Evaluation of Legionella pneumophila contamination in Italian hotel water systems by quantitative real-time PCR and culture methods

Quantification of Legionella pneumophila by real-time quantitative PCR from samples with humic acid and ferric ion

This study determined and overcame the influences of humic acid (HA) and/or ferric ion (Fe) on quantification of Legionella pneumophila by real-time quantitative PCR (qPCR). Four commonly used DNA isolation methods, QiAamp DNA Mini Kit (Q), Q with Sepharose 4B gel column (Q/G), freeze-thaw/phenol-chloroform lysis (FT-PC), and FT-PC/G, were adopted to isolate L. pneumophila DNA from samples containing Fe alone (0-30 mg l(-1)) or Fe/HA (0/0-3/100 mg l(-1)). Among the four DNA isolation methods, Q removed HA effectively and obtained the greatest DNA yield regardless of Fe and HA concentration (P<0.05). For samples containing Fe (0.3-3 mg l(-1)) or Fe/HA (0.3/10-3/100 mg l(-1)), qPCR inhibition was found in all isolated DNA, especially in those obtained by Q/G and FT-PC/G. DNA dilution at either 10 or 100 folds reduced qPCR inhibition and increased cell recovery (P<0.05). Under 10-fold dilution, Q acquired the highest concentrations of L. pneumophila determined by qPCR. Consequently, Q with post 10-fold dilution is suggested prior to qPCR for quantifying L. pneumophila from water containing Fe (≤ 3 mg l(-1)) or Fe/HA (≤ 3/100 mg l(-1)).

Detection of Legionella by quantitative-polymerase chain reaction (qPCR) for monitoring and risk assessment

Background

Culture and quantitative polymerase chain reaction (qPCR) assays for the detection of Legionella were compared on samples from a residential area before and after two interventions. A total of 84 samples were collected from shower hoses and taps as first flush samples and at constant temperature. Samples were grouped according to the origin of the sample, a) circulation water b) water from empty apartments c) water from shower hoses. The aims were to investigate the usefulness of qPCR compared to culture for monitoring remedial actions for elimination of Legionella bacteria and as a tool for risk assessment.

Results

In water collected from the apartments Legionella spp were detected by qPCR in the concentration range from LOQ to 9.6*10⁵GU/L while L. pneumophila were detected in a range from LOQ to 6.8*10⁵ GU/L. By culturing, the legionellae were detected in the range from below detection limit (> 10 CFU/L) to 1.6*10⁶ CFU/L. In circulating water and in first flush water from shower hoses, culture and qPCR showed the same tendencies. The overall correlation between the bacteria number detected by culture and the two developed qPCR assays (L. spp and L. pneumophila) was relatively poor (r² = 0.31 for culture and Legionella spp. assay, r² = 0.20 for culture and L. pneumophila assay).

Conclusion

Detection by qPCR was suitable for monitoring changes in the concentration of Legionella but the precise determination of bacteria is difficult. Risk assessment by qPCR only on samples without any background information regarding treatment, timing, etc is dubious. However, the rapid detection by qPCR of high concentrations of Legionella - especially Legionella pneumophila - is valuable as an indicator of risk, although it may be false positive compared to culture results. On the other hand, the detection of a low number of bacteria by qPCR is a strong indication for the absence of risk.

Usefulness of real-time PCR as a complementary tool to the monitoring of Legionella spp. and Legionella pneumophila by culture in industrial cooling systems

AIMS

This study was designed to evaluate the usefulness of quantification by real-time PCR as a management tool to monitor concentrations of Legionella spp. and Legionella pneumophila in industrial cooling systems and its ability to anticipate culture trends by the French standard method (AFNOR T90-431).

METHODS AND RESULTS

Quantifications of Legionella bacteria were achieved by both methods on samples from nine cooling systems with different water qualities. Proportion of positive samples for L. pneumophila quantified by PCR was clearly lower in deionized or river waters submitted to a biocide treatment than in raw river waters, while positive samples for Legionella spp. were quantified for almost all the samples. For some samples containing PCR inhibitors, high quantification limits (up to 4·80 × 10(5) GU l(-1) ) did not allow us to quantify L. pneumophila, when they were quantified by culture. Finally, the monitoring of concentrations of L. pneumophila by both methods showed similar trends for 57-100% of the samples.

CONCLUSIONS

These results suggest that, if some methodological steps designed to reduce inhibitory problems and thus decrease the quantification limits, could be developed to quantify Legionella in complex waters, the real-time PCR could be a valuable complementary tool to monitor the evolution of L. pneumophila concentrations.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study shows the possibility of using real-time PCR to monitor L. pneumophila proliferations in cooling systems and the importance to adapt nucleic acid extraction and purification protocols to raw waters.