Occurrence and expression of bacterial human virulence gene homologues in natural soil bacteria.
The presence and in vitro expression of homologues to 22 bacterial human virulence determinants amongst culturable soil bacteria were investigated. About 25% of the bacterial isolates contained virulence gene homologues representing toxin (hblA, cytK2), adhesin (fimH), regulator (phoQ) and resistance (yfbI) determinants in pathogenic bacteria. The homologues of the toxin genes were found in Actinobacteria and Firmicutes (hblA), and in Firmicutes and Alpha- and Gammaproteobacteria (cytK2). The homologues to the type 1 fimbrial adhesin gene, fimH, and the L-Ara4N transferase gene, yfbI, were observed in Actinobacteria, Firmicutes and Gammaproteobacteria. The regulator gene, phoQ, was only found in Gammaproteobacteria. The presence of cytK2 in Alpha- and Gammaproteobacteria, fimH in Actinobacteria and Firmicutes, and hblA in Actinobacteria has not previously been described. A close sequence similarity (84-100%) was observed between the genes of environmental and clinical isolates, and expression assays suggested that the genes in some cases were expressed in vitro. The presence of functional virulence gene homologues underpins their importance for the survival of environmental bacteria. Furthermore, the high degree of sequence conservation to clinical sequences indicates that natural environments may be 'evolutionary cribs' of emerging pathogens.