Assessing Heavy Metal-Resistant Microorganisms and Health Risks in Landfill Leachate and Soil: A Systematic Review

Background Improper municipal solid waste management, particularly in unengineered landfills, leads to the accumulation of toxic heavy metals in leachate and soil. These environments select for heavy metal-resistant microorganisms, which pose both bioremediation opportunities and potential human health risks. Aim This systematic review assessed the distribution, diversity, and minimum inhibitory concentrations (MICs) of heavy metal-resistant bacteria in leachate and soil from unengineered landfills, evaluated their bioremediation potential, and examined associated human health risks. Methods A systematic review following PRISMA 2020 guidelines was conducted. PubMed, Scopus, Web of Science, and Google Scholar were searched for articles published between January 2012 and September 2022. Studies reporting isolation and characterization of heavy metal-resistant bacteria from unengineered landfill leachate or soil were included. Data were extracted and synthesized narratively. Quality assessment was performed using the Joanna Briggs Institute checklist for laboratory-based cross-sectional studies. Results Sixteen studies met the inclusion criteria, originating from 11 countries (31.3% Nigeria, 18.8% India). The most frequently isolated genera were Pseudomonas (12 studies), Bacillus (8 studies), Escherichia coli (6 studies), and Klebsiella (5 studies). MICs ranged from 700–1500 µg/mL for lead, 800–1000 µg/mL for chromium, 900–1200 µg/mL for nickel, and 700–900 µg/mL for cadmium. Bacterial isolates reduced leachate metal concentrations by 27–37% and achieved up to 99% mercury removal under optimized salinity. Eleven studies had moderate methodological quality, and five had low methodological quality. Opportunistic pathogens (P. aeruginosa, K. pneumoniae, S. aureus) were identified in 10 studies, and co-selection of metal- and antibiotic-resistance genes was reported. Conclusion Unengineered landfill leachate and soil harbor diverse heavy metal-resistant bacteria with high tolerance to lead, chromium, nickel, and cadmium. While these bacteria show promise for bioremediation, their pathogenicity and potential to carry antibiotic resistance genes pose human health risks. Field-scale validation, standardized MIC protocols, and integrated health risk assessment are urgently needed