Occurrence of Pseudomonas aeruginosa in several sources of drinking water

Introduction Pseudomonas aeruginosa is an environmental ubiquitous bacterium that may cause opportunistic waterborne infections in immunocompromised host. It can be isolated in several aquatic habitat where it shows the ability to survive in a quiescent metabolic state under stressful conditions and to revert to a metabolically active status when environmental conditions allow. In domestic environment it is not rare to find it associated with the water piping system, producing a thick layer of biofilm. Many strains that are biofilm producers express virulence factors and resistances to several antibiotic classes. This study aims to isolate and characterize P. aeruginosa and other Gram-negative bacteria in a range of types of drinking water. Materials and Methods For each water source, 1 liter sample was aseptically collected in sterile flasks containing sodium thiosulfate to neutralize chlorine. The samples were kept at +4 °C and analyzed within 24 hours. Isolation and enumeration of P. aeruginosa was carried out by membrane filtration technique according to the UNI EN ISO 16266: 2008. The isolated strains were identified and tested for antibiotic resistance by Vitek 2 (Biomérieux). All Pseudomonas isolates were tested for biofilm production by violet crystal staining in multiwell 96 plates and their growth was compared to those of strongly producing biofilm reference strains. All strains were stored at -80 ° C for further studies. Results A total of one hundred water samples from different sources was analyzed: tap water, domestic deionizer water (from home filtration devices), water dispensers, ground water and mineral water springs. In most of the samples directly from the drinking water system a microbial load < 10 CFU/L has been detected. In 1/3 of the samples till now analyzed, Gram-negative species, resistant to the chlorine, have been identified. A higher percentage of samples from domestic deionizer and dispenser waters resulted positive for the presence of P. aeruginosa than ground and tap water samples. Discussion and Conclusions Our preliminary data highlight a critical issue related to the home filtration devices installed in the terminal part of taps. Although the water suppliers constantly check the ensuring compliance with the microbial load parameters, the last part of the private pipes frequently hosts microorganisms organized in biofilms mono or multi species able to alter the microbiological quality of water. Deionized water samples have been resulted a large reservoir of microbial diversity. Indeed, the great filtering surface provides perfect substrate to sessile forms triggering the biofilm production and enhancing the growth of bacteria routinely living in drinking water. Further sampling and analysis are necessary to drive reliable conclusions.