**1. Introduction**

Air, water and soil which are the essential elements of life are contaminated rapidly due to increasing population, urbanization, mining activities and industrialization [1]. Heavy metals toxicity is causing problem to humans, animals, aquatic animals, plants and even microbes too.

Various methods are introduced to remove the heavy metal pollution like chemical techniques such as chemical precipitation, oxidation or reduction method, electrochemical treatment. Physical techniques such as ion exchange, evaporation, filtration, membrane technology, reverse osmosis. Biological techniques like microorganisms such as bacteria, fungi, algae, cynobacteria, lichens, etc.

Heavy metals damage cell membranes, alter functioning of enzymes, inhibit protein synthesis, denature protein and damage the structure of DNA. Toxicity is mainly created by the dislocation of essential metals from their real binding sites or ligand interactions [2]. Bioremediation is cost-effective, safe and eco-friendly; can be virtually restored a result to the heavy metal pollution issue as it is natural process. Biological methods are best to control short term or long term environmental pollution. Various heavy metals are accumulated with the help of bacteria, fungi, cyanobacteria, lichens, etc. and helps in bioremediation and used as bioindicators. They are not harmful human heath as well as ecosystem. Such organisms are used for indication and controlling heavy metal pollution. Mostly genes encoded by heavy metal resistant bacteria are located on plasmids. Biosorption is environmentally safe and low cost methodology of removing metals from the ecosystem. Various analysis were observed throughout previous 5 decades provided quantity of data regarding differing kinds of biosorbents and their mechanism of absorption of heavy metal. Additional research is to explore new biosorbents from surroundings [3].

Since last few years, various physical and chemical methods are used to remove heavy metals but it is expensive, needs laboratory and inefficient. According to various studies bioremediation and biosorption techniques are much more beneficial, cheap, non-toxic, natural process.

Minimum inhibitory concentration (MIC) is the lowest concentration at which the isolate or antimicrobial agent is completely suppressed is recorded. Microorganisms correspond to heavy metals using various defense systems, such as exclusion, compartmentalization [4], complex formation and synthesis of binding proteins, such as metallothioneins [5].

Bioremediation strategies have been proposed as an attractive alternative owing to their low cost and high efficiency [6].

Different methods are used to study characterization of heavy metals on microbes by 16S RNA sequence, biodegradability test, siderophore assay, biochemical test, morphological test, antibiotic resistance, nucleotide sequencing, etc. Microbial pigmentation and enzymatic activities like catalase, gelatin hydrolysis, oxidase, nitrate reductase, were characteristics selected to examine their outcomes.

Bioremediation is of two types: in-situ bioremediation and ex-situ bioremediation. In-situ bioremediation process is mainly used due to its ability in decreasing disturbance of ecosystem at the heavy metal polluted sites whereas ex-situ bioremediation, it takes place inside bioreactors, bio-piles and land farming. In-situ bioremediation is much more efficient and eco-friendly (**Figure 1**).

Metal microbe interactions developed by microbial cells are bio-transformation, bio-leaching, bio-degradation, bio-mineralization, bio-adsorption and bio-accumulation in bioremediation method.

Biofilm used as efficient bioremediation tool and stabilization too. Even at harmful conditions, they show high resistance towards heavy metals. With the help of genetic engineering one can insert desired characters like ability to resist heavy metals, tolerate metal stress, etc. For example: engineered *Chlamydomonas reinhardtii* shows increased resistance to cadmium toxicity. *Corynebacterium glutamicum* was genetically modified using ars (operon) to accumulate arsenic polluted sites. Biofilm combines or work with biosorbent or any exopolymeric substance which consist of surfactants or emulsifier properties. The study was conducted on *Rhodotorula mucilaginosa* shows efficiency in heavy metal removal and develops 91.7–95.4% biofilm cells. Biosurfactants studied were surfactin, rhamnolipid and sophorolipid for removal of several heavy metals.

*A Review on the Resistance and Accumulation of Heavy Metals by Different Microbial Strains DOI: http://dx.doi.org/10.5772/intechopen.101613*

#### **Figure 1.**

*Bioremediation (enzyme-catalyzed destruction) of contaminants. The use of power ultrasound in biofuel production, bioremediation and other applications [7].*

The aim of the review is to study the source of the heavy metals on earth, consequences of the heavy metals on plants as well as on animals, various isolated microbial strains from bacteria, fungi and algae tolerance towards heavy metals and to study mechanism adapted by strain to accumulate heavy metals.

Future approaches in bioremediation are genetic modification of microbes or genetic engineered microbes, genetic technologies and forms specificity using biofilm by optimization process and immobilization process can be attained, biofilm mediated remediation, formation of microbial fuel cell (MFC), use of nano-particles with algae and bacteria, gene transfer within biofilm, transgenic cynobacteria, modify gene or enzyme in microbes. In Rhizo-remediation technique, *rhizosphere* bacteria and *mycorrhizae* combine for uptake.

#### **2. Source of the heavy metals**

High amount of heavy metals in the soil, water and air arise from various sources, which consist of natural sources include natural emission, atmospheric decomposition, sea salt spray, forest fires, rock weathering, biogenic means and wind borne soil particles and artificial sources such as mining activities, agricultural waste, domestic effluents, smelters, sewage sludge irrigation, improper stacking of the industrial solid waste, the excess utilization of pesticides, insecticides and fertilizers, etc. [8, 9].

#### **2.1 Lead in environment**

Lead (Pb) is unnecessary metal on the crust. It is a important contaminant that is present in the soil, water and air as a dangerous waste. It is extremely injurious to the human, animals, plants and even microbes too. The crucial sources of lead metal are children toys, drinking water, dust, petroleum, electronic industries, water pipes, battery, pottery, paint, stained glass, cosmetics and biocide preparation [10, 11].
