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This study was designed to examine the in vitro bactericidal activity of hydrogen peroxide against Legionella. We tested hydrogen peroxide (Peroxy Ag+) at 600 ppm to evaluate Legionella survival in a simulated dental treatment water system equipped with Water Hygienization Equipment (W.H.E.) device that was artificially contaminated. When Legionella pneumophila serogroup (sg) 1 was exposed to Peroxy Ag+ for 60 min we obtained a two decimal log reduction. High antimicrobial efficacy was obtained with extended periods of exposure: four decimal log reduction at 75 min and five decimal log reduction at 15 h of exposure. Involving a simulation device (Peroxy Ag+ is flushed into the simulation dental unit waterlines (DUWL)) we obtained an average reduction of 85% of Legionella load. The product is effective in reducing the number of Legionella cells after 75 min of contact time (99.997%) in the simulator device under test conditions. The Peroxy Ag+ treatment is safe for continuous use in the dental water supply system (i.e., it is safe for patient contact), so it could be used as a preventive option, and it may be useful in long-term treatments, alone or coupled with a daily or periodic shock treatment.

BackgroundWorldwide, L. pneumophila sg 1 is the most common agent of Legionnaires’ disease ( 80 to 90% of the reported cases). In contrast, L. pneumophila sg 2–14 account for only 15 to 20% of community-acquired cases, although they account for over 50% of the environmental isolates. The discrepancy between environmental isolates and clinical cases of disease suggested that there are differences in virulence.We decided to subtype the environmental Legionella strains isolated from health care facilities (HCFs) and to compare the distribution of strains with the occurrence of hospital-acquired legionellosis.MethodsObservational ecological study based on the data provided by the regional surveillance of legionellosis and on data obtained from hospitals environmental monitoring.Using the monoclonal antibody MAb 3/1 of the Dresden Panel we collected and typed environmental strains of L. pneumophila sg 1 obtained during routine testing in 56 health care facilities from 2004 to 2009.The results of the laboratory analyses of the environmental samples were compared with the number of cases that each health care facility reported during the study period.ResultsThe association between the type of colonisation (L. pneumophila sg 1 vs others serogroups) and the incidence of reported cases was statistically significant (p = 0.03 according to the χ2 test).Legionella strains with the virulence–associated epitope recognised by MAb 3/1 were isolated in 8 of the 26 HCFs colonised by L. pneumophila sg 1; 7 of the HCFs colonised by MAb 3/1-positive strains accounted for 85% of the cases of hospital-acquired legionellosis reported during the 6-year study period. There was a statistically significant association (p = 0.003) between the presence of cases and colonisation by MAb 3/1-positive Legionella strains.ConclusionThis study suggests that hospitals colonised by more virulent strains should be aware of the increased risk and consider the opportunities of increase their monitoring efforts and implement more effective contamination control strategies.

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.