Consideration of some implications of the resistance of biofilms to biocides

Biofilms are considered as the growth of cells at a surface with the production of extracellular polymeric substances. Biofilm formation has serious implications in industrial, environmental, public health and medical situations. There is now a large body of evidence and thus concern that microorganisms within biofilms (including certain pathogenic microorganisms) are less susceptible to the activity of biocides than their planktonic counterparts. The mechanisms of bacterial resistance to some antibacterial agents such as many antibiotics are reasonably well-understood. The extensive exopolysaccharide polymer associated with biofilms is considered as a potential barrier which can hinder or prevent biocides from reaching target organisms within the biofilm, but the actual physical barrier properties of the biofilm may be of less importance and it now seems that perhaps the intrinsic (phenotypic) resistance to biocides, shown by cells within a biofilm, is of primary concern to researchers in the field. Nutrient limitation and reduced growth rates resulting from the position of bacteria within a biofilm are considered to influence the physiology of bacteria which in turn alters their sensitivity to biocides. When these phenomena are coupled with the ability of both Gram-positive and Gram-negative bacteria to communicate with each other in a cell-density-dependent or growth phase manner via diffusible communication molecules, it is becoming obvious that individual biofilm organisms may be able to behave as sociable, collective communities to regulate their gene expression in order to control various physiological processes and responses, in aid of the ‘common good’. Since most antimicrobial agents have been developed as a consequence of their activities against relatively fast-growing planktonic organisms, this approach is unlikely to be entirely suitable for the development of biocides against sessile biofilm organisms. Thus, the generation of biofilms which are representative of their true nature in the environment and which are suitable for the evaluation of the effects of biocides is essential in biocide research and development but it is not an easy task, as every application involving problematic biofilm formation will probably represent a unique niche which will require individual solutions. Disruption of a biofilm prior to assessing the viability of individual members by conventional microbiological methods is not considered ideal for the evaluation of biocides. There is a need for the development of non-disruptive methods for the measurement of biofilm formation.