# Structure, Biology, and Therapeutic Application of Toxin-Antitoxin Systems in Pathogenic Bacteria
## 3 TA Modules from Pathogenic Bacteria
### Gram-Negative Bacteria  
_Neisseria gonorrhoeae_ is the causative agent of the sexually transmitted disease, gonorrhea [149]. After survival inside epithelial cells, _N. gonorrhoeae_ can penetrate the epithelial monolayer to infect the stromal tissue of the subepithelial, giving rise to purulent inflammation characteristic of gonornrhoea [150; 151]. A bioinformatics search has revealed that _N. gonorrhoeae_ species have type II TA pairs including two HicBA, two MazzEF, three RelBE, and two VapBC (see Supplementary Table S1). In addition, the FitAB TA system was identified through genetic screens, not bioinformatics [149]. This system could facilitate a dormant state of the pathogen within epithelial cells, which circumvents human immune attacks. This state also allows the host to be an asymptomatic carrier, promoting the spread of the disease. FitAB deletion causes _N. gonorrhoeae_ to grow faster than wild-type inside host cells. _R. felis_ is a flea-borne pathogenic bacterium that causes spotted fever in humans [152]. The _R. felis_ genome harbours at least 13 TA systems predicted to act as addiction systems [153]. There exist type II TA pairs in _R. felis_ organisms: one HicBA, one MazzEF, 12 RelBE, and six VapBC (see Supplementary Table S1). Among these, the VapC toxin was shown to exhibit a ribonuclease activity and thereby inhibits the growth of _E. coli_ and _Saccharomyces cerevisiae_[154]. _Shigella flexneri_ is a pathogenic bacterium that mainly causes diarrhoea in humans [134]. This pathogen has type II TA pairs including one VapBC, one MazEF, one ParDE, and six RelBE (see Supplementary Table S1). VapC (MvpT) toxin encoded on virulence plasmid pMYSH6000 cleaves the initiator tRNA\({}^{\text{Mdet}}\) specifically in the anticodon region and therefore inhibits the global cellular translation [58], which is similar to the function of VapC toxins from enteric bacteria [58]. In addition, in the YeeUW(V) system from _S. flexneri_, the YeeU antitoxin represses the expression of the YeeV toxin without direct physical interaction with it [70]. The action mechanism of this system is fundamentally different from those of other well-known bacterial TA systems. _E. coli_ O157 is a human enterohemorrhagic pathogen that causes haemorrhagic diarrhoea and kidney failure [155]. _E. coli_ O157 infection occurs primarily through consumption of contaminated and raw foods [156]. The _E. coli_ O157 genome harbours type II TA pairs including two CcdAB, two HicBA, two MizEE, two ParDE, and nine RelBE (see Supplementary Table S1). There are two three-component TA operons related to the parDE system, the EcPaaR1-EcPaA1-EcParE1 and EcPaaR2-EcPaA2-EcParE2 modules on the prophage region [157]. A three-component TA system is also observed for the \(\omega\)-\(\epsilon\)-\(\zeta\) module on the plasmid of _B. subtilis_[158]. In contrast to typical type II TA systems, the antitoxin of the three-component system lacks a DNA-binding domain, which is instead adopted by a third protein that acts as transcriptional repressor of the TA operon. The DNA-binding domain of EcPaaR1 and EcPaaR2 belongs to the DicA transcriptional repressor family. _S. typhimurium_ is a foodborne pathogen that causes inflammatory disease in the gastrointestinal tract of humans [134]. After penetrating the intestinal barrier and spreading via the lymphatic and hematogenous routes, this bacterium settles in the spleen, bone marrow, Peyer's patches, liver, bile ducts, etc., causing typhoid fever. The TA database has suggested that _S. typhimurium_ genome harbours type II TA pairs including three HigBA, three Phd-Doc, 13 RelBE, and five VapBC (see Supplementary Table S1). _Salmonella_ species can be transformed into non-replicating persister cells inside macrophages, which are involved in at least 14 putative type II TA families [28]. Three of the 14 TA families have toxin components that share sequence similarity with Gcn5 N-acetyltransferase (GNAT) enzymes, and one of the three toxins, named TacT, was structurally and mechanistically characterized [159].