Phase Variable O Antigen Biosynthetic Genes Control Expression of the Major Protective Antigen and Bacteriophage Receptor in <em>Vibrio cholerae</em> O1

The Vibrio cholerae lipopolysaccharide O1 antigen is a major target of bacteriophages and the human immune system and is of critical importance for vaccine design. We used an O1-specific lytic bacteriophage as a tool to probe the capacity of V. cholerae to alter its O1 antigen and identified a novel mechanism by which this organism can modulate O antigen expression and exhibit intra-strain heterogeneity. We identified two phase variable genes required for O1 antigen biosynthesis, manA and wbeL. manA resides outside of the previously recognized O1 antigen biosynthetic locus, and encodes for a phosphomannose isomerase critical for the initial step in O1 antigen biosynthesis. We determined that manA and wbeL phase variants are attenuated for virulence, providing functional evidence to further support the critical role of the O1 antigen for infectivity. We provide the first report of phase variation modulating O1 antigen expression in V. cholerae, and show that the maintenance of these phase variable loci is an important means by which this facultative pathogen can generate the diverse subpopulations of cells needed for infecting the host intestinal tract and for escaping predation by an O1-specific phage.霍乱弧菌（Vibrio cholerae）的脂多糖O1抗原（lipopolysaccharide O1 antigen）是噬菌体（bacteriophages）与人类免疫系统的核心作用靶点，同时亦是疫苗设计的关键靶标。本研究以一株O1特异性裂解性噬菌体为研究工具，探查霍乱弧菌调控其O1抗原的能力，并鉴定出该菌可调节O抗原表达、呈现菌株内异质性的全新机制。我们成功鉴定出两个参与O1抗原生物合成的相变异基因（phase variable genes）：manA与wbeL。其中，manA坐落于此前已探明的O1抗原生物合成基因座（biosynthetic locus）之外，其所编码的磷酸甘露糖异构酶（phosphomannose isomerase）是O1抗原生物合成初始步骤的关键功能因子。本研究证实，manA与wbeL的相变异株毒力显著减弱，这为O1抗原在致病菌感染过程中的核心作用提供了功能性佐证。此外，本研究首次报道了霍乱弧菌中相变异调控O1抗原表达的现象，并发现维持此类相变异基因座的稳定性，是这类兼性致病菌（facultative pathogen）生成多样化细胞亚群的重要途径——这些细胞亚群可协助致病菌侵染宿主肠道，并逃避O1特异性噬菌体的裂解捕食。