According to Vidyasagar (2016), biofilms are a collective of one or more types of microorganisms that can grow on many different surfaces, and bacteria, fungi, and protists are the most common microorganism that forms this type of biofilm. Biofilm is also defined as a cluster of microorganisms that stick to non-biological surfaces, such as rocks in a stream, as well as to surfaces on plants or in animals. These clusters are often encased in an outer polymer layer that can be produced by the microorganism or by the defensive mechanisms of the colonized host. This biofilm can be found on minerals and metals underwater, underground, and above the ground. Biofilm also can grow in on plant tissues and animal tissues, and implanted medical devices such as catheters and pacemakers (Donlan, 2002).
A biofilm forms when certain microorganisms adhere to the surface of some object in a moist environment and begin to reproduce. The microorganisms form an attachment to the surface of the object by secreting a slimy, glue-like substance. In a study on biofilm, Stoodley (2002) showed that biofilm formation seems to be an ancient and fundamental part of the life cycles of many microorganisms and essential for survival in diverse environments. Biofilm formation represents a protected mode of growth that not only allows cells to survive in hostile environments but also to colonize new niches by dispersal of microorganisms from the microbial clusters. According to Rasmussen and Givskov (2006), biofilm can enhance the tolerance of bacteria to harsh environmental conditions. Bacteria can avoid being washed away by water flow or bloodstream by simply attaching to a surface or tissue. In research on biofilm Jefferson (2004) mentions that the formation of biofilm help bacteria in four ways, such as i. defense which means protection from harmful conditions in the host, ii. colonization or sequestration to a nutrient-rich area, iii. the community which indicates utilization of cooperative benefits and finally iv. biofilms as the default mode of growth mean that biofilms normally grow as biofilms and planktonic cultures are an in vitro artifact.
Biofilms are mainly made up of three parts: the organisms themselves, the slime they produce, and the water molecules trapped between the slime particles. The chemical compositions of the cellular components are mainly made up of polysaccharides (40%–95%), proteins (1%–60%), lipids (1%–40%), and nucleic acids (1%–10%), which have a significant effect on growth and metabolism of the biofilm. In a biofilm, microbial cells stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular polymeric substances (EPS). The structure of the extracellular polymeric substance (EPS) matrix of biofilms is composed of one or more extracellular polysaccharides, DNA, and proteins. Channels in the biofilm allow for water, air, and nutrients to get to all parts of the structure. We can divide the formation of biofilm into five-step. i. Attachment ii. Colonization iii. Development iv. Maturation v. Active dispersal.
The formation of biofilm is mainly regulated by some extracellular cues. Quorum sensing, C-di-GMP, and sometimes s-RNA regulate the biofilm formation. Subramani (2019) said that Quorum sensing (QS) is a process of intercellular signaling or cell-cell communication and a vital regulatory mechanism for coordinating biofilm formation including common activities and physiological processes such as symbiosis, formation of spores or fruiting bodies, antibiotics synthesis, genetic competence, apoptosis, and virulence in many bacterial species using extracellular QS signaling molecules, which is often referred to as autoinducers. He also mentions that microorganisms produce a wide variety of QS signaling molecules that can be self-recognized in a concentration-dependent manner and subsequently induce or suppress the expression of QS-controlled genes. Bacterial QS regulation is established through a wide range of signals such as oligopeptides, N-acyl homoserine lactones, furanose borate, hydroxy palmitic acid methyl ester, and methyl decanoic acid. Besides this, Cyclic di-GMP has been shown to regulate biofilm formation, motility, virulence, the cell cycle, differentiation, and other processes. Most c-di-GMP-dependent signaling pathways control the ability of bacteria to interact with abiotic surfaces or with other bacterial and eukaryotic cells (Romling et al, 2013). It is a second messenger that modulates a variety of bacterial growth phenotypes including biofilm formation. Bacterial small RNAs (sRNAs) are known regulators in many physiological processes. sRNAs usually respond to specific environmental stress conditions and regulate several genes participating in stress adaptation. many sRNAs modulate outer membrane or surface proteins in E. coli OR Salmonella enterica. it is speculated that sRNAs are involved in regulatory networks linking environmental cues and metabolic changes during the development of biofilm (Bak & Lee, 2015).
There was believed that biofilm has negative effects on the environment as it can harbor human infectious agents in the environment. After that scientists figure out that biofilm can promote remediation of contaminated groundwater and soils. Bioremediation is an environmentally friendly, cost-effective, sustainable technology that utilizes microbes to decontaminate and degrade a wide variety of pollutants into less harmful products. Relative to free-floating planktonic cells, microbes existing in biofilm mode are advantageous for bioremediation because of greater tolerance to pollutants, environmental stress, and the ability to degrade varied harsh pollutants via diverse catabolic pathways. In biofilm mode, microbes are immobilized in a self-synthesized matrix which offers protection from stress, contaminants, and predatory protozoa. Contaminants ranging from heavy metals, petroleum, explosives, pesticides have been remediated using microbial consortia of biofilms. In the industry, biofilm-based bioremediation is used to decontaminate polluted soil and groundwater (Mitra & Mukhopadhyay, 2016). Bacterial biofilm can efficiently remove metals and dyes by the bioaccumulation and biosorption mechanisms due to their high biomass density and can also reduce some metals and dyes to a lower toxicity level by their enzyme activities (Shukla, 2017). Biofilm possesses various physicochemical and biological processes to bind and transform toxic pollutants including metals and dyes with the help of EPSs and biosurfactants. Polyconic characteristics of the biofilm EPS can help to bind the metal ions resulting in the formation of organometal complexes by the action of the electrostatic force of attraction (Shukla, 2017).
According to Shukla (2014), metals or metalloids at molecular densities higher than 5 g/cm3 are known as heavy metals and when they exert a toxic effect on the biological environment, they are termed toxic heavy metals. Most of the heavy metals are cationic with an incomplete orbital and are known as d-block transition elements. Some of the heavy metals are essential for biochemical and physiological functions when present in trace amounts, called trace elements, and are considered vital cellular micronutrients. However, when present in slightly higher concentrations, they may be toxic and cause serious health and environmental hazard as they form nonspecific complexes with cell components. According to the Comprehensive Environmental Response Compensation and Liability Act (CERCLA), the United States, the maximum permissible limit for some important toxic heavy metals in the water system is 0.05, 0.01, 0.015, and 0.002 mg/L for Cd, Cr, Pb, and Hg respectively. In India, the permissible standard for some soil heavy metals is 3–6, 135–270, 75–150, 250–500, and 300–600 mg/kg for Cd, Cu, Ni, Pb, and Zn respectively (Ayangbenro and Babalola, 2017). Different heavy metals create toxicity by a wide range of mechanisms, for example, lead causes free radical imbalance and oxidative stress; chromium, arsenic, and mercury from harmful thiol or methyl derivatives; cadmium and aluminum can replace essential cofactors and bind to vital proteins; Cr causes disorders in ion channels, membrane permeation, DNA and protein damage; Iron has a corrosive effect with organ incursion and lipid peroxidation.
Besides this, petroleum hydrocarbons are still among the major and most commonly occurring environmental pollutants. Crude oil-derived hydrocarbons constitute the largest group of environmental pollutants worldwide. We all know that hydrocarbons are toxic substances that exert a negative impact on the environment. The production of crude oil, its transport, chemical processing, and distribution are considered as the main sources of anthropogenic hydrocarbon pollution. According to Sinha (2018), textile and pigment-based industries including paints, photography, plastics, printing, tannery, rubber, paper, pharmaceutical, and cosmetics manufacturers significantly release different color wastes as a major pollutant into the environment and also harm the environment.
Bioremediation has been highlighted as an emerging technology that can be widely applied in situ for the removal of environmental pollutants using biological systems efficiently and cost-effectively. Microbial degradation/transformation is one of the key approaches used for the bioremediation of heavy metals and organic compounds such as hydrocarbon and industrial dyes. Biofilms can potentially be used in the remediation process as they have the inherent capacity to survive in most toxic environments with in-built greater resistance mechanisms due to the presence of certain genes. Bacterial biofilm can efficiently remove metals and dyes by the bioaccumulation and biosorption mechanisms due to their high biomass density and can also reduce some metals and dyes to a lower toxicity level by their enzyme activities (Shukla, 2017).
Changes in the industrial sector and unplanned waste management have increased heavy metal contamination in the soil, sediment, and aquatic environment. Although several techniques have been developed for the removal of metal ions depending upon need and availability, recently, bioremediation using bacterial biofilm has gained attention due to its greater advantages (Shukla et al., 2017). Many researchers have studied and reported the biosorption of different heavy metals such as Cd (II), Cr(VI), Pb(II), Hg(II), Ni(II), Zn(II), Cu(II), and Mn(II) by using different types of bacterial biomass. Application of the indigenous bacterial biomass such as Arthrobacter sp. SUK 1205, Escherichia coli, Staphylococcus epidermidis, Leptothrix cholodnii SP-6SL, Pseudomonas sp. Lk9, Bacillus subtilis, Bacillus cereus, Shewanella oneidensis, Penicillium simplicissimum, Stenotrophomonas spp, and Pseudomonas putida CZ1 isolated from different metal-contaminated sites in the bioremediation processes seems to be effective and potentially removed the heavy metal load from the contaminated waste.
Biofilms have a great contribution to the remediation of hydrocarbon and its derivatives. In today’s world, one of the major environmental problems is hydrocarbon contamination resulting from activities related to the petrochemical industry. Bacteria absorb hydrocarbon and make them non-toxic compounds with the help of biofilm. Bioremediation functions basically on biodegradation, which may refer to complete mineralization of organic contaminants into carbon dioxide, water, inorganic compounds, and cell protein or transformation of complex organic contaminants to other simpler organic compounds by biological agents like microorganisms (Das & Chandran,2011). Several bacterial biomasses were applied for the bio removal of toxic hydrocarbon and dyes from the dye-contaminated waste, water, and soil. It was found that the bacteria such as Vibrio fischeri, Bacillus amyloliquefaciens, Bacillus sp. AK1, Lysinibacillus sp. AK2, Kersteasia sp. VKY1, Enterococcus faecalis, P. aeruginosa, Aeromonas spp., Pseudomonas luteola, E. coli, B. subtilis, Staphylococcus aureus, and Pseudomonas sp. strain DY1 were efficiently able to remove different types of hydrocarbons and different types of organic dye. Their biofilms play a key role in the removal process (Shukla, 2017). Bacterial biofilms can be the potential tools for the bioremediation of polycyclic aromatic hydrocarbons. It is an environmentally friendly process as it reduces the harmful effects of toxic hydrocarbon elements.
Most of the bacteria that live in adverse polluted environmental conditions can form a biofilm for better survival. Due to having a complex structured extracellular polymeric substances matrix, intercellular gene transfer, quorum sensing, and chemotaxis characteristics, they have proved a potential candidate for the bioremediation of various toxic recalcitrant compounds. Bacterial biofilms comprising single or multiple species of different habitats can be successfully applied for both in situ and ex situ bioremediation processes. It is possible to use biofilms in the remediation of heavy mantles and hydrocarbons and also their derivatives. Biofilms can be an essential tool to remove different pollutants from our environment to make our environment clean and safe for the survival of humans and other animals. But still, our knowledge of biofilm and its potential applications is limited. So, we need more and more research on biofilm to maximize its potential application of it. if we can properly utilize and use biofilms, it can give a pollution-free environment. we can use all the potential of biofilms cost-effectively to make the environment harmful element-free.
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