The Hha-TomB Toxin-Antitoxin System Shows Conditional Toxicity and Promotes Persister Cell Formation by Inhibiting Apoptosis-Like Death in S. Typhimurium

Abstract

Toxin-antitoxin (TA) modules are two component “addictive” genetic elements found on either plasmid or bacterial chromosome, sometimes on both. TA systems perform a wide range of functions like biofilm formation, persistence, programmed cell death, phage abortive infection etc. Salmonella has been reported to contain several such TA systems. However, the hemolysin expression modulating protein (Hha) and its adjacent uncharacterized hypothetical protein TomB (previously known as YbaJ), have not been listed as a TA module in Salmonella. In this study we established that Hha and TomB form a bonafide TA system where Hha serves as a toxin while TomB functions as an antitoxin. Interestingly, the toxicity of Hha was conditional causing cell death under acid stress. The antitoxin attenuated the toxicity of Hha by forming a TA complex through stable interactions. The Hha-TomB TA system was found to increase persistence and inhibit programmed cell death under antibiotic stress where a phenotypically diverse population expressing differential level of TA components was observed. Therefore we propose that Hha and TomB prevent cells from committing suicide thereby promoting persister cell formation.

Prediction of Toxin Antitoxin System (TA System) as a Novel Potent Target in Salmonella typhi by Using Bioinformatics Analysis

“…Many factors are involved in the pathogenesis of bacteria specially S. typhi [9]. One of these factors is toxin-antitoxin (TA) system which has been considered today [7]. Since the TA system plays an essential role in the pathogenesis of bacteria [10], we decided to analyze the potent TA loci by bioinformatics method.…”

“…Control and treatment of S. typhi infection has become a challenge [6]. On the other side, according to the most recent studies, TA system is significant as a potent antimicrobial target and plays an important role as a basic principle in the pathogenesis of bacteria [7].…”

Persister Cells – a Plausible Outcome of Neutral Coevolutionary Drift

“…The phenomenon of bacterial persistence (persister bistability) – a non-inherited antibiotic tolerance in a minute fraction of the cellular population 1 – 11 is a proven example of the existence of phenotypic multiplicity in a genetically homogeneous population of bacterial cells 12 . This phenomenon was discovered in 1944 13 and its essence lies in the fact that an antibiotic-sensitive population of bacterial cells practically always, even after prolonged antibiotic therapy, contains a very small number of cells (10 −3 –10 −6 ) possessing tolerance to the antibiotic and therefore is capable of restoring cellular population after withdrawal of treatment.…”

“…Analysis of the functional activity of persister cells revealed a significantly higher expression of genes belonging to the stress response systems, including the toxin-antitoxin (TA) systems 8 , 11 , 15 , 16 . The study of mutant genes belonging to various TA systems confirmed their participation in the formation of persister cells 12 , 22 – 25 .…”

Plausible role of bacterial toxin–antitoxin system in persister cell formation and elimination

“…For example, bioinformatics analysis suggested about 11-19 type II TA modules in Salmonella Typhimurium of which, the phd/doc and TacAT systems(Cheverton et al, 2016) induce multidrug-resistant persisters in this pathogen after being activated by nutrient-deprived environment in vacuoles. Similarly, a study byJaiswal et al (2016) suggested that Hha (toxin) and TomB (antitoxin), belonging to the bonafide type II TA system, crucially play role in persister cell formation by inhibiting apoptosis like cell death of Salmonella Typhimurium.TA also plays a significant role in mediating persistence of oraldwelling bacteria such as S. mutans and Aggregatibacter actinomycetemcomitans. S. mutans, present inside the biofilm in oral cavity,…”