Isolation and Assessment of Metal-tolerant Bacteria and their Potential for Heavy Metal Removal

Abstract

Heavy metal contamination is one of the major global ecological concerns; it is commonly found to contaminate soil, sediments, and wastewater, where they remain persistent and become toxic to many species exceeding certain threshold concentrations.  Bacteria resistant to heavy metals can be used for detoxification and prevent further deterioration of contaminated sites.  Soil samples were collected from two different auto-mechanic workshops: one located at Kofar Ruwa Market (main activity here is car battery charging), and the other one located at Unguwa Uku Motor park (main activities here include welding and soldering), and also a control site at Ecological Study Area of Bayero University, Kano, Nigeria (this area is free from any human activities).  The physicochemical parameters of the soil samples were initially determined.  Bacterial enumeration and isolation were conducted.  Ten-fold dilutions of the soil samples were made in which an aliquot was plated in nutrient agar amended with nystatin (0.5-1 μg/ml) to suppress fungal growth.  The bacterial isolates were screened for chromium, lead, Zinc, Copper, and Iron resistance by plate diffusion method.  The maximum tolerable concentration of the isolates was measured in terms of O.D. at 595 nm, and optimization of temperature and pH was carried out.  Atomic absorption spectrophotometric analysis was carried out for the removal of chromium, lead, Zinc, Copper, and Iron by the isolates confirmed by Scanning electron microscopy (SEM).  Of the 7 isolated bacteria, 4 bacterial isolates with maximum tolerance trends for Cr, Pb, Zn, Cu, and Fe were selected and identified as Pseudomonas aeruginosa, Escherichia coli, Staphylococcus aureus and Klebsiella pneumoniae.  The highest removal percentage was observed at 72 and 96 hours of incubation using AAS analysis, and the efficiency of removal of the five heavy metals in decreasing order was P. aeruginosa (97.5% Cu, 76.5% Pb, 72.6% Fe, 72.2% Zn, 50.2% Cr) >E.coli (93.8% Cu, 65% Pb, 64.2% Fe, 64.5% Zn, 19.4% Cr) >K. pneumonia (75.7% Cu, 49.3% Pb, 49.3% Fe, 27.1% Zn, 34.9% Cr) >S. aureus (55.5% Cu, 57.5% Pb, 19.9% Fe, 64.5% Zn, 17.3% Cr). Scanning electron microscopy (SEM) showed changes in the surface morphology of all four bacterial isolates after metal treatment.  These results suggest that all four identified heavy metal-tolerant bacteria can be useful for the bioremediation of soil environments contaminated with heavy metals.