Abstract
Legionella spp. are ubiquitous in aquatic habitats and water distribution systems, including dental unit waterlines (DUWLs). The aim of the present study was to determine the prevalence of Legionella in DUWLs and tap water samples using PMA-qPCR and standard culture methods. The total viable counts (TVCs) of aerobic heterotrophic bacteria in the samples were also determined. Legionella spp. were detected and quantified using the modified ISO 11731 culture method. Extracted genomic DNA was analysed using the iQ-Check Quanti Legionella spp. kit, and the TVCs were determined according to the ISO protocol 6222. Legionella spp. were detected in 100% of the samples using the PMA-qPCR method, whereas these bacteria were detected in only 7% of the samples using the culture method. The number of colony forming units (CFUs) of the TVCs in the DUWL and tap water samples differed, with the bacterial load being significantly lower in the tap water samples (p-value = 0). The counts obtained were within the Italian standard range established for potable water in only 5% of the DUWL water samples and in 77% of the tap water samples. Our results show that the level of Legionella spp. contamination determined using the culture method does not reflect the true scale of the problem, and consequently we recommend testing for the presence of aerobic heterotrophic bacteria based on the assumption that Legionella spp. are components of biofilms.
Highlights
Legionella spp. are ubiquitous in aquatic habitats and water distribution systems, including dental unit waterlines (DUWLs)
Our results show that the level of Legionella spp. contamination determined using the culture method does not reflect the true scale of the problem, and we recommend testing for the presence of aerobic heterotrophic bacteria based on the assumption that Legionella spp
Consistent with the results of other studies [31,34,35], we demonstrated that a significant proportion of the DUWLs tested were disinfected, samples taken from them were more heavily contaminated with bacteria compared with tap water samples collected from the same offices and that their bacterial levels were much higher than the standards set for drinking water
Summary
Legionella spp. are ubiquitous in aquatic habitats and water distribution systems, including dental unit waterlines (DUWLs). The concentration of Legionella spp. in the water of dental units may reach 1000 organisms per mL [1]. The presumed natural reservoirs for this pathogen are amoebae that Legionella cells can invade and in which they can replicate. Amoebae are the natural hosts of legionellae in the environment; the relationship between these organisms is unique in that, amoebae which generally use other bacteria as food, are parasitized by legionellae [10]. Direct contact with or exposure to aerosols containing free-living amoebae may cause infections, and it has been suggested that amoebae can act a “Trojan horses” for pathogens such as Legionella [11,12]. Freshwater amoebae have been detected in dental unit water samples [13]. The intensive use of instruments such as air-water syringes, ultrasonic
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