ABSTRACT
There has been considerable argument as to what exactly constitutes a biofilm and a number of definitions have emerged. A clear view as to what it is not is a homogenous collection of microbes in a liquid culture. Biofilm is an amorphous group of microbial communities that fall loosely under the umbrella of microbial aggregates. The latter can be divided into those with roughly spherical co-ordinates, for example sludge floc, anaerobic digester granules, mycelial balls and marine snow. On the other hand is the family of aggregates that appear on surfaces and/or at phase interfaces. These are biofilms. They are roughly two-dimensional, ranging from one or a few micrometres (a bacterial monolayer) to millimeters or more in thickness. The structure can vary from smooth to rough with obvious frond like projections. They may be dense, opaque structures like dental plaque or translucent gelatinous associations. Coverage may range from very patchy to continuous, even and unbroken. Biofilm forms most commonly at water/solid interfaces although it can appear at an interface between two immiscible liquids like oil and water, at air-water interfaces (neuston on the surface of water bodies) and at gas-solid surfaces (lichen and other microbial associations).
