BMC Genomics 2007, 8:110 http://www.biomedcentral.com/1471-2164/8/110 conditions [33]. Whilst lacking the Cj0264c gene or its close homologues, C. jejuni 81–176 carries an alternative DMSO reductase gene cluster (dmsABC-torD), and an additional cytochrome C biogenenesis gene cluster (cytC locus; cju02-09) [10]. Cytochrome C may be an impor- tant link between the menaquinine pool and alternative terminal electron acceptors such as DMSO and TMAO [33]. It has been suggested that the capacity of Campylo- bacter isolates to utilise alternative electron acceptors may contribute to selective advantages in specific ecological niches [16], and that the presence of additional respira- tory capabilities may contribute to the efficiency of colo- nisation of highly pathogenic strains such as C. jejuni 81– 176 [10]. Thus, it is possible that the presence or absence of particular genes or islands contributing to growth in microaerophilic environments may influence host prefer- ence. Hofreuter et al. (2006) recently provided some evi- dence for this when demonstrating that a dmsA mutant of C. jejuni 81–176 colonised mice less well than its wild- type equivalent in a mixed infection model. We identified several strain-variable SSH sequences matching putative enzymes with roles in electron trans- port using alternative terminal electron acceptors. The SSH library constructed from strain 629 (rabbit isolate) included SSH sequences with best BLASTX matches against genes in the cytochrome C biogenesis cluster of C. jejuni 81–176 (including SSH629-C10). The SSH library constructed from strain 1967 (cattle isolate) included numerous SSH sequences (including SSH1967-D9) with a best BLASTX match against the dmsABC genes of C. jejuni 81–176. The SSH libraries constructed from strains 1967, 670 and strain 504 each contained SSH sequences (SSH1967-C2, SSH670-B10 and SSH504-C10) matching a pyridine nucleotide disulfide oxidoreductase from C. jejuni 84-25 belonging to a family of enzymes that can play a role in electron transport. SSH sequences SSH670- B10 and SSH504-C10 were identical to each other. In order to test the hypothesis that the distribution of such metabolic genes may be related to host preference, we chose to screen a panel of isolates varying in MLST type and source for the presence or absence of four representa- tive sequences (Tables 3 and 4). SSH1967-H9 and SSH1967-C10 were chosen to represent the alternative DMSO reductase gene cluster (dmsABC-torD), and the additional cytochrome C biogenenesis gene cluster (cytC locus; cju02-09), of C. jejuni 81–176 respectively. SSH1967-C2 was chosen to represent the pyridine nucle- otide disulfide oxidoreductase described in C. jejuni 84- 25. In addition, the Cj0264c gene of strain NCTC11168, encoding a reductase, was included. Interestingly, of the 81 C. jejuni isolates screened, only two (both ST-42) shared the sequences SSH1967-H9 and SSH1967-C2, whilst a further 13 lacked both sequences, including all of the ST-48 isolates. For most of the clonal complexes (ST-21, ST-61, ST-45 and ST-257) the sequences SSH1967-H9 and SSH1967-C2 were mutually exclusive. There was a strong correlation between the distribution of the two reductase-related sequences (SSH1967-H9 and 1967-C2) or the cytochrome C biogenesis-related sequence (SSH629-C10) and the MLST clonal complex. It has been reported previously that the C. jejuni NCTC11168 ORF Cj0264c, which encodes the sole TMAO and DMSO reductase in this strain [33], was absent or highly divergent in 10 strains amongst a panel of 18 C. jejuini strains from diverse sources [16]. Our data indi- cated that this gene was widespread amongst all clonal complexes in our panel of strains, with the exception of the ST-257 complex (Tables 3 and 4). In our study, there was no evidence for an association between the presence of a particular gene associated with metabolism using alternative electron acceptors and the source of the isolate, suggesting that the presence or absence of these alternative metabolic genes does not play a significant role in niche preferences. However, it should be noted that our strain panel was dominated by isolates from cattle and human sources. Thus we cannot com- pletely rule out such associations with other host species. In a study of gene expression variations by different vari- ants of C. jejuni NCTC11168 it was reported that many of the differences in gene expression were in respiration and metabolism genes [36]. The authors suggested that adap- tation to different oxygen tensions may influence coloni- sation potential. The gene expression profiles compared were those of C. jejuni NCTC 11168-GS, the genome- sequenced isolate, and NCTC 11168-O, the original iso- late from which NCTC11168-GS was derived. Of the two, isolate NCTC 11168-O is a much better coloniser of chicks and invades tissue culture cells far more efficiently [36]. Under microaerobic and severely oxygen-limited conditions there were marked difference in the expression of genes associated with metabolism and respiration. Although ORF Cj0264c was not amongst those loci expressed differently between the two variants of NCTC 11168, such observations support the notion that rather than the presence or absence of metabolic and respiratory genes per se, variations in expression may be more rele- vant to niche preference. Conclusion In this study we have demonstrated that by broadening the range of clonal complexes and host sources of C. jejuni isolates submitted to genetic interrogation, we did not greatly increase the pool of identified strain-variable genes. This suggests that the current database already con- Page 8 of 11 (page number not for citation purposes)