# Emerging Roles of Toxin-Antitoxin Modules in Bacterial Pathogenesis
## 4 Biofilm Formation is Influenced by the Action of Multiple TA Systems
The expression of _mqsR_ is stimulated by the cross-species bacterial communication signal autoinducer 2 (AI-2), which controls chemotaxis, flagellar synthesis, and motility in _E. coli_[60; 62; 69]. MqsR acts via the motility regulator system QseBC, which transcriptionally induces _flhDC_, the master regulator of flagella and motility genes [69]. In addition, MqsR stimulates expression of the transcription factor McbR [69], which, in turn, inhibits the synthesis of McbA, thereby preventing colonic acid production [70]. Colanic acid is an exopolysaccharide that is important for the formation of three-dimensional architecture of mature biofilms [71; 72]. Moreover, the GhoST type V TA pair, which is regulated post transcriptionally by the MqsR toxin, impacts early biofilm formation and swimming motility [18]. The antitoxin DinJ of the YafQ-DinJ TA pair also affects the general stress response by indirectly decreasing RpoS levels. It has been demonstrated that DinJ reduces the expression of RpoS by repressing the _cspE_ gene that encodes the cold-shock protein CspE. CspE is an enhancer of translation of _rpoS_ mRNA [73]. Moreover, the YafQ toxin mediates tolerance of _E. coli_ biofilm cells to multiple, but specific, antibiotics [74]. Further insights into the relation between TA modules and biofilm development are provided by the Hha-TomB (also known as Hha-Yba)) pair [62; 75]. Transcription of both genes is highly induced in biofilms [60]. Hha inhibits biofilm formation by preventing the synthesis of type I fimbiae via transcriptional inhibition of _fimA_ and _ibfA_ genes as well as by inhibition of their translation via rare tRNAs [75] (Figure 3). Thus, Hha functions as a biofilm inhibitor that promotes dispersal of cells from the mature biofilm, possibly to promote colonization in distant niches. Hha also activates several prophage genes that are involved in lysis of cells, for example the _vifZ_ gene coding for YfiZ toxin of the YpjF-YfjZ TA pair found in a cryptic prophage of _E. coli_ K-12 [75; 76] (Figure 2). Moreover, the expression of ClpXP and Lon proteases, which are responsible for degradation of many antitoxins, is stimulated by Hha. As a consequence the cognate toxins are liberated to exert their inhibitory effects on bacterial development [75]. Other data have demonstrated that Hha overproduction activates genes encoding the toxins and antitoxins of the YefM-YoeB, Dinj-YafQ and RelBE systems [77]. Moreover, in an _E. coli_ strain deleted of five type II TA cassettes (_mazF-mazE, relE-relB, chpB, yoeB-yefM, and yafQ-dini_) biofilm development was influenced via control of fimbiae and reduced dispersal through the YigK (TabA) protein [77]. However, contrasting data were obtained by [PERSON] and co-workers, showing that only _mazEF_ and _diniJ-yafQ_ among these five cassettes participated in the promotion of biofilm development through a mechanism related to their role in mediating cell death [78]. The HipBA TA system influences _E. coli_ biofilm formation through the production of extracellular DNA (eDNA) [79]. eDNA is an important structural component of the biofilm matrix and directly enhances biofilm adhesion [80; 81]. The production of the HipA toxin affects the integrity of _E. coli_ cells  
Figure 3: Schematic representation of a complex and multilayered protein network engaged in biofilm development in _E. coli_ via TA systems. The central part illustrates Lon protease as an example of antitoxin degrading proteases, which also include ClpXP and ClpAP. Only the relevant, most well-described interactions are displayed. Coloured boxes denote toxins (red), antitoxins (blue), Lon protease (yellow), other proteins (dark grey), small molecules (light grey), and cellular processes and structures (purple). Arrows, stimulation; broken lines ending in balls, inhibition. and may induce the lysis of a subpopulation of cells, which thereby increases eDNA concentrations in biofilms [79]. HipA also is implicated in biofilm formation in the presence of several drugs [82; 83].