## 9 Information from Toxin-Antitoxin-DNA Cestructures
For example, one-third of the world's population is infected with _Mycobacterium tuberculosis_, the causative agent of tuberculosis, which causes two million deaths globally per annum. The emergence of multidrug resistant strains, then extensively drug resistant isolates, and more recently totally drug resistant strains of _M. tuberculosis_ threatens even further devastation in developing countries, as well as the deadly re-emergence of an ancient scourge in the Western world [122]. Similarly, multiresistant strains of Gram-negative _Pseudomonas aeruginosa_, Acinetobacter spp., and Enterobacteriaceae that produce extended-spectrum beta-lactamases are serious hospital-acquired pathogens that are becoming evermore difficult to treat with existing antibiotics, as are Gram-positive species including methicillin-resistant _Staphylococcus aureus_ and vancomycin-resistant enterococci [119, 120, 123]. Despite the impending crisis, only one new class of antibiotic has come to market in the past 50 years. Moreover, pharmaceutical companies are withdrawing from antimicrobial development in pursuit of more lucrative therapeutics: a perilous gap has opened in the race for novel antibacterials [124]. Ectopic expression of toxin genes induces severe bacterial growth defects, including cell death. Moreover, genes for TA complexes have no known human homologues and are abundant on chromosomes and plasmids of most bacteria, including pathogenic species, which contributes to their attractiveness as potential targets for novel antibacterial agents. There is considerable interest in identifying natural and artificial molecules that destabilize TA complexes to promote toxin activation [125]. A variety of different strategies may be implemented to exploit TA systems to this end. For example, the disruption of the toxin-antitoxin interaction may free the former to induce cell death [126]. Similarly, interference with antitoxin oligomerization will destabilize the TA complex thereby liberating the toxin. In both cases, perturbation of TA complex assembly or organization also is predicted to interfere with correct transcriptional autoregulation. Moreover, compounds that bind the TA promoter region will directly inhibit transcription of TA genes. Replenishment of the labile antitoxin will be prevented thereby releasing the more stable toxin to induce bacterial cell suicide [90]. Similarly, ligands that block antitoxin interaction with the operator site may derepress TA gene expression, imbalance the TA ratio and, again, cause inappropriate toxin release. Persister cells comprise a rare, antibiotic tolerant fraction of bacterial populations. TA loci are implicated in bacterial persistence as the toxin factors reduce metabolism and render cells insensitive to many antibiotics [11]. This phenomenon needs to be considered carefully when assessing the effect of artificial toxin release on bacterial survival. Studies on disruption of the streptococcal \(\varepsilon\)-\(\zeta\) interaction and inhibition of TA complex formation in the anthrax agent of _Bacillus anthracis_ suggest that designer peptides may be useful in modulating TA system interactions [127; 128; 129]. Peptides based on the structure of the ParE toxin have been used as inhibitors of bacterial topoisomerases [130]. Analogously, the Extracellular Death Factor is an endogenous pentapeptide activator of the MazF toxin [60] which indicates the existence of natural ligands that may be exploited to moderate TA complex function. Although these initial studies suggest that peptidomimetics may be potentially powerful in disrupting TA function, designing effective molecules is not straightforward [131]. Developing molecules that alter TA transcriptional regulation directly is an alternative approach for controlled toxin release. Sequence-specific DNA binders already function, for example, in anticancer therapies [132; 133; 134]. Potential antibacterial drugs targeting TA systems could alter interactions of antitoxins with DNA regulatory sites by a variety of different mechanisms.