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Microorganisms exposed to heavy metal stress are vulnerable to the toxic conditions caused by reactive oxygen species. The aim of this study was to investigate the tolerance of stressed bacteria and detect the ability of some resistant bacterial strains to produce antioxidant enzymes under heavy metal stress. The minimum inhibitory concentration (MIC) of heavy metals and antibiotic sensitivity were investigated for some bacterial strains isolated from a heavy metal-polluted area. Moreover, the activities of antioxidant enzymes produced in the resistant bacterial strains were measured. The most toxic heavy metals found using the MIC test were Cd2+ followed by Co2+. The MIC of all heavy metals except Pb2+ and Zn2+against the most metal-resistant bacteria species ranged from 300 to 500 mg/L. In addition, 100 mg/L Zn2+ or Pb2+ increased and induced the activity of the antioxidant enzymes catalase, peroxidase and ascorbate peroxidase. The strain Pseudomonas putida KNU5 greatly induced ascorbate peroxidase (1,030%) under the stress of 100 mg/L Zn2+, while the activity of ascorbate peroxidase decreased to 115% under 100 mg/L Pb2+. Thus, heavy metals cause oxidative stress in P. putida and antioxidant enzymes appear to play a pivotal role in combating oxidative stress, providing protection to P. putida. Furthermore, the results of this study show that antioxidant enzymes can be used to biologically monitor heavy metal pollution.       Key words: Heavy metals, resistant bacteria, antioxidant enzymes.

Heavy metal resistance of bacteria and its impact on the production of antioxidant enzymes