Antibiotic susceptibility patterns of heavy metal-tolerant Gram-negative bacteria isolated from a major dumpsite in Ibadan, Nigeria

Objective: To determine the antibiotic susceptibility patterns of heavy metal-tolerant Gram-negative bacteria isolated from a major dumpsite in Ibadan, Nigeria. Methods: Gram-negative bacteria were isolated using MacConkey and eosin methylene blue agar and presumptive identification to species level was done using conventional methods. The isolates were screened for their tolerance to heavy metals. Antimicrobial susceptibility was tested by Kirby-Bauer disc diffusion method with the following antibiotics: tetracycline (10 μg), cotrimoxazole (25 μg), gentamicin (10 μg), cefuroxime (30 μg), chloramphenicol (10 μg), ceftriaxone (30 μg), cefotaxime (30 μg), ciprofloxacin (5 μg), amikacin (30 μg), ceftazidime (30 μg), and meropenem (10 μg). The zones of growth inhibition were interpreted based on the Clinical Laboratory Scientific Institute. Identification of the most metal-tolerant isolates was done using 16S rRNA. Results: A total of 79 Gram-negative bacteria were isolated, 31 of which tolerated a minimum of three heavy metals (Pb at 10 350 μg/mL; Cu at 750 μg/mL; Cr at 750 μg/mL in the wet season and Pb at 2 350 μg/mL; Cu at 450 μg/mL; Cr at 300 μg/mL in the dry season). All 31 isolates showed varying degrees of multiple antibiotic resistance indexes. The isolates showed the highest resistance to cefuroxime and meropenem (100%) and the least resistance to chloramphenicol (51.6%). The most metal-tolerant isolates were identified as Aeromonas hydrophila and Pseudomonas aeruginosa . Conclusions: This study revealed that the waste dumpsite contains significant amounts of heavy metals and antibiotic-resistant bacteria which may pose serious public health risks if they enter the food chain. The study highlighted the health risks of using untreated dumpsite soils for vegetable farming due to pathogen and heavy metal transfer, and it is unsuitable for agricultural use.