environmental change? Microbial biofilms are notoriously difficult to eradicate. My lab is attempting to overcome this barrier by investigating TA modules as potential target for the development of successful anti-fouling strategies. Using the state-of-the art optical and genomic tools to elucidate the physiology and gene activities of biofilms, my research has the potential to contribute to the development of cost-effective biocidal strategies for the control and eradication of microbial biofilms.