Molecular determinants of bacterial stress responses to reactive chlorine species

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Bacteria are able to survive under a variety of often harmful environmental conditions such as heat or cold, nutrient limitation, low ph or antimicrobial treatment due to a multitude of adaptation processes and stress responses. One important adaptation and survival strategy exhibited by a wide range of bacteria is the formation of biofilms. Biofilms are communities of microorganisms attached to a surface and embedded in a self-produced, polymeric matrix. Biofilm bacteria have been shown to possess unique characteristics including increased stress resistance and higher antimicrobial tolerance leading to failures in bacterial eradication in many technical settings including drinking and waste water industries. During a screening of different environmental stressors, we identified that the oxidative stress agents and commonly used reactive chlorine species (RCS) and disinfectants sodium hypochlorite [NaClO, household bleach], calcium hypochlorite [Ca(ClO)2] and chloramine [NH2Cl] strongly stimulated Pseudomonas aeruginosa stress responses and increased biofilm formation at sublethal concentrations. This enhanced formation of biofilms was very specific to reactive chlorine species such as bleach and chloramine, but did not occur in response to other tested oxidants likes hydrogen peroxide [H2O2] or the herbicide paraquat. We hypothesize that Pseudomonas aeruginosa biofilm stimulation triggered by reactive chlorine species such as bleach and chloramine is a specific, tightly controlled stress response and defense mechanism and that inadequate chlorination could lead to increased stress resistance and survival rates. We will test these hypotheses with the following objectives: 1) Identify and characterize key genes involved in the bacterial stress response and defense against reactive chlorine species, 2) Elucidate the regulatory networks involved in RCS-induced biofilm formation, and 3) Evaluate the biological effects of RCS-induced stress responses and biofilms. The long-term goal of my research program is to understand bacterial stress responses and survival strategies in a hostile environment. This work will greatly advance our basic knowledge on fundamental mechanisms by which bacteria sense and adapt to widely used reactive chlorine oxidants including the enhanced formation of biofilms, and will suggest new ways to control bacterial growth and biofilm formation in many important natural, domestic, industrial or clinical settings.