Effects of Heavy Metals on Bacterial Growth, Biochemical Properties and Antimicrobial Susceptibility

Abstract

Heavy metals are metallic elements that occur naturally and have a high atomic weight and a density which is five times greater than the density of water. Several studies have demonstrated that metal contamination in natural environments could have an important role in the development and spread of antimicrobial resistance. This is of particular concern, considering that heavy metals usually occur at higher levels than pharmaceutically produced antimicrobials. Also, the bio-accumulative and non-biodegradable nature of heavy metals may result in long- term antimicrobial resistance selection pressure. Therefore, this study assessed the effects of Copper, Cobalt, Cadmium, Zinc and Lead salts on growth, biochemical properties and antimicrobial susceptibility of Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus to commonly used antimicrobials. The selected bacteria were exposed to varying concentrations of the heavy metal salts ranging from 1 ppm to 1000 ppm after which growth was measured using a spectrophotometer. The effect on biochemical reactions and antimicrobial susceptibility were also tested by the use of conventional biochemical tests and Kirby-Bauer disk diffusion methods respectively. Different heavy metals affected the growth of each individual microorganism differently. Despite that, the growth of E. coli, P. aeruginosa and S. aureus was observed to be inversely proportional to the concentrations of the heavy metals used. Following exposure to heavy metals, E. coli and S. aureus showed no changes in their biochemical properties but P. aeruginosa gave a positive urease result contrary to the control. E. coli developed resistance to Levofloxacin, Meropenem and Tetracycline while S. aureus to Azithromycin and Gentamicin and P. aeruginosa developed resistance only to Meropenem. These results confirm the adverse effects of heavy metals on bacterial growth and reveals that heavy metals can cause changes in some biochemical properties of bacteria that produce any of these properties under stressful growing conditions. This research also supports the findings that heavy metals enhance the development of antimicrobial resistance in otherwise antimicrobial sensitive strains of bacteria. This study therefore, gives insight into the likely state of the proliferation of heavy metal induced antimicrobial resistance in heavy metal contaminated areas.