Influence of flow on the structure of bacterial biofilms

Bacteria attached to surfaces in biofilms are responsible for the contamination of industrial processes and many types of microbial infections and disease. Once established, biofilms are notoriously difficult to eradicate. A more complete understanding of how biofilms form and behave is crucial if we are to predict, and ultimately control, biofilm processes. A major breakthrough in biofilm research came in the early 1990’s when confocal scanning laser microscopy (CSLM) showed that biofilms formed complex structures which could facilitate nutrient exchange. We have recently found that biofilms growing in turbulent flow can also be temporally complex. Structures such as cell clusters and ripples can migrate downstream along solid surfaces. Further, biofilm viscoelasticity allows the biofilm to structurally deform when exposed to varying shear stresses. Biofilm accumulation Biofilms have been defined in numerous ways by various researchers. These definitions are usually constructed to be inclusive of the many environments in which biofilms are found and the disciplines that the subject covers. Characklis and Marshall defined a biofilm as consisting of “cells immobilized at a substratum and frequently embedded in an organic polymer matrix of microbial origin” [3]. Costerton et al. supplied a broader definition of biofilms as “matrixenclosed bacterial populations adherent to each other and/or to surfaces or interfaces” [4]. But what is it that physically distinguishes biofilm cells from their planktonic counterparts? Some salient features include the association with a surface, a high population density, and the presence of an extracellular polysaccharide (EPS) glycocalyx. However, it may be argued that it is structural organization which best distinguishes biofilm cultures from conventional well mixed suspended cultures [13]. Characklis [2] identified up to 8 different processes describing the progression of biofilm accumulation. These can be condensed into three main steps. First, the attachment of cells to a surface; second, cell growth and EPS production; and third, detachment of single cells (erosion) or large pieces of biofilm (sloughing) from the surface. Complexities in biofilm structure revealed by confocal microscopy Over the last decade the development of improved imaging techniques, such as confocal scanning laser microscopy (CSLM), has allowed biofilms to be studied in the fully hydrated state. The combination of CSLM and fluorescent molecular staining has revealed detailed information on the three dimensional structure of biofilms and the spatial arrangement of microbial species within the biofilm [10, 11]. The use of flow cells and time lapse image