INTRODUCTION

In most textbooks, biofilm formation on surfaces has been depicted as a sequence of events. Depending on the particular interest of the author, these events are split up into four or more steps (Escher & Characklis, 1990; Van Loosdrecht et al., 1990). The sequence is presented in Fig. 1. Traditionally, biofilm formation is said to begin with mass transport of micro-organisms towards a substratum surface (Fig. 1b), but in almost all environments, the mass transport step is preceded by the adsorption of conditioning film components (Fig. 1a), such as: an adsorbed tear film on a contact lens (Baguet et al., 1995; Landa et al., 1998); the salivary pellicle on surfaces in the oral cavity (Busscher et al., 1989; Bradshaw et al., 1997); a film of adsorbed urinary components on urogenital surfaces (Reid et al., 1998); and adsorbed macromolecules on marine surfaces (Schneider & Marshall, 1994). Many more examples of the formation of a film conditioning a surface as the first step in biofilm formation can be given. Once transported to a substratum surface, organisms may or may not adhere, depending on the interaction forces (Rutter & Vincent, 1980). This initial adhesion (Fig. 1c) is generally reversible (Norde & Lyklema, 1989), but even in the absence of exopolymer production, becomes less reversible within minutes due to the progressive removal of water from in-between the interacting surfaces (Meinders et al., 1995). The unfolding of binding molecules and other non-metabolic mechanisms eventually lead to the strong anchoring of the initially adhering organisms (Fig. 1e). At this stage, there is often a neglected step which occurs almost simultaneously, that is, coadhesion (Fig. 1d).