MgtC is a virulence factor in Salmonella typhimurium, being required for intramacrophage survival and growth in low Mg2+ medium. The mgtC gene is carried by the SPI-3 pathogenicity island, that is likely to have been acquired through horizontal gene transfer. The acquisition of mgtC occurred probably prior to the Salmonella genus diversification because mgtC sequences were detected by Southern experiments in all eight subspecies of S. enterica. In contrast, Southern experiments indicated a narrow and sporadic distribution of the mgtC gene in 10 enterobacteria. A MgtC homologue, that is also required for intramacrophage survival and growth in low Mg2+, is found in Mycobacterium tuberculosis, a pathogen not phylogenetically related to Salmonella. We have investigated further the presence of mgtC-like sequences in eubacterial genomes. Analyses of MgtC-like proteins phylogeny and mgtC-like chromosomal context support the hypothesis that mgtC has been acquired by horizontal gene transfer repeatedly throughout bacterial evolution. In addition, the phylogenetic analysis revealed a subgroup of proteins, that includes the S. typhimurium and M. tuberculosis MgtC proteins, as well as MgtC-related proteins from other pathogens that are able to survive in macrophages, B. melitensis and Y. pestis. We propose that MgtC has a similar function in all these distantly related pathogens, most likely providing the ability to grow in a low Mg2+ environment.