How Disinfectants React With Biofilms
Chlorine based disinfectants have significant issues in controlling bacterial biofilm populations. The reason for this is not fully understood but it is likely to be due to an inability to fully penetrate the biofilms protective layers (Jang et al., 2006), with reduced disinfectant concentrations found inside the protective layers. De Beer et al., 1994, measured the concentration of chlorine in biofilms of Pseudomonas aeruginosa and Klebsiella pneumoniae during disinfection and found that concentrations were 20% or less of the concentration in the bulk liquid. They concluded that this limited ability of chlorine to penetrate biofilms is likely to be responsible for its lesser activity when comparing with planktonic cultures.
This phenomenon is not only applicable to chlorine containing biocides and the reasons for this lack of penetration are multifactorial and steric hinderance has been suggested as a possible factor. Disinfectants by their nature are generally highly chemically reactive, and organic compounds which are components of biofilm layers, either in bacterial cells or in the EPS react with groups on disinfectants, negating their ability to penetrate the layers effectively and reducing their concentration (Bridier et al., 2011).
Biofilm populations have also demonstrated the ability to adapt to the stresses of biocide exposure. When bacteria in the inner layers of biofilms are subjected to levels of disinfectant that are not sufficient to kill them, they have shown the ability to develop responses which make them more resistant in comparison to planktonic cultures. Mangalappalli-Illathu, Vidovic and Korber, 2008, in their study, examined phenotypes of Salmonella enterica serovar Enteritidis biofilm and planktonic cultures that were adapted to benzalkonium chloride (BC) by constant low-level exposure (1 g/ml-1 of BC for 144 h). They found that of the BC adapted biofilm and planktonic cells, the biofilm cells were 4.6-fold more likely to survive a lethal dose of BC and were significantly more likely to survive heat shock treatment. They also found that when comparing the response to a lethal dose of adapted and non-adapted cells, the adapted cells were 18.3 (biofilm) and 3.2 (planktonic) fold more likely to survive than their non-adapted counterparts. They found upregulation of specific proteins in the biofilm adapted cells, alterations in cell surface roughness and changes in fatty acid concentration which gave them a greater survival ability when compared with the planktonic cells. For bacillus subtilis, it has been demonstrated that exposure to non-lethal doses of ClO2 can stimulate biofilm growth (Shemesh, Kolter and Losick, 2010). In the same study it was shown that strains hypersensitive to ClO2 were unable to form an extracellular matrix suggesting that biofilm formation can be initiated as a protectionist defensive response against biocidal treatment.
The act of adhesion of cells on to a surface when a biofilm is being formed has also been shown to lead to changes which can result in the formation of resistant phenotypes within cells.
