The cad locus of Enterobacteriaceae: More than just lysine decarboxylation

The cadBA genes are transcribed as a transcriptional operon and activated by CadC in a low pH- and lysine-dependent manner. In general, their expression is associated with decarboxylation of lysine and production of the polyamine cadaverine and CO₂. However, the absence of these genes in some of the pathogenic members of the Enterobacteriaceae family results in a benefit for their virulence phenotype. Furthermore, Cad proteins had been found regulating the expression of genes associated with colonization, and production of polyamines has been also associated with protection of some pathogenic Escherichia coli against stress conditions. Therefore, in this review, we discuss recent evidence indicating that the cad locus encodes more than a mechanism to degrade lysine in Enterobacteriaceae and instead, in some pathogenic strains, and throughout evolution; the cad locus has become part of a "pathoadaptive" mutation which confers an advantage for survival and pathogenesis.