7th joint conference "Joint Microbiology & Infection Conference 2024" of the DGHM and VAAM

Abstract: Our knowledge on bacteria in drinking water is mainly limited to cold water (potable water cold, PWC). Bacteria in hot water (potable water hot, PWH) are understudied as routine diagnostics does typically not include incubation of nutrient agar plates with PWH samples at elevated temperatures like e.g. 50°C or 60°C. These temperatures would however be needed to enable growth and detection of thermotolerant bacteria that do not form colonies at 22°C or 36°C. Recently the bacterial diversity of PWH received more attention. We established a flow cytometry-based method to rapidly assess the heat tolerance of bacteria subjected to a heat gradient. The indirect viability criteria comprised maintenance of (1) cellular integrity, (2) enzymatic activity and (3) intracellular homeostasis when challenged with an acidic pH. When applied to different Legionella species, results demonstrated that the group of Legionella is highly diverse and that different Legionella differ greatly in their heat resistance. Legionella micdadei or representatives of Legionella pneumophila displayed a substantially greater resistance to heat than for example Legionella anisa or Legionella dumoffii. When applied to real water samples, bacteria in the hot water circulation displayed much greater thermotolerance than bacteria contained in cold drinking water. Also, the passage of PWC through a tube reactor kept at 36°C markedly increased the number of bacteria resistant to elevated temperatures. Apart from quickly assessing the heat tolerance of individual species or entire water microbiomes, the method is useful as a diagnostic guide when performing thermal disinfection and when being challenged with the question which temperature is needed and how long it should be maintained to achieve a satisfactory disinfection result. 

Andreas Nocker

Invited Speaker