Campylobacter jejuni, a gram-negative motile bacterium, secretes a set of proteins termed the Campylobacter invasion antigens (Cia proteins). The purpose of this study was to determine whether the flagellar apparatus serves as the export apparatus for the Cia proteins. Mutations were generated in five genes encoding three structural components of the flagella, the flagellar basal body (flgB and flgC), hook (flgE2), and filament (flaA and flaB) genes, as well as in genes whose products are essential for flagellar protein export (flhB and fliI). While mutations that affected filament assembly were found to be nonmotile (Mot ؊ ) and did not secrete Cia proteins (S ؊ ), a flaA (flaB ؉ ) filament mutant was found to be nonmotile but Cia protein secretion competent (Mot ؊ , S ؉ ). Complementation of a flaA flaB double mutant with a shuttle plasmid harboring either the flaA or flaB gene restored Cia protein secretion, suggesting that Cia export requires at least one of the two filament proteins. Infection of INT 407 human intestinal cells with the C. jejuni mutants revealed that maximal invasion of the epithelial cells required motile bacteria that are secretion competent. Collectively, these data suggest that the C. jejuni Cia proteins are secreted from the flagellar export apparatus.Campylobacter jejuni, a gram-negative motile bacterium, is a frequent cause of human gastrointestinal infections (39). The spectrum of disease observed in C. jejuni-infected individuals ranges from asymptomatic to severe enteritis characterized by fever, severe abdominal cramping, and diarrhea with blood and mucus (2, 4). By analogy with other more extensively characterized bacterial pathogens, the mechanism of C. jejunimediated enteritis is proposed to be multifactorial. Previous work has indicated that motility as well as the presence of the flagellum contributes to the ability of C. jejuni to colonize the intestinal tract of animals (33,36,42).The flagellum of C. jejuni is composed of a basal body, hook, and filament. The flagellar filament is comprised of two proteins, FlaA and FlaB, although it appears that FlaA is the preferred subunit (3). While the C. jejuni FlaA and FlaB flagellin proteins are transcribed concomitantly (16), the flaA gene is regulated by 28 and the flaB gene is regulated by 54 (3, 16). Hendrixson et al. (16) noted that a C. jejuni isolate deficient in 28 , which is encoded by the fliA gene, is able to assemble a truncated filament composed exclusively of the flagellin protein FlaB. This result indicates that the regulation of flagellar gene expression within C. jejuni differs from the regulation in more intensely studied systems such as that of Salmonella enterica. Unlike flagellar gene expression in C. jejuni, flagellar gene expression in S. enterica is initiated by a master regulator, while late gene expression and motility require 28 (1). Previous work in our laboratory has demonstrated that C. jejuni synthesizes a set of proteins during coculture with epithelial cells, a subset of which are secreted. The sec...

BackgroundThe complex microbiome of the ceca of chickens plays an important role in nutrient utilization, growth and well-being of these animals. Since we have a very limited understanding of the capabilities of most species present in the cecum, we investigated the role of the microbiome by comparative analyses of both the microbial community structure and functional gene content using random sample pyrosequencing. The overall goal of this study was to characterize the chicken cecal microbiome using a pathogen-free chicken and one that had been challenged with Campylobacter jejuni.Methodology/Principal FindingsComparative metagenomic pyrosequencing was used to generate 55,364,266 bases of random sampled pyrosequence data from two chicken cecal samples. SSU rDNA gene tags and environmental gene tags (EGTs) were identified using SEED subsystems-based annotations. The distribution of phylotypes and EGTs detected within each cecal sample were primarily from the Firmicutes, Bacteroidetes and Proteobacteria, consistent with previous SSU rDNA libraries of the chicken cecum. Carbohydrate metabolism and virulence genes are major components of the EGT content of both of these microbiomes. A comparison of the twelve major pathways in the SEED Virulence Subsystem (metavirulome) represented in the chicken cecum, mouse cecum and human fecal microbiomes showed that the metavirulomes differed between these microbiomes and the metavirulomes clustered by host environment. The chicken cecum microbiomes had the broadest range of EGTs within the SEED Conjugative Transposon Subsystem, however the mouse cecum microbiomes showed a greater abundance of EGTs in this subsystem. Gene assemblies (32 contigs) from one microbiome sample were predominately from the Bacteroidetes, and seven of these showed sequence similarity to transposases, whereas the remaining sequences were most similar to those from catabolic gene families.Conclusion/SignificanceThis analysis has demonstrated that mobile DNA elements are a major functional component of cecal microbiomes, thus contributing to horizontal gene transfer and functional microbiome evolution. Moreover, the metavirulomes of these microbiomes appear to associate by host environment. These data have implications for defining core and variable microbiome content in a host species. Furthermore, this suggests that the evolution of host specific metavirulomes is a contributing factor in disease resistance to zoonotic pathogens.

SummaryCampylobacter jejuni, a Gram-negative bacterium, is a common cause of gastrointestinal disease. By analogy with other enteric pathogens such as Salmonella and Shigella, the ability of C. jejuni to bind to host cells is thought to be essential in the pathogenesis of enteritis. Scanning electron microscopy of infected INT407 cells suggested that C. jejuni bound to a component of the extracellular matrix. Binding assays using immobilized extracellular matrix proteins and soluble fibronectin showed specific and saturable binding of fibronectin to C. jejuni. Ligand immunoblot assays using 125 I-labelled fibronectin revealed specific binding to an outer membrane protein with an apparent molecular mass of 37 kDa. A rabbit antiserum, raised against the gel-purified protein, reacted with a 37 kDa protein in all C. jejuni isolates (n ¼ 15) as tested by immunoblot analysis. Antibodies present in convalescent serum from C. jejuni-infected individuals also recognized a 37 kDa protein. The gene encoding the immunoreactive 37 kDa protein was cloned and sequenced. Sequencing of overlapping DNA fragments revealed an open reading frame (ORF) that encodes a protein of 326 amino acids with a calculated molecular mass of 36 872 Da. The deduced amino acid sequence of the ORF exhibited 52% similarity and 28% identity to the root adhesin protein from Pseudomonas fluorescens. Isogenic C. jejuni mutants which lack the 37 kDa outer membrane protein, which we have termed CadF, displayed significantly reduced binding to fibronectin. Biotinylated fibronectin bound to a protein with an apparent molecular mass of 37 kDa in the outer membrane protein extracts from wild-type C. jejuni as judged by ligandbinding blots. These results indicate that the binding of C. jejuni to fibronectin is mediated by the 37 kDa outer membrane protein which is conserved among C. jejuni isolates.

Presented here is the first evidence that Campylobacter jejuni secrete proteins upon co‐cultivation with host cells and in INT 407 cell‐conditioned medium. A C. jejuni gene designated ciaB for Campylobacter invasion antigen B was identified, using a differential screening technique, which is required for this secretion process and the efficient entry of this bacterium into a host cell. The C. jejuni ciaB gene encodes a protein of 610 amino acids with a calculated molecular mass of 73 154 Da. The deduced amino acid sequence of the CiaB protein shares similarity with type III secreted proteins associated with the invasion of host cells from other more extensively characterized bacterial pathogens. In vitro binding and internalization assays revealed that the binding of C. jejuni ciaB null mutants was indistinguishable from that of the parental isolate, whereas a significant reduction was noted in internalization. Confocal microscopic examination of C. jejuni‐infected cells revealed that CiaB was translocated into the cytoplasm of the host cells. Culturing C. jejuni with INT 407 cells or in INT 407‐conditioned medium resulted in the secretion of at least eight proteins, ranging in size from 12.8 to 108 kDa, into the culture medium. C. jejuni ciaB null mutants were deficient in the secretion of all eight proteins, indicating that CiaB is required for the secretion process. The identification of the C. jejuni ciaB gene represents a significant advance in understanding the molecular mechanism of C. jejuni internalization and the pathogenesis of C. jejuni‐mediated enteritis.

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