Hidden risks of biofilm-forming and multidrug-resistant Gram-negative bacteria in water vending machines

Water vending machines are widely utilised in various locations, including workplaces, universities and urban areas due to their convenience and accessibility. However, the microbial quality of the water dispensed from these machines, particularly in residential colleges, has not been thoroughly studied. This study aims to analyse the physicochemical and microbiological quality of drinking water collected from water vending machines located in 14 residential colleges around Universiti Malaya, Kuala Lumpur. Our study revealed that all drinking water samples met WHO acceptable limits for pH, TDS and electrical conductivity. However, dissolved oxygen and free chlorine levels were slightly below the acceptable range. No faecal contamination was detected in the drinking water samples, as indicated by the absence of enterococci and coliform bacteria, including faecal coliforms. Nevertheless, Gram-negative non-coliform bacteria were identified in water samples from 11 residential colleges. The identified bacteria included Pseudomonas spp., Acinetobacter spp., Bosea sp., Stenotrophomonas sp., Achromobacter sp., Roseatels sp. and Cupriavidus sp. Using CLSI guidelines as the standard, antibiotic sensitivity tests showed that all bacterial isolates were resistant to penicillin G (100%), while most were susceptible to ofloxacin (> 80%). Notably, Pseudomonas spp., Acinetobacter spp., Stenotrophomonas sp., Achromobacter sp. and Cupriavidus sp. were identified as multidrug-resistant bacteria, showing resistance to three or more categories of antimicrobial agents tested in this study. The biofilm assay confirmed that Bosea sp., Roseateles sp. and Acinetobacter sp. possess biofilm formation capacity. Overall, this study highlights that while the water from vending machines generally meets physicochemical standards and is free of faecal contamination, multidrug-resistant Gram-negative bacteria are still prevalent. Although these bacteria may not pose immediate health risks upon consumption, their presence poses long-term risks due to biofilm accumulation, resistance gene transfer and poor maintenance. This underscores the need for proper maintenance measures of water vending machines, including regular cleaning and effective disinfection, to ensure the microbial safety of drinking water.