## 2 Persisters of Foodborne Pathogens
### Persister Types and Mechanisms of Formation
#### 2.3.2 Toxin-Antitoxin System Induced Persisters  
TA systems are defined as ubiquitous small operons containing two genes that separately express a stable toxin molecule to slow down/block certain metabolic processes and an unstable antitoxin to neutralize this toxicity. Under certain stress, the antitoxin will be selectively degraded, which consequently leads to toxin accumulation and growth arrest that ultimately contributes to persister formation [66]. Currently, there are six kinds of TA systems (type I-VI) and persistence-associated TA systems belong to the first two types (Figure 4) [66,67]: (1) a type I TA system contains a small RNA (sRNA) antitoxin inhibiting the translation of toxin that decreases ATP synthesis by disrupting membrane potential, (2) the best-studied type II TA system includes a DNA-binding protein antitoxin that can repress toxin gene transcription and/or directly inactivate protein toxin by forming a TA complex. This type II toxin inhibits several essential metabolic processes such as DNA or protein synthesis. Most type II toxins can also co-repress transcription with an antitoxin [68]. _E. coli_ is an important species to study the function of type II TA systems in persister formation. The first discovered gene that directly associated with persister levels is _E. coli hipA_ gene encoding high persister protein A. _HipA_ is part of a type II TA locus, _hipBA_, with its inhibitor gene _hip B_[69]. The HipA toxin attenuates cell growth by inhibiting glutamyl tRNA synthetase (GltX), and then stimulates  
Figure 3: Functions of ppGpp associated with persister formation after being generated by a RelA/SpoT homolog family. RPDP model: ribosome dimerization persister model. ppGpp synthesis and, ultimately, promotes persister formation [70; 71]. In addition, _hipA_-induced _E. coli_ persisters highly rely on ppGpp [72]. In addition to _hip BA_, there are 10 other type II ppGpp-independent TA loci in _E. coli_ that encode mRNAs involved in the reduction of cellular metabolic activity [73]. In 2013, [PERSON] et al. [74] provided strong evidence that _E. coli_ mutants without these 10 TA loci display sharply decreased persister levels. However, it was proven in 2017 that these mutant cells were contaminated by a \(\varphi\)80 bacteriophage and the article was retracted [75]. Later, [PERSON] et al. [76] reassessed the role of mRNAs-encoding TA loci and demonstrated that they are not directly linked to antibiotic persistence. Despite this, evidence still suggests an essential role of TA systems in _E. coli_ persister formation. For example, [PERSON] et al. [77] proved that toxin MazF-mediated growth arrest leads to high levels of persisters under antibiotic exposure. [PERSON] et al. [78] revealed the importance of the _yafQ_ toxin gene in inducing persisters in antibiotic-exposed biofilms. Although reports about _E. coli_ persisters and their TA systems are conflicting, positive relationships between the TA toxin and persister formation are proposed in _Salmonella_. [PERSON] and colleagues discovered that the majority of 14 ppGpp-dependent type II TA operons play key roles in intramacrophage persister formation [79]. Later, acetyltransferase toxin TacT and three TacT-like toxins were revealed and their ability to induce persisters by inhibiting translation was further proven [80; 81]. On the opposite side, TA II systems do not have a clear effect in _S. aureus_[52] and _L. monocytogenes_[82]. In addition to type II TA systems, type I TA systems have also been proven to mediate persister formation. The overexpression of type I TA toxin _tisB_ activated by the SOS response lead to a significant increase in _E. coli_ persister formation [83]. In 2020, [PERSON] et al. [84] first reported a novel type I TA system (termed TMCS) in _S. aureus_ that promotes persister formation by inhibiting the efflux pump NorA under antibiotic treatment. In summary, TA systems are likely linked to the molecular physiology of persister formation.