In nature, microorganisms are generally found attached to surfaces as biofilms such as dust, insects, plants, animals and rocks, rather than suspended in solution. Once a biofilm is developed, other microorganisms are free to attach and benefit from this microbial community. The food industry, which has a rich supply of nutrients, solid surfaces, and raw materials constantly entering and moving through the facility, is an ideal environment for biofilm development, which can potentially protect food pathogens from sanitizers and result in the spread of foodborne illness.
Biofilms in the Food Environment is designed to provide researchers in academia, federal research labs, and industry with an understanding of the impact, control, and hurdles of biofilms in the food environment. Key to biofilm control is an understanding of its development. The goal of this 2nd edition is to expand and complement the topics presented in the original book. Readers will find:
* The first comprehensive review of biofilm development by Campylobacter jejuni
* An up-date on the resistance of Listeria monocytogenes to sanitizing agents, which continues to be a major concern to the food industry
* An account of biofilms associated with various food groups such as dairy, meat, vegetables and fruit is of global concern
* A description of two novel methods to control biofilms in the food environment: bio-nanoparticle technology and bacteriophage
Biofilms are not always a problem: sometimes they even desirable. In the human gut they are essential to our survival and provide access to some key nutrients from the food we consume. The authors provide up-date information on the use of biofilms for the production of value-added products via microbial fermentations.
Biofilms cannot be ignored when addressing a foodborne outbreak. All the authors for each chapter are experts in their field of research. The Editors hope is that this second edition will provide the bases and understanding for much needed future research in the critical area of Biofilm in Food Environment .
Anthony L. Pometto III, Ph.D., Professor, Department of Food, Nutrition, and Packaging Sciences, Clemson University, Clemson, South Carolina, USA.
Ali Demirci, Ph.D., Professor, Department of Agriculture and Biological Engineering, The Pennsylvania State University, University Park, Pennsylvania, USA.
Current Knowledge and Perspectives on Biofilm Formation and Remediation
Lynne A. McLandsborough
Department of Food Science, University of Massachusetts, Amherst, MA, USA
Biofilms are formed by almost every type of microorganism under suitable conditions. Biofilm food associated organisms include food spoilage microorganisms, such as Pseudomonas sp. and thermophilic sporeformers, and pathogens, including the genera of Bacillus, Cronobacter, Campylobacter, Vibrio, Listeria, Escherichia, and Salmonella (Burgess et al. 2010; Hartmann et al. 2010; Marriott 1999; Poulsen 1999; Sommer et al. 1999). Simplistically, biofilms are microorganisms growing on a solid surface. However, biofilms are generally defined as matrix-enclosed bacterial populations that adhere to a surface and/or to each other producing a dynamic environment in which the component microbial cells appear to reach homeostasis, optimally organized to make use of all available nutrients (An and Friedman 1997; Doyle 2001; O'Toole et al. 2002; Poulsen 1999; Sutherland 2001).
Throughout natural ecosystems, biofilms can be found on almost any surface with a high enough level of moisture to support growth (Kim and Frank 1995). Interfaces where biofilms may grow in food processing environments include solid/liquid, gas/liquid, or in the case of solid foods at the gas/ solid interfaces (Jenkinson and Lappin-Scott 2001; Poulsen 1999). Over the past 15 or more years, researchers have realized that bacteria growing on surfaces, either alone or in a community containing a diversity of different organisms, have a greater resistance to a large variety of environmental stresses (Costerton 1995; Jessen and Lammert 2003). Thus, the biofilm physiology and organization enables organisms to survive within the food processing environment. In order to control environmental bacterial contamination, cleaning and sanitation of this environment is indispensable in order to ensure safety of all commercially produced foods.
1.1.1 General properties of biofilms
When growing as a biofilm, bacteria are known to have a different growth rate and physiology than their planktonic (free growing broth cultures) counterparts, and may exhibit varied physiological responses to nutrient conditions (Hodgson et al. 1995; Kim and Frank 1995; Kuchma and O'Toole 2000; O'Toole and Kolter 1998; Sauer et al. 2002). Although gases and liquid nutrients are transported to, from, and through the biofilm matrix via diffusion, studies have indicated that biofilm-forming bacteria can grow with less oxygen and fewer nutrients than cells in suspension. Surprisingly, this leads to advantages in growth, altered physiology, and increased resistance to a variety of stress compared to their planktonic forms (Fox et al. 2011; Frank and Chmielewski 1997; Frank and Koffi 1990; Sutherland 2001; Vatanyoopaisarn et al. 2000). Through diffusional mass transport, biophysical interactions, and cell-to-cell interactions, commensal and mutual communities of organisms survive in the low nutrient and decreased temperature conditions that are often found in food processing and storage environments. The ability to resist antimicrobial agents is of particular concern to both the medical and food processing communities, since once a biofilm has been established on a surface, it becomes exceedingly difficult to clean and sanitize (Bolton et al. 1988; Bower and Daeschel 1999; Bridier et al. 2011; Carpentier and Cerf 2011; Donlan 2002; Donlan and Costerton 2002; Frank and Koffi 1990; LeChevallier et al. 1988; Lowry 2010; Simoes et al. 2010; Tompkin 2002).
1.1.2 Biofilm formation and propagation
There are several steps in the formation of bacterial biofilms: (1) transport (2) initial adhesion, (3) substrate attachment, and (4) microcolony formation (cell-cell adhesion) leading to mature biofilms consisting of cells and a surrounding extracellular polymer matrix with the last step being the dissemination or disruption of the biofilm (Figure 1.1) (O'Toole et al. 2002; Purevdorj-Gage et al. 2005; Simoes et al. 2010; van Loosdrecht et al. 1997). The first step in biofilm formation consists of the transport of the organism to a solid surface. This can occur via motility of the organism, diffusion of the organism through the environment, or natural and forced convection in the system. Biofilm-forming bacteria may use all of these mechanisms at one time or another. It is well documented that cells with flagella often have lower biofilm forming capability under static conditions indicating that, under these conditions, flagella are involved in active cellular transport to surfaces and this has also been observed in listerial biofilm formation (Lemon et al. 2007; Vatanyoopaisarn et al. 2000). The role of flow conditions on the attachment and growth of cells was investigated by various authors (Millsap et al. 1996; Pereira et al. 2002; Sasahara and Zottola 1993). Contrary to expectations, greater deposition of bacteria under both laminar and turbulent flow conditions has been observed when compared to static conditions (Rijnaarts et al. 1993). It has been speculated that turbulent flow may thrust bacterial cells onto the surface, thus enhancing probability of adhesion and biofilm formation (Donlan 2002); however, the number of planktonic cells coming off the biofilm is not thought to be dependent upon flow and shear stress (Bester et al. 2013).
Figure 1.1 Stages in biofilm formation.
Once bacteria approach a surface, physical interaction forces are thought to influence the initial adhesion of the organisms. Typical interactions that can take place include van der Waals interactions (>50 nm from the surface), repulsive or attractive electrostatic interactions (2-10 nm from the surface), and hydrophobic interactions (0.5-2 nm from the surface) (Fletcher 1996). van der Waals forces are due to dipole-dipole, induced dipole-dipole, and induced dipole-induced dipole interactions and are always attractive (Israelachvili 1992). Electrostatic interactions arise, because the cells and the surface may carry a positive or negative charge leading to the formation of a diffuse electrostatic layer. Bacteria, as well as most natural solid surfaces, generally have an overall gross negative charge, but the origin of the overall charge is due to the combination of various charges from functional groups on the membrane constituent molecules, such as amino, carboxyl, phosphate, and less commonly, sulfate groups and capsular macromolecules (James 1991). Ultimately, the magnitude of the electrostatic interactions is influenced by the nature of the environment (e.g., pH, ionic strength, valency of present counter-ions, and nature of the solvent) (Israelachvili 1992). Hydrophobic interactions in water are much stronger than van der Waals attraction at small separation distances (Israelachvili 1992) and it has been suggested that hydrophobic interactions between the cell surface and the solid substrate may be responsible for overcoming the repulsive electrostatic interactions. This strict physicochemical approach, however, should not be over interpreted. The bacterial surface is an extremely complex entity and contains a multitude of molecules that not only carry a variety of charges, but are also more or less hydrophobic. In addition, the nature and composition of bacterial surfaces can vary greatly between different species. The fact that a single bacterial strain can adhere to a variety of surfaces with differing surface energies indicates that this simplified physicochemical interaction model is most likely not entirely correct. Strategies that attempted to prevent bacterial attachment by engineering the surface to be more or less hydrophobic have not led to the desired results. A large variety of bacterial cells have no difficulty attaching to both hydrophobic and hydrophilic surfaces (Fletcher 1996).
After the initial adhesion occurs, bacteria begin to anchor themselves to the surface by synthesizing extracellular polymeric substances (EPS) that facilitate irreversible bacterial attachment to a surface and help maintain the microcolony and biofilm structure (Doyle 2001; O'Toole et al. 2002; Sutherland 2001; Wimpenny et al. 2000; Wirtanen and Mattila-Sandholm 1993). Azaredo and Olivera (2000) found that the exopolymers produced by Sphingomonas paucimobilis possess surface-active properties that aided bacteria in their attachment to hydrophilic surfaces (Azaredo and Oliveira 2000). Interestingly, the presence of pre-adsorbed proteins on a surface prior to inoculation generally reduced the adhesion of Listeria monocytogenes regardless of the surface composition or free energy (Al-Makhlafi et al. 1994; Al-Makhlafi et al. 1995; Barnes et al. 1999; Cunliffe et al. 1999), although others have reported that the type of food soil or preconditioning film can influence the final cell density of this organism within the biofilm (Verghese et al. 2011); therefore, there may be a difference upon initial adhesion and ultimate density of biofilm formation. EPS have been shown to enhance nutrient capture and resistance to environmental stress and anti-microbial agents (Costerton 1995; Costerton et al. 1987; Jenkinson and...