Several mechanisms conferring bacterial resistance to biocides have been described;
some are inherent to the bacterium, other to the bacterial population. In addition, some
of the resistance mechanisms are intrinsic (or innate) to the micro-organism while others
have been acquired through forced mutations or through the acquisition of mobile
genetic elements. Innate mechanisms can confer high-level bacterial
resistance to biocides. In this case, the term unsusceptibility is used
The most described intrinsic resistance mechanism is changes in the permeability of the
cell envelope, also referred to as "permeability barrier". This is not only found in spores
, but also in vegetative bacteria such as
mycobacteria and to some extent in Gram-negative bacteria. The permeability barrier
limits the amount of a biocide that enters the cell, thus decreasing the effective biocide
concentration . In mycobacteria the presence of a mycoylacylarabinogalactan layer accounts for the
impermeability to many antimicrobials . In addition, the presence and composition of the
arabinogalactan/arabinomannan cell wall also plays a role in reducing the effective
concentration of biocide that can penetrate within mycobacteria .
The role of the lipopolysaccharides (LPS) as a permeability barrier in Gram-negative
bacteria has been well documented. There
have also been a number of reports of reduced biocide efficacy following changes in other
components of the outer membrane ultrastructure including proteins , fatty acid composition and
phospholipids. It must be noted that in the above mentioned
examples, an exposure to biocides was followed by changes in ultrastructure related to a
decrease in biocidal susceptibility, usually at a low concentration (under the MIC value).
The charge property of the cell surface also plays a role in bacterial resistance
mechanisms to positively charged biocides such as QACs . It is
likely that bacterial resistance emerges from a combination of mechanisms , even though single
specific mechanisms are often investigated.