Physicochemical Quality, Microbiological Contamination, and Antibiotic Susceptibility of Bacteria in Rooftop-Harvested Rainwater in Ajdabiya City, Libya

Abstract

Rainwater harvesting is widely used as an alternative water source in arid and semi-arid regions. However, the quality of stored rainwater may be affected by environmental contamination and storage conditions. This study assessed the physicochemical and microbiological quality of rooftop-harvested rainwater in Ajdabiya City, Libya, and evaluated the antibiotic susceptibility patterns of isolated bacteria. A total of 70 rainwater samples were collected from household storage tanks between February and May 2025. Standard analytical methods were used to evaluate physicochemical parameters, total bacterial counts, coliform detection, bacterial isolation, and antimicrobial susceptibility testing. Most physicochemical parameters complied with the World Health Organization (WHO) drinking-water guidelines. Electrical conductivity ranged from 186 to 1186 µS/cm, while total dissolved solids were generally below 500 mg/L. pH values ranged between 5.9 and 8.5, and turbidity was not detected in any sample. Microbiological results revealed bacterial counts ranging from 30 to 295 CFU/mL. Coliform bacteria were detected in the majority of samples, with only five samples free from contamination. The most frequently isolated bacteria were Pseudomonas aeruginosa (38.57%), Escherichia coli (34.28%), and Klebsiella pneumoniae (27.14%). Antibiotic susceptibility testing demonstrated different resistance patterns among the bacterial isolates: E. coli showed susceptibility only to amikacin (87%), with complete resistance to all other antibiotics tested, while P. aeruginosa and K. pneumoniae showed susceptibility to a wider range of antibiotics. The results suggest that, despite the acceptable physicochemical quality of the harvested rainwater, microbiological contamination and the detection of multidrug-resistant E. coli may limit its suitability for drinking without appropriate treatment.