## 2 GidA/MnmE-Associated Virulence of Gram-Negative Bacteria  
GidA and MnmE have been implicated in the pathogenesis of members of the Enterobacteriaceae family including _E. coli_ and _Salmonella_. MnmE was first described in _E. coli_ as a molecular switch GTPase which assumes different conformations depending on whether it is bound to GTP or GDP [18]. Deletion of _mmmE_ has been shown to be lethal in some _E. coli_ strains, but shows no lethal consequences in other strains of _E. coli_[18]. MnmE has also been shown to activate the transcriptional regulator _gadE_, which is important in glutamate-dependent acid resistance in _E. coli_[40]. Furthermore, the study by [PERSON] _et al._ shows that GidA is a potential regulator of cytotoxic necrotizing factor 1 (CNF1) which is an important toxin in meningitis-causing _E. coli_ K1 [47]. In their study, _gidA_ was identified in a transposon screening for genetic requirements of CNF1 production and secretion. A _gidA_ deletion mutant had a drastic effect on CNF1 production, and they showed that GidA inhibited CNF1 at the translation level and is most likely part of a key mechanism in the modulation of CFN1 translation [47]. The most comprehensive investigation into the role of GidA and MnmE in pathogenesis was performed in _Salmonella_. In an initial study by [PERSON] _et al._, a _gidA_ deletion mutant of the STM 14028 strain was significantly attenuated _in vitro_ as indicated by reduced motility, replication in macrophages, invasion of intestinal epithelial cells, and cytotoxicity of macrophages [38]. Furthermore, a _gidA_ deletion mutant was highly attenuated in mice as indicated by a significantly higher lethal dose 50 (LD\({}_{50}\)). In a mouse infection model of salmonellosis, a _gidA_ mutant showed reduced colonization of the liver and spleen, reduced severity of histopathological lesions in the liver and spleen, and decreased levels of the proinflammatory cytokines when compared to mice infected with the wild-typeSTM 14028 strain [38]. Additional analysis into the virulence mechanism was performed using global transcriptional and proteomic profiling. Microarray analysis, deposited at the GEO database of the National Center for Biotechnology [59] under series number GSE30787, showed altered expression of numerous genes in the _gidA_ mutant with genes of numerous functions being affected. Several virulence genes were shown to be down-regulated like the type 3 secretion system apparatus genes (_invAEG_, _spaPQ_, and _prgHJ_), flagellar genes (_fliACDJKZ_, _fglLM_, and _ffh4B_), flagellar motor genes (_motAB_), and many others [38]. 2D gel electrophoresis and semi-quantitative Western blot analysis showed numerous proteins were altered in a _gidA_ deletion mutant most notably a significant down-regulation of virulence proteins including PrgH, FliC, and MotB [38]. The down-regulation of these virulence genes and proteins provides a plausible correlation to the attenuated phenotypes observed in the _gidA_ mutant, and identified virulence factors potentially regulated by GidA. In a follow up study, [PERSON] _et al._ characterized a _gidA_, _mmmE_, and _gidA _mmmE_ deletion mutants of _Salmonella_ to examine the relative contribution of GidA and MnmE in virulence. A _gidA_ deletion mutant showed the same attenuated phenotypes as in their previous study [30,38]. The _mmmE_ deletion mutant was significantly attenuated in both _in vitro_ and _in vivo_ models of _Salmonella_ infection, but not to the extent of a _gidA_ deletion mutant [30]. Meanwhile, a _gidA _mmmE_ deletion mutant was more attenuated than either single mutant, particularly _in vivo_ as no mice died during the LD\({}_{50}\) experiment that were infected with the _gidA _mmmE_ deletion mutant [30]. Additional data from this study showed a rather significant growth defect in the _gidA_ and _gidA _mmmE_ deletion mutants with a modest growth rate decrease in the _mmmE_ mutant when compared to the wild-type _Salmonella_ strain [30].