Legionella in drinking water: the detection method matters

Area of Science:

• Environmental microbiology
• Public health
• Water quality analysis

Background:

• Legionella concentrations in drinking water plumbing systems (DWPS) are regulated internationally, with varying action levels.
• Germany's Federal Environment Agency (UBA) revised its recommended methods for Legionella detection (ISO 11731) in March 2019, altering culture media.
• This change impacts the detection spectrum of Legionella colonies in drinking water.

Purpose of the Study:

• To assess the impact of the revised UBA recommendation on the evaluation of Legionella contamination in DWPS.
• To compare Legionella detection rates and action level exceedances before and after the method change.
• To determine if current action levels accurately reflect health risks under the new methodology.

Main Methods:

• Analysis of 17,270 individual drinking water samples from routine monitoring over 6 years.
• Comparison of Legionella detection results and action level exceedances using pre- and post-March 2019 methods.
• Statistical assessment of the significance of changes in reported exceedances.

Main Results:

• The revised UBA method resulted in a significantly higher frequency of reported action level exceedances.
• The alteration in culture media broadened the detection spectrum, leading to more positive results.
• A substantial increase in reported exceedances was observed under the new detection protocol.

Conclusions:

• The revised Legionella detection method leads to more frequent action level exceedances in German DWPS.
• Current action levels are likely inadequate for the new detection method, potentially overestimating health risks.
• A revision of the action levels is necessary to prevent a misleading impression of increased health risks from Legionella in drinking water.