Biofilms biocide resistance

An intuitive explanation of biofilm drug resistance is that antimicrobial compounds are physically excluded from the community by the barrier properties of the glycocalyx. Such intuition however envisages that the glycocalyx functions as a biocide-impermeable umbrella, but since it generally possesses a diffusivity approximating that... 

All release systems that liberate an immobilized biocide into the surroundings will exhaust rather quickly. Furthermore, the constant release is an environmental issue and supports the building of biocide-resistance in microbial strains. If a release system is the only possible option, then it would be desirable to release the biocide on demand, e.g., in cases of infection or the start of biofilm formation. This can be achieved by either degrading or swelling the matrix with an infection-specific enzyme or metabolite, or by cleaving the linker between biocide and surface with a biochemical factor. 

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. 

These observations indicate that biofilm populations may always contain a sub-set of organisms which ensure survival of the species by the ability to adopt transiently a biocide-resistant phenotype. 

Resistance to antimicrobial agents is of concern as it is well known that bacterial resistance to antibiotics can develop. Many bacteria already derive some nonspecific resistance to biocides through morphological features such as thek cell wall. Bacterial populations present as part of a biofilm have achieved additional resistance owkig to the more complex and thicker nature of the biofilm. A system contaminated with a biofilm population can requke several orders of magnitude more chlorine to achieve control than unassociated bacteria of the same species. A second type of resistance is attributed to chemical deactivation of the biocide. This deactivation resistance to the strong oxidising biocides probably will not occur (27). 

Bacterial resistance to biocides (Table 13.2) is usually considered as being of two types (a) intrinsic (innate, natural), a natural property of an organism, or (b) acquired, either by chromosomal mutation or by the acquisition of plasmids or transposons. Intrinsic resistance to biocides is usually demonstrated by Gram-negative bacteria, mycobacteria and bacterial spores whereas acquired resistance can result by mutation or, more frequently, by the acquisition of genetic elements, e.g. plasmid- (or transposon-) mediated resistance to mercury compounds. Intrinsic resistance may also be exemplified by physiological (phenotypic) adaptation, a classical example of which is biofilm production. 

Physical methods for the control of microbial biofilms, although often effective, are in many situations impractical. In this context it is notable that an almost universal feature of the biofilm mode of growth is their profound resistance to antibacterial compounds. Conventional chemical control methods, developed for use against fastgrowing planktonic cultures are only poorly effective against biofilm bacteria. Large doses of biocide or antibiotics, which are either environmentally undesirable or above toxic thresholds respectively, are required to eradicate biofilms in industry and medicine. 

Membrane autopsy of desalination RO membranes that had been in service for 2.5 yr in Saudi Arabia revealed bacterial deposits that are slimy and very adherent (64). This is primarily due to accumulation of extracellular polysaccharides excreted by microorganisms, thus resulting in biofilm formation (54). Bacteria embedded in a biofilm are found to be more resistant to biocides than freely suspended ones (65). 

Pseudomonas aeruginosa Gram-negative, micro-aerobic rod General environmental contaminant Quintessential opportunist pathogen High resistance to antibiotics and biocides Biofilm-former... 

Other advantages that accrue for biofilms on surfaces include protection from short-term fluctuations in pH, salt and biocide concentrations, and dehydration. Because of the mass-transfer resistance of the biofilm to the transport of nutrient, conditions will change throughout the biofilm depth. Where several... 

Chronic infections in contrast tend to be focal infections, limited in size, that wax and wane for long durations and are only partially destructive to tissues. The strategies of a single-cell, mobile, free-floating bacterium versus those of a community of bacteria encased in a self-secreted protective matrix (biofilm) are radically different and may one type of infections "chronic." Biofilm is intrinsically resistant to host immunity, antibiotics, and biocides, different treatment strategies will be required. Chronic infections such as chronic wounds, surgical-site infections, and infected implants will yield only to repetitive evaluation and multiple simultaneous therapies that require much persistence from the physician. 

As previously indicated, many microorganisms produce EPS or slime, and several researchers have investigated the role of EPS in corrosion [86-88]. EPS consist of polysaccharides and proteins, plus significant amounts of nucleic acids, (phospho) lipids and humic substances [89-91]. The final composition of the EPS matrix results from a combination of the following active secretion, shedding of cell surface material, cell lysis, and/or adsorption of substances from the environment. EPS are usually acidic and contain functional groups, such as carboxylic and amino acids that, as mentioned earlier, readily bind metal ions. EPS can bind metal ions from the substratum or from a liquid medium and control interfacial chemistry at a metal/biofilm interface. EPS are also implicated in increased resistance of biofilm cells to biocides and other antimicrobial compounds [9]. 

Resistance of the sessile bacteria (and thus, biofilms) to biocides is a known factJ - There are three mechanisms that have been proposed to explain this behavior of biofilms ... 

Szomolay et have developed a model to describe the interaction between biofilm thickness and biocides. They have postulated that in addition to the known mechanisms mentioned above (mechanisms 1, 2, and 3), some of the bacteria in a biofllm seem to be able to sense the biocidal material and actively respond to it. In this model, it is proposed that there is a relationship between the thickness of the biofilms and the activity of the biocide for the sessile bacteria. Put in simple words, the thicker the biofilm becomes, the more the bacteria learn to resist it. 

Intrinsic resistance is the most common form of resistance to biocides and is considered to involve exclusion of a biocide as a consequence of impermeability. Additionally, physiological adaptations of an organism in response to changes in the growth environment can modulate sensitivity to biocides, a classical example of which is biofilm production [236]. 

Bacterial spores are invariably the most resistant forms of bacteria to bio-Table 4.3. RESISTANCE OF BIOFILMS TO BIOCIDES... 

It is important to note that antimicrobial and biofilm resistance are two different characteristics though some materials show both properties at the same time. Antimicrobial materials do not automatically prevent biofilm formation and vice versa. Antimicrobial surfaces could kill bacteria on contact but if dead bacteria cell debris blocks the active biocidal surface, biofilm formation could eventually occur. For example, quaternary anunonium polymers can effectively kill bacteria but when the surface is fouled with dead bacteria debris, biofilm formation is inevitable [188]. Materials with antibiofilm properties will repel the bacterial adhesion very effectively but may not kill the bacteria when they do colonize the surface. PEG surfaces are well known to repel bacteria adhesion. However, PEG surfaces show little antimicrobial activity. Quantitative antibiofilm efficacy tests can be divided into two categories static (minimum biofilm eradication concentration assay, MBEC) and dynamic (flow cell assay). In addition, SEM is a semiquantitative assay, which is discussed in Section 2.5. 

Biofilms are known to exert enhanced resistance to biocides (LeChevallier et al., 1988 a b). There are different mechanisms of resistance, depending upon the biocide, the biofilm and the environmental conditions. Some of the major factors are discussed in the following sections. 

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