Isolation and characterization of zinc resistant bacteria from a coil coating industrial wastewater treatment plant

Wastewater effluents from metal coil coating industry containing priority pollutants including chromium, cyanide, zinc, aluminum, cadmium, lead, nickel, selenium, tin, and mercury; oil and grease, or total suspended solids are often discharged with limited or no treatment into the environment thereby threatening microbial diversity and health of plants, animals and humans. This study was carried out to assess the incidence of Zn²⁺ resistant bacteria from a coil coating industrial wastewater treatment plant in Ota, Southwest Nigeria. In this study, ten Zn²⁺ resistant bacterial strains were recovered from treated wastewater samples on nutrient agar supplemented with 1mM concentration of zinc sulphate heptahydrate by the standard spread plate method. The isolates were studied based on their cultural, morphological and biochemical characteristics and were identified as strains of Bacillus azotoformans, Bacillus megaterium, Micrococcus varians, Serratia marcescens HM1, Serratia marcescens HM2, Proteus mirabilis, Aeromonas hydrophila HM3, Aeromonas hydrophila HM4, Aeromonas caviae and Citrobacter spp. Further testing of the bacterial isolates in nutrient agar supplemented with different concentrations (2, 4, 6, 8 and 10 mM) of hydrated zinc sulphate showed that eight of the isolates (80%) exhibited resistance to Zn²⁺ concentration of 6 mM and above. Co-resistance of four bacterial isolates to Pb²⁺ was studied and they showed high resistance to the heavy metal with minimum inhibitory concentration (MIC) in the range of 2 to 8 mM. Out of the ten bacterial strains, seven (70%) exhibited resistance to several antibiotics including amoxicillin, cotrimixazole, augmentin, nitrofurantoin and tetracycline, and intermediate or full sensitivity to ofloxacin, nalidixilic acid and gentamicin. The bacterial isolates obtained in the present study enrich studies on microbial diversity of zinc resistance and could be exploited for biotechnological remediation of zinc, lead and other heavy metal contaminated ecosystems.