The Mouse Colonizing Helicobacter pylori Strain SS1 May Lack a Functional cag Pathogenicity Island

The SS1 strain of Helicobacter pylori was first described in 1997 [1], and has since then been extensively used in experimental infections of mice. The strain colonises the gastric mucosa at a high density and retains colonising ability after subculture for 20 passages [1]. The strain is cagA positive as determined by PCR [1], and generally assumed to be a good strain for analysis of the function of the cag pathogenicity island ( cag PAI) in experimental models. It is well documented that epithelial cell signalling responses induced by cag PAI positive strains of H. pylori , which are characterised by NFκ B activation and IL-8 induction, are dependent on multiple genes throughout the cag PAI [2–5]. It has been reported that SS1 failed to induce IL-8 secretion in gastric epithelial cells [6], and this was recently confirmed by Eaton et al. [7], and in two of the authors’ laboratories (JEC and RLF, unpublished data). Only one study to date has demonstrated that the SS1 strain induces IL-8 secretion in gastric epithelial cells [8]. We have previously demonstrated that inactivation of ORF7 (HP0521) reduced IL-8 transcription in gastric epithelial cells to the level observed with a total cag PAI deletion mutant [4]. Although polar effects on downstream genes can not be completely excluded, these data indicate that ORF7 is essential for epithelial cell signalling. Recently whole-genome microarray analysis demonstrated that SS1 lacks ORF7 (HP0521) in the left half of the cag PAI [9]. The absence of ORF7 in the SS1 isolate may therefore explain the observations on the lack of a functional cag PAI of the SS1 isolates tested in four of five independent laboratories around the world. Genomic comparisons have revealed considerable diversity in the ORF7 genes of H. pylori strains 26695 and J99 [10]. These genes were reported to be composed of 240 and 657 base pairs, respectively. The relatively short length of ORF7 in H. pylori 26695, however, can be partially attributed to the presence of a frame-shift in the published sequence, resulting in a gene with greater similarity to the 348-base pair homologue of H. pylori NCTC11638 [3]. Given that the published ORF7 sequence of H. pylori 26695 was used in the DNA microarray analyses of the SS1 strain [9], classical sequence analysis is required to confirm the absence of the ORF7 gene in the SS1 strain. Moreover, similar analyses on the various H. pylori SS1 isolates throughout the world would contribute to a clarification of the role of ORF7 in cag PAI function. These findings illustrate the importance of genome information for the interpretation of data emanating from DNA microarray analysis of H. pylori isolates. There is a marked diversity in host gene expression in gastric epithelial cells following exposure to cag PAI positive and cagPAI negative H. pylori strains [11]. Studies in animal models with isogenic cagPAI mutants and parental strains are critical to investigate the in vivo relevance of the cag PAI in pathogenesis. Recently, successful studies on the in vivo function of the cag PAI have been undertaken in the Mongolian gerbil model [12,13]. Given the high mutation rate of H. pylori during repetitive culture [8,14], and the large number of cagPAI genes required for induction of epithelial cell signalling responses [2–5], it is important that bioactivity of parental strains be examined before construction of isogenic mutants and animal experimentation. The H. pylori SS1 strain has proven to be an excellent strain for use in the mouse colonisation model, and has permitted in vivo studies on a variety of bacterial functions. Nevertheless, as suggested by recent studies, the likely absence of a functional cag PAI in this strain suggests that it is not suitable for determining the functional importance of the cag PAI in animal models. Recent studies suggest changes in cag genotype or in cag PAI related functions can occur during passage in mice [15,16]. Whilst the significance of such host induced adaptive changes is unclear, these data do indicate that a functional cag PAI may not be required for mouse colonisation. Additional evidence for this is in the recently reported isolation of a new mouse colonizing H. pylori strain (SS2000): from all 125 clinical H. pylori isolates tested, strain SS2000 persisted at highest numbers in mice, and in spite of the total absence of a cag PAI it clearly induced inflammation in both C57BL/6 and BALB/c mice [17]. Taken together this indicates that the mouse may not be the optimal species for investigating the functional importance of the cag PAI.