Non-typhoidal serovars of Salmonella enterica are an important cause of food-borne diarrhoeal illness in humans worldwide. Using active surveillance data from a catchment area of 44.5 million people, the FoodNet network has estimated that there are 1.4 million cases of human non-typhoid salmonellosis in the United States per annum, leading to 15,000 hospitalisations and 400 deaths . Over the past three decades S. enterica serovar Enteritidis has emerged as a significant cause of such infections . The consumption of undercooked poultry meat and eggs is a major risk factor for S. Enteritidis infection  and the phage types circulating in humans are commonly found in broilers  and layers . The incidence of S. Enteritidis infection in humans declined markedly following the implementation of control strategies, including vaccination for poultry, regulations on storage and preparation of food and improved education . Despite such measures, S. Enteritidis remains the most prevalent cause of non-typhoidal salmonellosis in many countries, including the United Kingdom http://www.hpa.org.uk/infections/topics_az/salmonella/data.htm, and improved vaccines are needed to achieve further reductions in the burden of human disease.
It is well established that S. Enteritidis is able to persist in the intestinal and reproductive tract of poultry in the absence of clinical signs ; however the molecular mechanisms mediating colonisation of these sites are ill-defined. Further, it is unclear why some S. enterica serovars are associated with enteric disease in a broad range of healthy out-bred adult hosts (e.g. Enteritidis and Typhimurium), whereas others are host-restricted or -specific and associated with severe systemic disease (e.g. Gallinarum in poultry and Typhi in humans). Targeted and genome-wide mutagenesis of the broad host range serovar Typhimurium has indicated that it uses both conserved and host-specific factors to colonise the intestines of chickens, cattle, pigs and mice [8–14]. Among the factors that influence intestinal colonisation are fimbriae; proteinaceous surface appendages that mediate interactions between bacteria and host cells.
Of the thirteen fimbrial loci predicted to be encoded by the S. Typhimurium genome, lpf, fim, bcf, stb, stc, std, sth and csg have been implicated in virulence in mice [11, 13, 15–17]. Screening of a library of signature-tagged mutants of S. Typhimurium indicated that pathogeniCity island (SPI)-6-encoded saf fimbriae may play a host-specific role in ileal colonisation of pigs , whereas the stbC, csgD and sthB fimbrial genes were implicated in colonisation of the avian gut . Separately Ledeboer et al described a role for lpfA-E, pefC, csgA and fimH, but not sthD or bcfF, in biofilm formation on chicken intestinal mucosa cultured ex vivo . Relatively few studies have probed the role of fimbriae in colonisation of poultry by S. Enteritidis. Allen-Vercoe and Woodward reported that a S. Enteritidis mutant lacking fimD, csgA, pefC, lpfC and sefA colonised the caeca at comparable levels to the parent strain following oral dosing of 1 or 5 day-old chicks  and was similarly invasive  and adherent to chicken gut explants . Furthermore, single mutants lacking fimA, csgA or sefA exhibited no significant defect in colonisation of chick caeca and were excreted in the faeces at comparable levels to the parent [22, 23]. Although roles for S. Enteritidis fimbriae in intestinal colonisation of poultry have so far been lacking, Type I fimbriae  and curli  have been implicated in egg contamination.
In the recent publication of the complete genome sequence of S. Enteritidis strain P125019  we have defined the full repertoire of fimbrial loci and identified a unique fimbrial operon, peg, present in S. Gallinarum, S. Enteritidis and also S. Paratyphi. The peg operon displays 60–70% sequence conservation with the stc operon of S. Typhimurium and is located in the same relative position. The peg operon belongs to the γ clade of fimbriae and is predicted to be assembled via the chaperone usher pathway .
The work herein examined the fimbrial gene conservation in the published genomes of other S. enterica serovars and also searched for traits associated with phase variation. Isogenic S. Enteritidis mutants with insertions in the major fimbrial subunit of each of the fimbrial operons were constructed using lambda Red recombinase-mediated linear recombination  followed by P22/int transduction. Mutant phenotypes were then evaluated and confirmed using an established chicken colonisation model.