, 1999) but may also be suspended in host material as seen in many chronic infections (Burmølle et al., 2010). Microbiologists have up until the last few decades focused and emphasized the planktonic state over the biofilm state. However, the importance of the biofilm mode of growth is becoming increasingly

recognized as improved methods to study sessile bacteria have become available, and hence the subsequent accumulation of evidence for its widespread presence. It has been suggested that bacteria are predominantly growing as sessile communities rather than as single cells (Costerton et al., 1987; Davey & O’Toole, 2000). Sessile growing bacteria are defined as an assemblage of cells embedded ‘in a self-produced polymeric matrix’. This matrix is Sunitinib mouse very important for the properties of the biofilm, because it offers structural stability and increased tolerance to antimicrobials and immune cells (Stoodley Sorafenib cost et al., 2002; Anderson & O’Toole, 2008; Mulcahy et al., 2008; Ma et al., 2009). To gain further information on this phenomenon, one has to investigate how a biofilm is established and propagated. The most

common method is the continuous-culture once-through flow system using the model organism Pseudomonas aeruginosa. In this system, media are slowly passed over the biofilm-growing bacteria, which have attached to a cover slip on a flow cell. This in vitro process of P. aeruginosa biofilm formation can be divided into at least five stages: in the first stage, planktonic cells reversibly attach to a vacant surface. Irreversible binding follows this attachment and then multiplication into microcolonies. The microcolonies produce an extracellular polymeric matrix, which in turn envelopes the colonies. After a couple of days, the microcolonies Interleukin-3 receptor attain tower- or mushroom-like structures measuring up to 50 μm in the flow cell (Costerton et al., 1995; Davey & O’Toole, 2000;

Stoodley et al., 2002). The extracellular matrix contains a mixture of polysaccharides, proteins, and DNA (Wingender et al., 2001; Whitchurch et al., 2002; Costerton et al., 2003). When the biofilm grows to a size not beneficial for bacterial survival and growth (e.g., owing to nutrient limitations), focal areas of the biofilm are sloughed off. It is hypothesized this enables the otherwise sessile biofilm bacteria to spread and colonize new surfaces and biofilms to spread. Hence, it seems that the biofilm lifecycle by P. aeruginosa is a dynamic process capable of renewing itself (Costerton et al., 1995; Davey & O’Toole, 2000; Stoodley et al., 2002). The biofilm lifecycle and the matrix components have preferably been investigated by means of confocal laser scanning microscopy (CLSM). This method has provided valuable insight into the biofilm development; however, the information on the detailed ultrastructure of the biofilm is difficult to image by light microscopes.