The lifestyles of all of the pathogens selected for this study are known to involve survival in phagocytes and of the genes down-selected, several are implicated in the survival of bacteria within phagocytic cells. SodC is known to be a virulence factor in several pathogenic bacteria including Neisseria meningitides , Burkholderia cenocepacia , Salmonella enterica serovar Typhimurium and Salmonella choleraesuis  and Brucella abortus , and disruption of the sodC gene in these bacteria have generated attenuated mutants. Further work has characterized the role of superoxide dismutase C in the virulence in Y. pseudotuberculosis . SodC is known to protect bacteria in phagosomes from the bactericidal action of superoxide anion, and SodC orthologs were found in most of the pathogens targeted for this study . Burkholderia mallei, Burkholderia pseudomallei and Bacillus anthracis are also known to have SodC, but these enzymes do not show significant sequence homology with the cluster of SodC orthologs in this study at the 50% identity and 90% coverage cut-offs used.
Both Mg2+ and Mn2+ are believed to be limiting in the phagosome, and transport systems for these ions are common to many of the pathogens targeted for this study . Bacterial MntH proteins are homologous to the eukaryotic NRAMP (natural-resistance-associated macrophage protein) family of proteins that transport either Mn2+ or Fe2+. Mutants of the mntH gene (manganese transport protein) have been shown to be attenuated in Salmonella typhimurium . In S. typhimurium, the magnesium transport ATPase gene, mgtB, is found on a pathogenicity island, SPI-3, and its expression is controlled by the PhoP/Q signal transduction system which is an essential system in Salmonella virulence .
HemY is a putative protoporphyrinogen IX oxidase that is found as part of the haem biosynthetic pathway in bacteria. Haem is a tetrapyrrole derivative commonly used as a prosthetic group in proteins such as cytochromes, catalases and peroxidases and is essential for respiration and defence against oxygen intermediates. This later function suggests a possible involvement as a host defense mechanism. Although no hemY gene has been associated with virulence to date, Staphylococcus aureus mutants of hemB have been shown to reduce virulence in the Caenorhabditis elegans infection model .
YPTB3827 is an uncharacterized protein which contains the COG domain COG2982, involved in outer membrane biogenesis [Cell envelope biogenesis, outer membrane]. The outer membrane of Gram-negative bacteria is made up of four major components: lipopolysaccharide, phospholipids, beta-barrel proteins, and lipoproteins . Together they play a number of roles including maintaining the integrity of the cell, uptake and secretion of solutes and interaction with the host cell. YPTB3827 has not previously been associated with virulence. However, it is possible that disruption of this protein may affect virulence though interactions with the host cell, or through secretion of certain proteins. Further work on this protein will be required to further characterize its role and to elucidate its role in virulence.
Of the 17 targets, four could not be constructed as mutants suggesting that their function may be essential to the virulence of the cell. Each of these were compared to the Database of Essential Genes (DEG: http://tubic.tju.edu.cn/deg) and two, YPTB2995 and YPTB2026, showed over 50% identity to proteins described as essential in this database .