Genetics of Coxiella burnetii

伯氏柯克斯体是一种专性胞内细菌，是人畜共患病Q(Query)热的病原体。急性Q热通常表现为一种自限性流感样疾病。罕见但严重的慢性感染可能会发生，通常表现为心内膜炎或肝炎。绝大多数人类Q热病例是通过接触受感染的家畜而获得的，在那里这种生物可能是地方性的。伯氏梭菌可慢性感染多种动物，并在各种分泌物和分娩产物中大量排出。除了生物体的隐匿性外，还有接近一个生物体的感染量和接近细菌孢子的显着的细胞外稳定性。环境抗性还与对吞噬酶体样寄生虫空泡(PV)降解条件的抵抗有关，PV是柯克斯体在宿主巨噬细胞内的生态位。 伯氏梭菌令人印象深刻的环境稳定性可能是由于一种被称为小细胞变体(SCV)的高抗性细胞形式的生物发生。这种形式出现在一个双相发育周期期间，可能是大多数环境获得性Q热病例的原因。一旦内化和隔离在光伏病毒中，SCV在形态上分化为更具新陈代谢和复制活性的大细胞变体(LCV)。成熟的光伏病毒含有SCV、LCV和中间型的混合物。伯氏华支睾吸虫形态分化的分子生物学尚不清楚。未来研究的一个重要领域包括确定细胞形态的转录能力和伯氏梭菌对溶酶体胁迫的反应。 限制性内切酶片段长度多态性分析表明，伯氏梭菌各菌株间基因组DNA具有相当大的异质性。此外，菌株可以根据与人类急性或慢性疾病的相关性进行分组，这表明群体具有独特的毒力潜力。尽管疾病的严重性和慢性化的可能性可能涉及患者因素，但伯氏梭菌分离株与疾病结局有明显的关联。菌株变异的程度和生物学相关性尚不清楚。毒力较低的菌株可能导致亚临床感染，延误诊断和治疗，导致几年或几十年后严重后果的慢性感染。 一个不同的基因组组由类似于9英里原型的急性疾病分离物代表，而来自山羊和绵羊流产的慢性分离物和分离物被组织成另外的组。所有分离株都携带一个内源质粒(~36-39kb)或整合的类似于质粒的序列。急性/慢性疾病分离株与特定基因组组的关联导致了这样的假设，即分离株可能含有独特的遗传决定因素，赋予不同的致病潜力。阐明慢性心内膜炎分离株的基因组序列，并与破译的9英里分离株基因组进行比较，将有助于更全面地了解伯氏柯萨奇杆菌的基因组结构和遗传多样性，从而提高我们模拟柯克斯体致病的能力。 伯氏梭菌的遗传多样性也通过产生抗原性和结构上独特的脂多糖(LPS)分子来揭示。内毒素是唯一已确定的伯氏弧菌毒力因子。已经描述了与特定基因组群相关的不同的伯氏弧菌内毒素化学型，并提出了内毒素化学型与伯氏弧菌毒力潜力之间的潜在联系。从自然来源和感染中分离的伯氏弧菌都能产生全长的内毒素，血清学上将其定义为I期。伯氏梭菌在胚胎卵子或组织培养中连续传代后，其分子量降低，最终导致无毒的II期生物的内毒素被严重截断。第二相脂多糖含有脂类A和一些核心糖，但缺少O-抗原糖，似乎代表了伯氏梭菌的最小内毒素结构。伯氏华支睾吸虫阶段变异的确切遗传机制尚不清楚。 互补、转座子突变和等位基因交换等遗传系统是研究细菌毒力的宝贵工具。这些方法需要将外来DNA导入细菌受体，通常是通过电穿孔。基因转化)通常需要抗生素抗性基因形式的强可选择标记。虽然已经描述了伯氏梭菌的遗传转化，但由于抗生素选择不足、引入的DNA不稳定以及缺乏有效的克隆分离方法，该系统受到阻碍。 目前，美国还没有获得许可使用的疫苗来预防Q热。实验疫苗和在其他国家获得许可的疫苗通常由灭活的全细胞伯氏梭菌组成。虽然有效，但这些疫苗在致敏个体中具有高度反应性，因此需要进行广泛的疫苗接种前筛查。保护性亚单位疫苗是必要的。

Coxiella burnetii is an obligate intracellular bacterium and the causative agent of the zoonosis human Q (query) fever. Acute Q fever normally manifests as a self-limiting influenza-like illness. Rare but serious chronic infections can occur that usually present as endocarditis or hepatitis. The vast majority of human Q fever cases are acquired through contact with infected domestic livestock where the organism can be endemic. C. burnetii can chronically infect a variety of animals and is shed in large numbers in various secretions and products of parturition. Adding to the insidious nature of the organism is an infective dose approaching one organism and a remarkable extracellular stability approaching that of a bacterial spore. Environmental resistance also correlates with resistance to the degradative conditions of a phagolysosome-like parasitophorous vacuole (PV), Coxiella's niche within host macrophages. The impressive environmental stability of C. burnetii is likely due to the biogenesis of a highly resistant cell form termed the small cell variant (SCV). This form arises during a biphasic developmental cycle and is likely responsible for the majority of environmentally acquired cases of Q fever. Once internalized and sequestered in a PV, SCV morphologically differentiate into more metabolically and replicatively active large cell variants (LCV). Mature PV contain a mixture of SCV, LCV and intermediate forms. The molecular biology of C. burnetii morphological differentiation is poorly understood. An important area of future research includes defining the transcriptional capabilities of cell forms and the response of C. burnetii to lysosomal stress. Restriction fragment-length polymorphism analysis reveals considerable genomic DNA heterogeneity among C. burnetii strains. Moreover, strains can be grouped according to association with human acute or chronic disease, suggesting groups have unique virulence potential. Although the severity of disease and the potential for chronicity may involve patient factors, there is a clear association of C. burnetii isolates with disease outcome. The extent and biological relevance of strain variation is unknown. Strains of lower virulence may result in subclinical infections where diagnosis and treatment are delayed, resulting in chronic infections with serious consequences years or decades later. A distinct genomic group is represented by acute disease isolates similar to the Nine Mile prototype, while chronic isolates and isolates from goat and sheep abortions have been organized into additional groups. All isolates carry an endogenous plasmid (~36-39 kb) or integrated plasmid-like sequences. The association of acute/chronic disease isolates with specific genomic groups has led to the hypothesis that isolates may harbor unique genetic determinants that confer distinct pathogenic potential. Elucidation of the genome sequences of chronic endocarditis isolates, and comparison to the deciphered genome of the Nine Mile isolate, will provide a more complete understanding of the genome architecture and genetic diversity of C. burnetii, thereby improving our ability to model Coxiella pathogenesis. Genetic diversity of C. burnetii is also revealed by production of antigenically and structurally unique lipopolysaccharide (LPS) molecules. LPS is the only defined virulence factor of C. burentii. Distinct C. burnetii LPS chemotypes have been described that are associated with specific genomic groups and a potential link between LPS chemotype and C. burnetii virulence potential has been proposed. Virulent C. burnetii isolated from natural sources and infections all produce a full-length LPS that is serologically defined as ?phase I?. Serial in vitro passage of phase I C. burnetii in embryonated eggs or tissue culture results in decreasing molecular weight LPS molecules, culminating in the severely truncated LPS of avirulent phase II organisms. Phase II LPS contains lipid A and some core sugars, but is missing O-antigen sugars, and appears to represent the minimal LPS structure of C. burnetii . The precise genetic mechanisms of C. burnetii phase variation are unknown. Genetic systems such as complementation, transposon mutagenesis, and allelic exchange are invaluable tools in the study of bacterial virulence. These methods require the introduction of foreign DNA into the bacterial recipient usually via electroporation. Genetic transformation) generally requires strong selectable markers in the form of antibiotic resistance genes. While genetic transformation has been described for C. burnetii, the system is hampered by inadequate antibiotic selection, instability of introduced DNA, and the lack of efficient cloning methods for clonal isolation. There are currently no vaccines licensed for use in the US to protect against Q fever. Experimental vaccines and vaccines licensed in other countries are generally comprised of inactivated whole-cell C. burnetii. Although efficacious, these vaccines suffer in being highly reactrogenic in sensitized individuals, thereby necessitating extensive pre-vaccination screening. Protective subunit vaccines are needed.