Heavy Metal Tolerance, Molecular Characterization and Plasmid-Mediated Resistance of Multidrug-Resistant Bacteria Isolated From Refuse Dumpsites in Delta State, Nigeria.

Heavy metal contamination of refuse dump soils poses serious environmental and public health concerns due to its role in selecting resistant microorganisms and promoting antimicrobial resistance. This study investigated heavy metal resistance, molecular characterization, plasmid-mediated resistance, and antibiotic susceptibility of bacterial isolates from refuse dump soils in Delta State, Nigeria. Soil samples were collected from municipal dumpsites, and bacterial isolates were obtained using standard microbiological methods. Resistance to chromium, copper, and cadmium was determined using the minimum inhibitory concentration (MIC) technique. Molecular identification of highly resistant isolates and plasmid curing were carried out to determine the genetic basis of resistance traits. A total of 61 bacterial isolates were recovered. All isolates (100%) showed resistance to chromium at 100–150 mg/L and copper at 100–200 mg/L. However, resistance decreased at higher concentrations, with only 9.8% surviving 200 mg/L chromium and 250 mg/L copper. Cadmium exhibited the highest tolerance, with all isolates surviving up to 900 mg/L and 6.6% remaining viable at 1000 mg/L. Molecular characterization identified the most resistant isolates as Pseudomonas aeruginosa, Bacillus subtilis, and Klebsiella species. Following plasmid curing, resistance to several antibiotics declined, while resistance to β-lactam antibiotics remained high. Bacillus subtilis retained resistance to multiple antibiotics including amoxicillin/clavulanic acid, ceftazidime, and ceftriaxone, whereas susceptibility was observed for gentamicin and ofloxacin. Klebsiella species and Pseudomonas aeruginosa maintained