**4. Biofilms**

A consortium of microorganisms composed of cells adhering to a surface is called a biofilm [51, 52]. The physical setting for cells to adhere to a surface occurs through the intermediacy of extracellular polymeric substances (EPS) which forms a slimy extracellular matrix **Figure 1** [53]. Microbial cells in the biofilm produce the EPS which are composites of extracellular polysaccharides, proteins, lipids, and DNA [54, 55]. The cellular agglomeration of biofilms forms a three-dimensional structure as a community that offers significant protection against the forces levied by the environment [56].

### **4.1 Structure**

Microbial cells composing a biofilm are distinct from the planktonic cells of the same organism, which are single-cell organisms that are free to float or swim in an aquatic medium [57]. Biofilm structures are formed in response to a variety of different factors enabling biofilm development [58, 59]. Surface recognition is important to specific or nonspecific attachment sites, toxic materials, or antibiotics, and nutritional stress may complicate biofilm growth **Figure 2** [60]. A cell that switches to the biofilm mode of growth undergoes a shift of observable behavior of the bacteria resulting from the interaction of its genotype with the environment that is required of a microbial cell in the transition from planktonic to sessile growth in the regulation genes of the biofilm. A biofilm can mimic a hydrogel, a three-dimensional (3D) network of hydrophilic polymers complex containing a

### **Figure 1.**

*Site of biofilm interactions.*

large quantity of water, which retains its structure through chemical or physical cross-linking polymer chains [61]. Biofilm formation can lead to the formation of a coordinated functional microbial community. The bacteria composing a biofilm can share nutrients due to their proximity in the biofilm and protection from harmful factors of the environment. Biofilms usually begin to form when a free-swimming bacterium attaches to a surface [62].

Colonization of a surface requires a significant transition from the free-living planktonic existence in the bulk aquatic phase to a surface-attached state. A biofilm life cycle is portrayed in **Figure 2** [63]. This process is initiated by the reversible adhesion of a few single cells to a surface leading to a reversible attachment where weakly attached cells are sloughed to the bulk medium, or irreversible attachment where interactions of the cells and a surface are reinforced [64]. Irreversibly attached cells at a surface continue to agglomerate to form microcolonies through cellular division and can proceed to form a mature biofilm when the conditions support growth [65]. As the biofilm matures, factors that will prevent sustainable growth can be triggered by limited nutrients supply or lowered oxygen concentrations may reverse biofilm formation through the dispersal of cells from the biofilm to the bulk aquatic phase. Released cells may attach to a new surface [66]. For single-cell adhesion, three factors leading to single-cell adhesion require attention: the chemical and physical composition of the aquatic environment, the solid surface, and the transitioning microbiota [67].

### **4.2 Characteristics**

Microorganisms form from attached phase growth structures (biofilms) or multicellular microbial communities by transitioning from planktonic (freelyswimming) biota to components of a complex, surface-attached community (**Figure 1**). These communities of adhering microorganisms in the form of biofilms provide protection to the microbes participating in its development. The process begins with planktonic microorganism encountering a surface where some adsorb followed by surface release to final attachment by the secretion of exopolysaccharides which act as an adhesive for the growing biofilm (**Figure 2**) [68]. Switching from a planktonic existence to an attached-life state (sessile) requires a complex

**309**

rotating polypropylene disk.

**Figure 2.** *Biofilm life cycle.*

**4.3 Plastic colonization and plastisphere communities**

*The Importance of Biofilms to the Fate and Effects of Microplastics*

process composed of several factors derived from biological, chemical, and physical properties of the environment, the surface, and the bacterial cell (**Figure 2**) [69]. Initial weak, reversible interactions between a bacterium and a surface lead to irreversible adhesion. New phenotypic characteristics are exhibited by the bacteria of a biofilm in response to environmental signals. Initial cell-polymer surface interactions, biofilm maturation, and the return to the planktonic mode of growth have regulatory circuits and genetic elements controlling these diverse functions. Studies have been conducted to explore the genetic basis of biofilm development with the development of new insights. Compositionally, these films have been found to be a single microbial species or multiple microbial species with attachment to a range of biotic and abiotic surfaces [70]. Mixed-species biofilms are generally encountered in most environments. With proper nutrient and carbon substrate provided, biofilms can grow to massive sizes. A biofilm can achieve large film structures that may be sensitive to physical forces such as agitation. Such energy regimes can lead to biofilm detachment. An example of biofilm attachment and utility can be found in the wastewater treatment sector where large polypropylene disks are rotated through industrial or agriculture wastewater and then exposed to the atmosphere to treat pollutants through the intermediacy of cultured biofilms attached to the

Plastic's role in freshwater and marine systems is poorly understood from many perspectives especially microbiology. Microscopic scrutiny and next-generation sequencing of PMD from locations in the North Atlantic were used to characterize attached microbial communities. A microbial community having a high degree of diversity was identified as the "Plastisphere" from the pitting of the debris surface which suggested bacterial shapes engaged in the utilization of the polymer by enzymatic means [71]. Opportunistic pathogens were observed as specific members of the genus *Vibrio* [72, 73]. Attached plastisphere communities were found to be distinct from surrounding surface water, suggesting that PMD could be a novel ecological habitat in the open ocean. Most natural floating marine substrates have shorter half-lives than PMD which is enhanced by a hydrophobic surface that assists

*DOI: http://dx.doi.org/10.5772/intechopen.92816*

*The Importance of Biofilms to the Fate and Effects of Microplastics DOI: http://dx.doi.org/10.5772/intechopen.92816*

*Bacterial Biofilms*

**Figure 1.**

*Site of biofilm interactions.*

large quantity of water, which retains its structure through chemical or physical cross-linking polymer chains [61]. Biofilm formation can lead to the formation of a coordinated functional microbial community. The bacteria composing a biofilm can share nutrients due to their proximity in the biofilm and protection from harmful factors of the environment. Biofilms usually begin to form when a free-swimming

Colonization of a surface requires a significant transition from the free-living planktonic existence in the bulk aquatic phase to a surface-attached state. A biofilm life cycle is portrayed in **Figure 2** [63]. This process is initiated by the reversible adhesion of a few single cells to a surface leading to a reversible attachment where weakly attached cells are sloughed to the bulk medium, or irreversible attachment where interactions of the cells and a surface are reinforced [64]. Irreversibly attached cells at a surface continue to agglomerate to form microcolonies through cellular division and can proceed to form a mature biofilm when the conditions support growth [65]. As the biofilm matures, factors that will prevent sustainable growth can be triggered by limited nutrients supply or lowered oxygen concentrations may reverse biofilm formation through the dispersal of cells from the biofilm to the bulk aquatic phase. Released cells may attach to a new surface [66]. For single-cell adhesion, three factors leading to single-cell adhesion require attention: the chemical and physical composition of the aquatic environment, the solid

Microorganisms form from attached phase growth structures (biofilms) or multicellular microbial communities by transitioning from planktonic (freelyswimming) biota to components of a complex, surface-attached community (**Figure 1**). These communities of adhering microorganisms in the form of biofilms provide protection to the microbes participating in its development. The process begins with planktonic microorganism encountering a surface where some adsorb followed by surface release to final attachment by the secretion of exopolysaccharides which act as an adhesive for the growing biofilm (**Figure 2**) [68]. Switching from a planktonic existence to an attached-life state (sessile) requires a complex

bacterium attaches to a surface [62].

surface, and the transitioning microbiota [67].

**4.2 Characteristics**

**308**

process composed of several factors derived from biological, chemical, and physical properties of the environment, the surface, and the bacterial cell (**Figure 2**) [69]. Initial weak, reversible interactions between a bacterium and a surface lead to irreversible adhesion. New phenotypic characteristics are exhibited by the bacteria of a biofilm in response to environmental signals. Initial cell-polymer surface interactions, biofilm maturation, and the return to the planktonic mode of growth have regulatory circuits and genetic elements controlling these diverse functions. Studies have been conducted to explore the genetic basis of biofilm development with the development of new insights. Compositionally, these films have been found to be a single microbial species or multiple microbial species with attachment to a range of biotic and abiotic surfaces [70]. Mixed-species biofilms are generally encountered in most environments. With proper nutrient and carbon substrate provided, biofilms can grow to massive sizes. A biofilm can achieve large film structures that may be sensitive to physical forces such as agitation. Such energy regimes can lead to biofilm detachment. An example of biofilm attachment and utility can be found in the wastewater treatment sector where large polypropylene disks are rotated through industrial or agriculture wastewater and then exposed to the atmosphere to treat pollutants through the intermediacy of cultured biofilms attached to the rotating polypropylene disk.

### **4.3 Plastic colonization and plastisphere communities**

Plastic's role in freshwater and marine systems is poorly understood from many perspectives especially microbiology. Microscopic scrutiny and next-generation sequencing of PMD from locations in the North Atlantic were used to characterize attached microbial communities. A microbial community having a high degree of diversity was identified as the "Plastisphere" from the pitting of the debris surface which suggested bacterial shapes engaged in the utilization of the polymer by enzymatic means [71]. Opportunistic pathogens were observed as specific members of the genus *Vibrio* [72, 73]. Attached plastisphere communities were found to be distinct from surrounding surface water, suggesting that PMD could be a novel ecological habitat in the open ocean. Most natural floating marine substrates have shorter half-lives than PMD which is enhanced by a hydrophobic surface that assists

microbial colonization and biofilm formation. The adhesion of individual bacteria to a surface-initiated biofilm formation is supported by a collection of factors arising from initial adhesion to the growth of a mature biofilm [74].

Bacteria communicate with one another using chemical signal molecules [75]. This process, termed quorum sensing, allows bacteria to monitor the environment to adjust community behavior at a population-wide scale in response to community changes in the number and species present [76]. The information conveyed by these molecules works to synchronize activities for a wide group of cells. This cell-to-cell communication is used by bacteria to coordinate population density-dependent changes in behavior. Quorum sensing involves the production of and response to diffusible or secreted signals, which can vary substantially across different types of bacteria and important to the first stage of encounter between a bacterium and a solid surface [77].

Initial bacterial adhesion to a surface, bacterial mass transport, the role of substratum surface properties in initial adhesion and the transition from reversible to irreversible adhesion have been analyzed through a physiochemical lens to yield great insight. Surface thermodynamics and Derjaguin Landau Verwey Overbeek analyses can describe bacterial support using smooth, inert colloidal particles to estimate bacterial cells. A depiction of initial bacterial adhesion to surfaceprogrammed biofilm growth was found to have four major stages: bacterial mass transport towards a surface, reversible bacterial adhesion, conversion to irreversible adhesion, cell wall deformation, and associated developing properties [78]. The production of EPS can be surface-programmed [79]. Initial bacterial adhesion to surfaces and biofilm growth at the solid surface is driven by aspects of physicochemistry [80].

Bacterial adhesion is important to the fate and transport of plastics in aquatic environments. There has been no systematic investigation of bacterial adhesion to different types of plastics. A limited evaluation of short-term and long-term adhesion for different types of bacteria and four types of plastics, polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), and polyvinyl chloride (PVC), was conducted [81]. The target physicochemical factors of surface charge, hydrophobicity/hydrophilicity, roughness, and plastic hardness were characterized. Surface hardness of the plastics was identified as a major factor dominating the adhesion of bacteria onto plastic surfaces in contrast to the other factors [82]. There were significant differences in bacterial cell adhesion for the types of plastics. The different plastic types influenced the bacterial adhesion due to intrinsic surface properties in both short- and long-term studies [83]. Generally, surface roughness, topography, surface free energy, surface charge, electrostatic interactions, and surface hydrophobicity are anticipated to be important to the process of biofilm attachment [84].
