1.1 Introduction
The green revolution has enhanced agricultural productivity to a great extent with the increased use of high-yielding crop varieties, heavy farm equipment, synthetic fertilizers, pesticide applications, improved irrigation, better soil management, and massive conversion of forest to agricultural lands [1, 2]. But there is a growing concern that intensive agricultural practices promote large-scale ecosystem degradation and loss of productivity. Adverse environmental effects include deforestation, soil degradation, large-scale greenhouse gas emissions, accumulation of pesticides and chemical fertilizers, pollution of groundwater, and decreased water table due to excessive irrigation [1, 3].
The world population is currently around 7 billion and is projected to approximately 8 billion by the year 2025 and 9 billion by 2050. Considering this population growth and the environmental damage due to ever-increasing industrialization, it is clear that feeding the world's population will be a daunting task over the next 50 years. Therefore, there is a need for new strategies and approaches to improve agricultural productivity in a sustainable and environmentally friendly manner [4]. The effective use of beneficial microorganisms in agriculture in an integrated manner is an attractive technology to address these problems. The role of soil microorganisms in agriculture to improve the availability of plant nutrients and plant health is well known [5]. However, the ability of root-associated microbes to improve nutrient supply and plant protection has yet to be fully exploited [6].
The colonization of the adjacent volume of soil under the plant root is known as rhizosphere colonization. Rhizosphere colonization not only works as a fundamental step in the pathogenesis of soil microbes but also plays an important role in the employment of microorganisms for beneficial purposes [7]. Beneficial rhizobacteria normally promote plant growth by establishing themselves on plant roots and suppressing the colonization or eliminating the presence of pathogenic microorganisms [8]. The competitive exclusion of deleterious rhizosphere organisms is directly linked to the ability to successfully colonize a root surface. However, disease suppressive mechanisms were shown by plant growthâpromoting rhizobacteria (PGPR) to be of no use until these microbes successfully colonized and established themselves on root surfaces [9, 10].
Bacterial root colonization is primarily influenced by the presence of the specific character of bacteria necessary for adherence and subsequent colonization. Moreover, several biotic and abiotic factors also play significant roles in bacterial-plant root interactions and colonization. When an organism colonizes a root, factors like water content, temperature, pH, soil characteristics, composition of root exudates, mineral contents, and other microorganisms may influence the process of root colonization. However, plants are the major determinant of microbial diversity [11]. Recent studies on the root-microbe interaction have indicated that rhizobacteria can colonize the root zone and form biofilm and biofilm-like structures. This phenomenon is considered to be a survival strategy by the rhizobacteria, which provides protection to the plant under stress conditions [12].
Traditionally, microbes have been characterized as freely suspended (planktonic) cells; although, many pioneering microbiologists recognized the surface-associated growth of microorganisms on tooth surfaces, aquatic environments, and other biotic and abiotic surfaces. However, a detailed examination of biofilms only became possible after observation under the electron microscope [13, 14]. Based on the observation of dental plaque and other sessile communities, in 1987 Costerton et al. put forth a theory on biofilms that explained the mechanisms of microbial adherence to living and nonliving material, and the benefits associated with this lifestyle. Since then, studies on biofilms in environmental, industrial, and ecological settings relevant to public health have increased significantly [15]. Much of the work on biofilms in the last few decades has demonstrated tremendous growth and understanding through the utilization of scanning electron microscopy, scanning confocal laser microscopy, and both standard microbiology cultural techniques and molecular-based investigation. The ultrastructures of biofilm, roles of various adhesins, genes, and regulatory pathways have all been explored in model organisms [16]. Our understanding of biofilms in natural settings has also substantially improved as new methods allow us to better distinguish different microbial species within complex communities [17â19].
According to Costerton, âthe father of biofilm,â a biofilm is defined as âa structural community of bacterial cells enclosed in a self-produced polymeric matrix and adherent to an inert or living surfaceâ [20]. However, this definition was later modified to include other characteristics of biofilm such as irreversible cell attachment, altered phenotype with respect to growth rate, and characteristic changes in gene transcription [21]. The composition of the self-produced polymeric material is mainly exopolysaccharide, protein, lipid, and DNA [19]. (Chapter 9 provides details of EPS composition.)
Biofilm formation is a complex process involving various steps such as initial adsorption or reversible attachment, irreversible attachment and the formation of a microbial monolayer on the substrate, early development of microcolonies, maturation of the biofilm structure, including the formation of characteristic architectural features, and lastly, the dispersion (or shedding) of planktonic cells from the biofilm [22]. Each of these stages is very distinct in their morphology and regulation [23]. The sessile growth of microorganisms has distinct phenotypes compared to planktonic cells and exhibits enhanced resistance to antimicrobial compounds and alterations in nutrient uptake [24].
Biofilms provide an important and fundamental strategy for adaptation and survival in the environment, as well as in the pathogenesis of various bacterial pathogens associated with humans, animals and plants [25]. Other applications of biofilms, which have been subsequently studied and are under active investigation, relate to the environmental sciences and food industry. However, in this chapter we will only address the roles of biofilm in plant and soil health, as well as briefly touch on their public health perspective.
