per liter, but currently, naturally Cr(VI) have been found above the WHO’s limit in ground and surface water [38]. The toxicity of chromium causes liver and kidney damage and skin ulceration and affects the central nervous system, and it is also connected with the effects on hematological problem and immune response in fresh water fishes. Its toxicity also associated with plant species, because it decreases the rate of photosynthesis [22].
3.2.1.4 Lead
Lead occupied sixth period and group IV in periodic table [23]. It is a metal generally occurring gray-bluish in color and found as a mineral form binding with other element like sulfur (PbS) or oxygen (PbCO3) and present in small amount in earth crust [16, 23]. It has occupied fifth rank in the production of heavy metals from industry and commonly used in lead storage batteries, solders, bearings, cable covers, ammunition, plumbing, pigments, and caulking [23]. It is also used in many different industrial, agricultural, and domestic applications [16]. Lead exposure to humans and animals occurs through lead contaminated dust particle and consumption of lead contaminated food stuffs and water [39, 40]. Adult peoples are taking 35% to 50% lead particles by drinking water but children are taking more than 50% [41]. In children, it causes toxicity of blood, deficient brainpower, poorer intelligence quotient-IQ, late or diminished growth development, neurobehavioral deformities, reduced auditory perception, speaking and wording disabilities, and unsocial and inattentive activities. In adults, it affects reproduction, such as, in men, reduces sperm count, and in women, continuous miscarriage have been reported due to long time exposure to lead contamination [16]. Sever expose to lead causes brain injury, malfunction of kidney, and different diseases in gastrointestinal track, while the prolonged expose may cause damage to blood, blood pressure, antagonistic effect on central nervous system, damage of kidneys, and trouble in metabolism of vitamin D [16, 39, 40, 42].
3.2.1.5 Mercury
In periodic table, mercury belongs to the transition element series. It occurs in three forms in the nature as elemental, inorganic, and organic form with individual toxicity character [43]. Generally, it exists in liquid form [23]. It can also exist as a cation through oxidation state of +1 (mercurous) or +2 (mercuric) [44]. It is used in electrical industry, dental amalgams, and in various industrial methods containing the manufacture of caustic soda, antifungal agent, manufacture of nuclear reactors, usage as a solvent for reactive and expensive metal, as a preservative of pharmaceutical products, etc. [45]. Mercury is released and contaminates the environment from combustion of coal, manometers at gas pressure measuring stations, and gas/oil pipelines as well. It exists in mercuric (Hg2+), mercurous (Hg2+2), elemental (HgO), or alkylated (methyl/ethyl mercury) form [23]. Human and animals expose to mercury and other chemicals by calamities, atmospheric pollution, contamination of food, dental repair, precautionary medical applications, farming, and industrial processes [17, 30]. Mercury entered to water by natural procedure of gassing from ground of earth and by industrial pollutants and accumulates in fish and tiny organisms inhabiting in the water bodies. Due to its lipid soluble nature, it can easily cross placenta and blood brain barrier. By eating methyl mercury affected fish, it enters to gastrointestinal tract and also affects kidney, neurological tissue and liver of human and it causes gastrointestinal toxicity, neurotoxicity, and nephrotoxicity [16].
3.2.2 Essential Heavy Metals
The essential heavy metals such as nickel, zinc, and copper are require for biological metabolism but in high concentration they show toxic and harmful effects and on living organisms.
3.2.2.1 Copper
Copper (Cu) is a transition element, occupied a place in group IB of period 4 in periodic table and ranked as third highest used metal in the world [23, 46]. It is a fundamental micronutrient for animal and plant development. In human, it helps for production of blood hemoglobin and in plant, Cu is essential for seed germination, resistant to disease and water regulation [23]. Spontaneously high amount of exposure of cupper dust causes eye, nose, and mouth irritation and often causes nausea and diarrhea [22]. Continuously exposure to high doses of Cu causes anemia, malfunction of liver, kidney, and impatience in stomach and intestine [23]. Generally, copper (Cu) is present in drinking water because flow pipes are made up of copper to control algal growth [23]. Mining, metallurgy, and industrial applications are the important causes of copper contamination in the environment [22].
3.2.2.2 Zinc
Zinc occupied a place in group IIB of periodic table. It is a transition element normally present in soil approximately 70 mg kg−1 in ground rocks [47]. It is an essential micro nutrient in our diet but higher concentration is toxic and may cause anemia and cholesterol complications in human beings and nausea and vomiting in children [22]. Currently, Zn concentration is found to be rising because of increasing anthropogenic activities. Mostly, industrial sectors and other human activities like mining, incineration of coal and waste, and steel dispensation are major causes for increase in Zn concentration in the environment. Other sources of Zn contamination are crops or drinking water stored in metal chambers [23].
3.2.2.3 Nickel
Nickel is the transition element that is present in environment only at very less amount and small doses are require for biological systems, but it can be hazardous when the concentration exceeds the permissible limit [23]. The higher concentration of Ni causes different types of cancer in various parts of animal body, mostly of those staying nearby industries [48]. It also causes damage to cells, reduces body weight, and damages the liver and heart [22]. Nickel is utilized in the steel industries, nickel-cadmium battery industries on a large scale, and in other metallic products [22, 48]. It is also found in paint formulation and cigarettes. The industrial wastes containing nickel enter into the water bodies contaminate the water and affect aquatic living organisms [49]. Earlier studies reported that microorganisms have problems for their growth and development in the presence of nickel. But, some studies in current days reported that some microorganisms have developed resistance to nickel [23].
3.3 Biofilm Bacteria
Biofilms are communities of one or more species of microorganisms living within the protection of an extracellular matrix composed of polysaccharides, proteins, DNA, and other molecules, collectively termed as the extracellular polymeric substances (EPS) [12, 19, 22].
Microbial EPS is crucial for the formation of biofilm and cell aggregates, which contribute to protect cells from hostile environments and can bind significant amounts of heavy metals [53–56]. Biofilm and planktonic cells have distinct heavy metal and metalloid susceptibility [57–59]. It is suggested that the complexation or sequestration of heavy metals and retarding their diffusion in to the biofilm may be responsible for protecting cells from heavy metal toxicity [58]. Microbial EPS are also of particular interest and relevance to the bioremediation process due to their involvement in flocculation and binding of heavy metals from solutions [53, 60–62].
3.4 Interaction of Metal and Biofilm Bacteria
The availability of heavy metal ions is the hazardous factor for environment. Availability of metal ions in different components of environment like water, soil, microorganisms, aquatic lives, and other forms of lives are dependent on several factors like industrial activity, natural sources like volcanic eruption, and unlimited anthropogenic activities. Again, its presence, concentration and effects are influenced by several environmental factors and circumstances like pH, alkalinity, redox potential, and action of microorganisms. Earlier, it was thought that metals have toxic effect only
