# Emerging Roles of Toxin-Antitoxin Modules in Bacterial Pathogenesis
## 7 Deciphering the Roles of TA Modules in Pathogenicity
### TAs in Other Gram-Negative Pathogens  
In addition to _E. coli_ and _S. typhimurium_, a handful of studies have described potential roles for TA complexes in the virulence of other Gram-negative pathogens. _Leptospira_ spp. are causative agents of the most globally widespread zoonotic infectious disease. Leptospirosis is especially prevalent in tropical climates and in its most severe form can impair multiple organs, notably the kidneys, liver, meninges and brain [187]. Expression of the _chtpBK_ and _mazEF_ TA genes was elevated in human macrophages infected with _Leptospira interrogans_ compared to cultures grown in laboratory medium. The wild-type strain and mutants deleted of either module induced early-stage macrophage apoptosis to similar levels. However, the mutants induced late-stage apoptosis less effectively than wild-type suggesting that the _chtpBK_ and _mazEF_ TA loci have roles in cellular necrosis by _L. interrogans_[188].  
_Vibrio cholerae_ is the etiologic agent of cholera, a potentially life-threatening, acute diarrheal disease with epidemic potential [189]. The superintegon (SI) of _V. cholerae_ is a genomic region with high genetic flexibility that is thought to allow a rapid response to environmental changes and niche adaptation. The SI harbors an unusually high concentration of type II TA modules: 17 cassettes are dispersed within only a \(\sim\)125-kb sequence. These TA loci are functional and may act in concert to ensure that the SI is maintained in _V. cholerae_ by a similar mechanism of post-segregational cell killing as established for plasmid-located TA genes [190]. Separate deletions of two _relBE_ modules within the SI impaired colonization by the mutants of the mouse intestine in competition with the wild-type strain. Deletions of the other five _relBE_ homologues had no discernible effect on virulence [86]. Deletion of the _relBE_ loci implicated in colonization also modestly perturbed biofilm formation by _V. cholerae_, although no effects on antibiotic resistance, the oxidative stress response or sensitivity to bile salts were observed in any of the mutants [86].  
_Haemophilus influenzae_ is a commensal resident of the normal airway microflora, but also is an opportunistic pathogen that causes a range of respiratory tract infections, including otitis media in children, as well as more systemic disease, including bacteriaemia and meningitis [191]. _H. influenzae_ strains are classified as encapsulated or unencapsulated based on the presence or absence, respectively, of a protective polysaccharide capsule that surrounds the cell. Unencapsulated strains also are termed nontypeable _H. influenzae_. Nontypeable _H. influenzae_ strains possess at least four _vap_ TA loci. The effects of the separate and simultaneous deletion of two of these loci were assessed in primary human respiratory epithelial tissue, which serves as an effective _in vitro_ model for the upper airway [192]. Survival of the single and double deletion _vap_ mutants in this model was attenuated in comparison with the wild-type strain. Similarly, the mutant strains persevered significantly less well and induced less middle ear inflammation than wild-type nontypeable _H. influenzae_ in a chinchilla model of otitis media [192]. Analogous results were obtained with deletion of a _higBA_ locus [193]. Thus, _vap_ and _higBA_ modules enhance the pathogenicity and survival of nontypeable _H. influenzae_ in both _in vitro_ and _in vivo_ infection models.  
Finally, Rickettsia are obligate intracellular bacteria that are transmissible from arthropods to humans in whom several _Rickettsia_ spp. can induce serious, sometimes fatal, infections [194]. _Rickettsia_ spp. with multiple type II TA modules promote higher levels of host cell apoptosis than a species with fewer TA genes. A VapC toxin homologue may be implicated in this effect [195; 196; 197].