The attachment of bacteria and other microorganisms on surfaces is the result of some physicochemical links and interactions and their own metabolical characteristics. Then occurs a second phase, called adhesion, linked to those factors in interaction with others of biological nature. These two phases could be eventually followed by a third one, the colonization if cleaning and/or disinfection of the surface at this place are inefficient … or not performed. This leads to the formation of a biofilm.
The attachment is due to interaction forces between both surfaces and microorganisms molecules : it is a reversible phase. These forces induce the linkage between different molecules or cohesion between identical molecules. They are due, by decreasing order of linkage’s energy, to the ionic bond (attraction between positive and negative ions), the covalent bond (sharing of electrons between atoms), the metal bond, the linkage between dipoles, the hydrogen bond, the linkage between dipoles and induced dipoles and the London force of dispersion between apolar molecules. In liquid media these forces induce, at the vicinity of the solid surface, some changes of the solute’s concentrations linked to their mode of interaction with the surface tension of the solvent. This induces pH and red-ox potential changes. Detergents will have a very important activity linked to their tensioactive potential ; they will tend to modify these interactions.
The microbial adhesion to the surface begins to be an irreversible attachment, sometime called « bonding ». This is due to the different factors enumerated previously if the distance microorganism-surface becomes sufficiently small, due to phenomenoms like sedimentation, brownian movement, flagellar movements, chimiotropic movements, hydrophobic effect, dessication, etc… Thus, adhesion may be passive or active if linked to a metabolical mechanism (coupling). Adhesion may also be more or less firm according to the possible interactions. This is linked to the solution’s composition, to the cellular metabolism (production of compounds able to induce adhesion) and to hydrophobic surface. Some microorganisms have a better adhesion than other inside a population when they are able to produce a capsule or exopolysaccharids like « slime », …. It is now perfectly known that a microorganism has a better growth attached on a surface than free in the environnement (= planktonic) .
The colonization of the surface will be linked to the microbial skills to growth or to survive depending of the local ecological conditions (humidity, temperature, surface characteristics, presence of protective and nutritive substances or inhibitors in addition to biological factors, etc..). At this stage the shape of the surface will also play a role (fissures, cracks, etc…) allowing or not the possibility of « nesting » of the microorganisms protected against a mechanical or/and detergent/disinfectant action.
Proliferation in synergy may also occur with a surface’s colonization with many layers called « biofilm ». Inside this biofim, the microorganisms located away from the surface will not be touched by disinfectants neither sampling methods.
Professor Philippe Hartemann