The present study deals with the isolation, the identification, and the characterization of heavy metal-resistant bacteria selected from several polluted sources with the aim of using them as bioremediation agents of contaminated sites by heavy metals. Four protocols were used in a different way to select the maximum of isolates resistant to heavy metals. Isolates obtained, according to the fourth selection protocol by using the free nutrients medium, showed the highest values of minimum inhibitory concentration (MIC), and it can be deduced from this last result that when the bacteria are subjected to stressful conditions they develop a system that will enable them to withstand high levels of heavy metals. The study of the MIC at all resistant isolates allowed determining that the order of toxicity of the different metals was as follows: Cu > Cr > Cd > Ni > Zn > Co. These metals might exert varying effects depending on their adsorption and their complexation capacities to the bacterial cell wall. In addition, we can assume from the results of different screening protocols that the mixture of heavy metals exerts a more significant stressful effect on the life of the bacterial cell than the metal used alone. Isolates are all bacilli and cocci of gram-negative and of gram-positive bacteria. Pseudomonas aeruginosa and Staphylococcus aureus appeared as the most dominant genera. Therefore, the presence of metals in the growth medium has as an effect to conserve and to keep the character of resistance in the bacteria, contrary to its or their absence allowed the direct loss of this resistance. The study on the morphology of resistant isolates showed that the contour and the size of colonies would change in the presence of heavy metals. The heavy metal tolerant isolates were as of two types, of gram-positive and of gram-negative bacteria, and isolates of gram-negative were generally more resistant to the mixture of metals than those of gram-positive, since they could tolerate a 30 mM of heavy metal mixture. In addition, the identification of isolates selected by the mixture of metals revealed the dominance of the P. aeruginosa. The study of the growth of P. aeruginosa in the presence of increasing concentrations of metals showed a significant toxic effect of the high concentrations. Low concentrations are essential as trace elements for the bacterial growth and metabolism. Considering P. aeruginosa metal resistant strains, the quantity of total proteins synthesized appeared variable and generally an increase of the metal concentration is accompanied with an inhibition of the protein synthesis. The study of the protein profile for heavy metals resistant isolates of P. aeruginosa is mainly shown by the decrease in the level of proteins. This decrease pointed out the toxic effect of metals on the bacterial metabolism.
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Mounaouer et al. (2013) studied this question.