Co-selection of antibiotic resistance and heavy metal tolerance in bacterial species isolated from automobile scrapyard soils

Abstract

This study investigates the physicochemical properties, heavy metal concentrations, and bacterial profiles of contaminated scrapyard soils compared to fertile agricultural soil. The results indicate that scrapyard soils exhibit higher bulk density (1.10 1.30 g/cm³), lower organic carbon (0.80 1.10%), and reduced organic matter (1.37 1.90%) levels, suggesting deterioration in soil fertility. Electrical conductivity is significantly elevated (0.70 0.80 dS/m), indicating increased ionic strength. Heavy metal analysis reveals significantly higher concentrations of Pb (14 18 mg/kg), Cd (3 7 mg/kg), Zn (20 40 mg/ kg), Cu (23 35 mg/kg), Ni (23 30 mg/kg), and As (3.85 5.5 mg/kg). Bacterial loads in scrapyard soils (3.08 3.76 Log CFU/g) are considerably lower than in control soil (7.88 Log CFU/g), alongside decreased soil enzyme activities, including dehydrogenase (130 170 μg TPF/g/h), urease (110 130 μg NH₄⁺/g/h), phosphatase (70–90 μg pNP/g/h), and catalase (85–95 μg O₂/g/h). The study isolates Sphingomonas paucimobilis, Klebsiella aerogenes, Acinetobacter baumannii, Klebsiella pneumoniae, Pseudomonas reactans, and Bacillus subtilis, which exhibit varying resistance to antibiotics and heavy metals. A strong correlation is observed between heavy metal tolerance and antibiotic resistance (r = 0.78 0.88, P < 0.05), suggesting a synergistic effect where metal exposure enhances bacterial resilience to antibiotics. Notably, the MICs of tetracycline and ampicillin increase substantially in Cd-exposed isolates (up to 35 μg/mL and 25 μg/mL, respectively), while Ni exposure in Pseudomonas reactans results in a two-fold increase in the MIC of chloramphenicol (from 10.5 μg/ mL to 20 μg/mL). The correlation between soil enzyme activity and dominant bacterial species further highlights the influence of heavy metal contamination on microbial function. These findings underscore the environmental risks posed by scrapyard contamination and the adaptive mechanisms of multi-resistant bacteria in response to heavy metal stress.