Oxidative stress in biofilms: Causes, role in diseases and biological effects

Abstract

Active oxygen species are produced as an inescapable by-product of normal aerobic metabolism, and their production is further enhanced by exposure to certain environmental conditions, through diet, or some diseases. Oxidative stress is defined as an imbalance between the production of free radicals and antioxidant defenses in favor of the former, which can result from an excess of substance or pro-oxidant free radicals, antioxidant deficiency or both factors. There are many kinds of free radicals, but can be grouped together under the name of reactive oxygen species (ROS) or nitrogen (RNI), determined by the ability of each species according to four basic characteristics as: reactivity, specificity, selectivity and diffusivity. The defense mechanisms of living organisms have developed mechanisms that prevent the formation of free radicals, recognize oxidative damage and can repair and antioxidant physiological defenses. The antioxidant defenses can be classified according to their nature as enzymatic or non-enzymatic antioxidants, and depending on their origin, as endogenous and exogenous antioxidants. The reactive species of oxygen not neutralized by antioxidant defenses can damage by oxidation lipids the cellular membranes, the critical enzymes of the intermediary metabolism, and also break DNA chains. Oxidative stress is involved in various disease processes such as cardiovascular and neurological disorders, diabetes, ischemia, cancer; and is one of the most important causative agents of mutagenesis, tumorigenesis, and even aging. It has been also associated with the chronic inflammation caused by biofilms of free radicals that are generated during bactericidal attack of macrophages and neutrophils. Biofilms are microbial communities containing sessile cells embedded in a self-produced extracellular polymeric matrix (containing polysaccharides, DNA and other components). In comparison with their planktonic (free-living) counterparts, sessile cells are often much more resistant to various stress conditions and this increased resistance has a considerable impact on the treatment of biofilm-related infections. In the first part of this review, the reactivates of active species of oxygen and the antioxidant system of oxidative stress responses will be highlighted briefly, and the physiological role in biofilms discussed. Subsequently, the similarities and differences between oxidative stresses responses in bacteria and eukaryotic microorganism biofilms will be reviewed and finally the phenotypic adaptation of microbial biofilms in response to stress their relationship to exposure to environmental pollutants, disinfectant and antibiotics is discussed. A better understanding of the genotypic and phenotypic adaptation of sessile (biofilm-associated) microorganisms to various forms of stress is required in order to develop more effective anti-biofilm strategies.