Two main mechanisms for development of resistance in bacteria are detoxification (transformation
of the toxic metal state and making it unavailable) and active efflux pumping of the toxic metal from
cells [72]. The basic redox (oxidation and reduction) reaction takes place in the soil between toxic metals
and microorganisms; microorganisms act as an oxidizing agent for heavy metals and cause them to lose
electrons, which are accepted by alternative electron acceptors (nitrate, sulphate and ferric oxides).
In aerobic conditions, oxygen acts as an electron acceptor, while in anaerobic conditions microbes
oxidize organic contaminants by reducing electron acceptors. The microorganism takes energy for
growth by oxidizing the organic compound with Fe (III) or Mn (IV) as an electron acceptor [73]. Anaerobic
degradation of organic contamination is stimulated with the higher availability of Fe (III) for microbial
reduction [74,75]. Metals being used as terminal electron acceptors is called dissimilatory metal
reduction [76]. Biodegradation of chlorines from contaminants takes place through reductive
dechlorination, where contaminants as chlorinated solvents acts as an electron acceptors in respiration.
Microorganisms reduce the state of metals and change their solubility, like the Geobaccter species, and
reduce the Uranium soluble state (U6+) to insoluble state (U4+) [77].