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All grown up: growth and maturation of the biofilm community

The stability and protection afforded to the biofilm community by the ECM is the cornerstone to continued growth and development. After the initial microcolony formation, there are two main routes by which the community continues to expand and grow in size. The first is by internal expansion of cells within the biofilm through bacterial replication. The biofilm can also expand by the attachment of new cells to the surface of the biofilm structure. The expansion in cell numbers leads to spatial expansion of the biofilm within the physical constraints of the surrounding environment. For example, if the biofilm is developing within a venous or a urinary catheter, bacteria can expand along the surface of the catheter (x/y-plane), as well as upward, toward the center of the catheter lumen (z-plane). As the biofilm community grows in size, the architecture of the community becomes more complex (Fig. 3.2(e); Stoodley et al., 2002), driven by cues in the immediate microenvironment, such as nutritional limitations, oxygen concentration, and bacterial population density. Gradients in these cues lead to phenotypic heterogeneity within the developing biofilm and the creation of a “division of labor” within the community.

Development of complex biofilm architecture coincides with the radial expansion of the growing population from the point of initial attachment. This serves as a mechanism to increase access to oxygen, nutrients, or other materials needed to maintain the health of the community that may be lacking or in lower concentrations beneath the surface level (Stoodley et al., 2002). Formation of water or nutrient channels in the expanding population can funnel the nutritional requirements throughout a community with extensive infrastructure (Stoodley et al., 2002; Hall-Stoodley and Stoodley, 2002; de Beer et al., 1994; Stewart, 2003). These nutrients are most likely to be found at the periphery of the biofilm that is closest to available nutrient sources.

Biofilm maturation in relation to contact with the human host can have numerous effects. In some instances, biofilm interactions with the human host can have positive outcomes. An example is the microbiome of the human gastrointestinal tract, which is vital for human health and immune system development and resides largely within a biofilm state (Macfarlane et al., 2011; Macfarlane and Dillon, 2007; Eckburg et al., 2005; Probert and Gibson, 2002). However, pathogenic biofilm formation on medical devices, ranging from urinary catheters (Tambyah and Oon, 2012; Tambyah and Maki, 2000; Tambyah, 2004), venous catheters (Bookstaver et al., 2009; Shanks et al., 2006; Bleyer et al., 2005; Raad et al., 2008), stents (Wol- lin et al., 1998; Reid et al., 1992; Speer et al., 1988; Keane et al., 1994), endotracheal tubes (Adair et al., 1999; Bauer et al., 2002; Sottile et al., 1986), dental implants (Lang et al., 2000; Shibli et al., 2008), and bone repair devices (i.e., artificial joints) (Zimmerli, 2014; Arciola et al., 2012; Clauss et al., 2010), has plagued the hospital setting for many decades. The term “biofouling” refers to the potentially hazardous formation of bacterial biofilms on a surface. Biofouling medical devices can lead to chronic infection states. Biofilm formation by pathogenic bacterial species on the aforementioned medical devices poses a great risk to the health of an individual (Wilson, 2001; Costerton et al., 1999, 2007; Fux et al., 2003; Parsek and Singh, 2003; Kaplan, 2010; Donlan and Costerton, 2002) and is discussed in depth in the following chapters.

 
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