Enterococcal Biofilm Structure and Role in Colonization and Disease

This chapter discusses biofilm formation in enterococci and its role in the biology of these organisms, especially in relation to opportunistic infections. As is the case for many microbes, the pace of enterococcal biofilm research has quickened in the past few years, and numerous genes and gene products affecting biofilm formation have been identified. At the same time, this research has not resulted in a comprehensive understanding of the critical steps in the process, particularly those steps that are involved in initiating the transition from planktonic growth to biofilm growth. Both the physical/chemical environment and the type of surface substratum on which the biofilm develops have a tremendous impact on the process, even with a single strain. A cursory scan of 15-20 years of publications on enterococcal biofilms indicates that the number of different experimental conditions employed to measure biofilm formation is comparable to the number of published papers on the topic. Clearly, these efforts have contributed to the discovery of different biofilm determinants and processes by different research groups, and in some cases, have provided contradictory reports and conclusions. Therefore, we begin this chapter with a brief summary of central principles, definitions, and questions to provide a framework for subsequent sections that contain more detailed descriptions of relevant research in this area. We conclude with a discussion of future research directions that may clarify and increase the level of understanding of this important microbial activity. Biofilms are microbial communities resulting from the adherence of planktonic organisms to an abiotic surface, usually followed by growth. It is generally recognized that microbial species spend much of their time in the biofilm state, and that biofilm formation plays a critical role in infections. Thus, research on enterococcal biofilms is driven by its potential to yield new insights into the pathogenesis of opportunistic infections, their treatment, and their prevention. It is commonly hypothesized that the formation of biofilms represents a developmental process that involves a shift in physiology from planktonic growth, organization of the adherent bacteria into structured communities physically linked by an extracellular matrix, and where communication between members of the community coordinates gene expression and metabolic activity (O'Toole, Kaplan, & Kolter, 2000). While this model has been a useful paradigm to guide experimental investigations, it has not been completely validated, especially for non-sporulating bacteria, and strong arguments have been made for considering alternatives (Monds & O'Toole, 2009). The developmental model implies that there are critical functions specific for biofilm formation, and it can also be inferred that there may be conservation of these functions among different bacteria. Examination of the published literature suggests that many, if not most, determinants of enterococcal biofilm formation identified to date are members of global networks important for adaptation to a variety of environments, and that factors such as the medium composition, physical/chemical conditions, and type of surface used for cultivation of biofilms may have a larger influence on the “biofilm functions” than biofilm growth itself. Investigators have also used different criteria to quantify biofilm formation. Conclusions about optimal biofilm formation can vary greatly depending on whether total biomass, enumeration of cell counts by microscopy or plate counting, or the ratio of biofilm biomass to bacterial growth in the planktonic phase is used as the primary criterion (Di Rosa, et al., 2006; Creti, Koch, Fabretti, Baldassarri, & Huebner, 2006; Kristich, Li, Cvitkovich, & Dunny, 2004; Hufnagel, Koch, Creti, Baldassarri, & Huebner, 2004; Sandoe, Witherden, Cove, Heritage, & Wilcox, 2003; Baldassarri, et al., 2001). Given these discrepancies, researchers interested in linking biofilm formation to pathogenesis need to carefully consider the ways in which their laboratory assays closely reflect conditions in an animal or human host, to compare results using different in vitro assays, and to examine the role of biofilm determinants of interest in the context of a relevant animal model. Pathogenesis and models of enterococcal infection discusses the use of various animal models to investigate enterococcal virulence in more detail. While it is conceivable that biofilm defects could result in the attenuation of virulence in any animal model, the rabbit cardiac catheterization model for experimental endocarditis (Frank, Barnes, Grindle, Manias, Schlievert, & Dunny, 2012; Singh K. V., Nallapareddy, Sillanpää, & Murray, 2010; Chuang, Schlievert, Wells, Manias, TRipp, & Dunny, 2009; Nallapareddy, Singh, & Murray, 2008; Nallapareddy S. R., et al., 2006) and the mouse urinary tract infection models (Sillanpää, et al., 2010; Guiton, Hung, Hancock, Caparon, & Hultgren, 2010; Singh, Nallapareddy, & Murray, 2007; Shankar, Lockatell, Baghdayan, Drachenberg, Gilmore, & Johnson, 2001) may be especially informative in examining the role of biofilm formation in the host on pathogenesis.