Biofilm-specific phenotype

Many different mechanisms of biofilm resistance are discussed in the literature, reflecting the different ways of biofilm organisms to withstand biocides. These mechanisms include physical and chemical diffusion-reaction barriers in the biofilm restricting biocide penetration of the biofilm, slow growth rate of biofilm cells due to nutrient limitation, activation of general stress response genes, the emergence of a biofilm-specific phenotype, and the presence of persister cells. 

Evidence is accumulating that the the process of attachment to surfaces and growth in a biofilms is associated with the activation and repression of genes, resulting in a biofilm-specific phenotype of the microorganisms within a biofilm community. It is assumed that this process includes the expression of a biocide-resistant phenotype in all or a subset of the biofilm cells (Mah and O Toole, 2001). Induction of this phenotype may be caused by nutrient limitation, environmental stress, exposure to sublethal amounts of biocides, high cell density or a combination of these factors. 

In order to understand current approaches for prevention and control of biofilms, we must first consider the reasons for the failure of conventional antimicrobial protocols. There are thought to be three main reasons as to why biofilm bacteria out-survive their planktonic counterparts during antimicrobial treatments (reviewed by McBain et a/.16).These are i) poor penetration of antimicrobial compounds due to the presence and turn-over of exopolymer slime (glycocalyx) ii) the imposition of extreme nutrient limitation within the depths of the biofilm community and the co-incident expression of metabolically-dormant, recalcitrant phenotypes and (iii) the expression of attachment-specific phenotypes that are radically different and intrinsically less susceptible than unattached ones. 

In recent years, it has become obvious that the biofilm mode of growth is associated with a specific expression of genes and altered growth rates. To take into consideration the adoption of characteristic biofilm phenotypes by planktonic bacteria, a modern definition of a biofilm has been given by Donlan and Costerton (2002), who described a biofilm as a microbially derived sessile community characterized by cells that are irreversibly attached to a substratum or interface or to each other, are embedded in a matrix of EPS that they have produced, and exhibit an altered phenotype with respect to growth rate and gene transcription . 

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