How Do Bacteria Know They Are on a Surface and Regulate Their Response to an Adhering State?

Abstract

Why Do Bacteria Adhere to Surfaces and Why Is Adhesion Considered a Virulence Factor?Bacteria adhere to virtually all natural and synthetic surfaces [1,2].Although there are a number of different reasons as to why bacteria adhere to a surface, the summarizing answer is brief: ''Adhesion to a surface is a survival mechanism for bacteria''.Nutrients in aqueous environments have the tendency to accumulate at surfaces [1,3], giving adhering bacteria a benefit over free floating, so-called planktonic ones.This is why mountain creeks may contain crystal clear, drinkable water, while stepping stones underneath the water may be covered with a slippery film of adhering microbes.In the oral cavity, adhesion to dental hard and soft tissues is life-saving to the organisms, because microbes that do not manage to adhere and remain planktonic in saliva are swallowed with an almost certain death in the gastrointestinal tract.Bacterial adhesion is generally recognized as the first step in biofilm formation, and for the human host, the ability of a bacterium to adhere is a definite virulence factor, especially in immunocompromised patients and in the growing number of elderly patients relying on biomaterials implants and devices for the restoration of function after (oncological) intervention surgery, trauma, or wear [4].Well-known examples of biomaterials implants are dental implants, vascular grafts, and prosthetic hips and knee joints.Bacterial adhesion is a virulence factor, because it stimulates the organism to produce extracellular polymeric substances (EPSs), such as polysaccharides, proteins, nucleic acids, and lipids [5], through which they embed themselves in a protective matrix.This protective matrix provides mechanical stability to a biofilm and constitutes the main difference between planktonic bacteria and bacteria adhering to a surface in a socalled biofilm mode of growth.Bacteria organized in biofilms are at least ten to 1,000 times more resistant to antibiotics [6] than bacteria in a planktonic state and can cope much better with unfavorable external conditions in the host immune system than their planktonic counterparts.Not surprisingly, the fate of an infection associated with a biomaterials implant is mostly removal and replacement of the implant [7], at high costs to the health care system and great inconvenience to the patient. How Do Bacteria Respond to Different Adhesion Forces?Based on available literature, we propose three adhesion force regimes dictating the bacterial response to a substratum surface, as schematically summarized in Figure 1.Recently, a link has been