Physiology of Microbes in Biofilms

A. M. Spormann

1 Introduction 17

2 Physical-Chemical Parameters Control the Physiology of Biofilm Cells 18

3 Global Regulators Determine the Physiology of Subpopulations of Biofilm Cells 24

4 Biofilm Stability and Cellular Detachment 27

  1. 1 Detachment of S. oneidensis Cells from Biofilms 28
  2. 2 Detachment of Pseudomonas Biofilms 29
  3. 3 Detachment of Vibrio cholerae Biofilms 30
  4. 4 c-di-GMP in Cellular Attachment and Detachment 31
  5. 5 Detachment Induced by Cell Lysis or Death 32

5 Conclusion 32

References 33

Abstract Microbial biofilms are governed by an intricate interplay between physical-chemical factors and the physiological and genetic properties of the inhabiting microbes. Many of the physiological traits that are exhibited in a biofilm environment have been observed and studied previously in detail in planktonic cultures. However, their differential and combinatorial phenotypic expression in distinct subpopulations localized to different regions in a biofilm is the cause for the overall biofilm heterogeneity. In this chapter, the causes and consequences of this interplay are elaborated with a special focus on processes controlling biofilm stability and dispersal.

1 Introduction

Research on microbial biofilms has been motivated mainly by a need to understand the mechanisms leading to the physical persistence of microbes on surfaces and the resistance of microbes to antimicrobial agents in biofilm environments (for reviews,

A. M. Spormann

Departments of Chemical Engineering, of Civil and Environmental Engineering, and of Biological Sciences, Clark Center E250, Stanford University, Stanford, CA 94305-5429, USA [email protected] stanford.edu

T. Romeo (ed.), Bacterial Biofilms.

Current Topics in Microbiology and Immunology 322.

© Springer-Verlag Berlin Heidelberg 2008

see Tolker-Nielsen and Molin 2000; Costerton 1999; Costerton et al. 1999; Watnick and Kolter 2000; O'Toole et al. 2000). The persistence of microbes in biofilms, on one hand, provides a reservoir for these microbes, and, on the other hand, is the cause for the build-up of biomass, which by itself is of great medical and industrial concern (clogging of catheters and pipes, creating drag in ships, etc.). In the past, research on biofilm-forming microbes has been focused largely on identifying the molecular processes that define the initial phases of biofilm formation, such as adhesion via pili, flagella, exopolysaccharide (EPS) production, and perhaps a role for quorum sensing (see the chapter by Y. Irie and M.R. Parsek, this volume). However, the physiological and genetic responses of biofilm microbes to external and self-induced stresses, including the competition for resources, determine the fate of a biofilm and its diverse subpopulations to a large extent. Thus, one of the most consequential challenges for microbes in a biofilm is how to deal with these conditions. One strategy is to simply reduce the growth rate or exhibit a behavior similar to that found in sporulating microbes. However, another one may be to leave and exit a biofilm. This review provides a physiological view of micro-bial life in a biofilm and attempts to reveal unifying principles in the physiology of cell populations in a biofilm environment. First, the current status on the physiological states of microbes in a biofilm environment and then the physiological and molecular mechanism(s) involved in cellular detachment will be reviewed.

2 Physical-Chemical Parameters Control the Physiology of Biofilm Cells

In most environmentally, medically, and industrially relevant systems, biofilms form at the interface of an aqueous phase and a substratum surface or a gaseous (air) phase. Common to all biofilm systems is that metabolic substrates leading to growth need to be available to the cells. These compounds can be present exclusively in the aqueous phase, such as in drinking water pipes or medical catheters, or be partitioned between the aqueous and a solid phase, such as in minerals or insoluble organic matter (e.g., cellulose, chitin, or protein). In addition, the absolute amount as well as the ratio of the different nutrient components in the bulk liquid are important, as these parameters determine which nutrient, for example electron donor, electron acceptor, phosphorous, etc., may become growth limiting (for review, see van Loosdrecht et al. 2002). Many environmental and laboratory biofilm systems are often limited by molecular oxygen (Huang et al. 1998; Xu et al. 1998; Barraud et al. 2006). Which compound may become growth-limiting matters, as this identifies the kind of stress biofilm cells might be experiencing. In any case, there is a net transport of at least some essential nutrient(s) from the aqueous phase to the immobilized cells in a biofilm.

Net transport of substrates into, as well as of metabolites out of, a biofilm is determined by the flow rate of the bulk liquid and by molecular diffusion (Picioreanu et al. 2001; van Loosdrecht et al. 2002) (Fig. 1). Depending on the laboratory or

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