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Legionella and free-living amoebae interactions: an ecological perspective of drinking water safety and controlling legionellosis

  • Author / Creator
    Shaheen, Md
  • Legionnaires’ disease (LD) outbreaks have been a growing concern for public health as well as for the water industry and building management. The number of outbreaks has significantly increased over the last decades worldwide, especially in the United States and Europe. This disease is mainly caused by a Gram-negative opportunistic water-based pathogen, Legionella pneumophila. The primary sources of human exposure to this bacterium are the aerosols from engineered water systems and drinking water premise plumbing (PP) where the pathogenic Legionella spp. regrow. Legionella spp. colonize the water-biofilms and grow intracellularly in the free-living amoebae (FLA) and other microeukaryotes that are present in built aquatic environment and even in high-quality drinking water. Therefore, the relationships of L. pneumophila with FLA and other microorganisms within the drinking water-biofilms under premise plumbing conditions were examined to aid developing safe water management strategies and better public health protection from this pathogen. This study identified a novel risk of Legionella-FLA interactions in drinking water by demonstrating that the Legionella-containing vesicles of respirable size may contain 23-873 Legionella cells per vesicle, thus could potentially serve as a single dose for human infection. Moreover, this study demonstrated that FLA-bacteria interactions may result in a natural selection of L. pneumophila within water-biofilms due to the preferential feeding behavior of FLA. These interactions also help L. pneumophila to persist within the water-biofilms for years. The feeding behavior of FLA and their reluctance to feed on L. pneumophila was later confirmed by real-time monitoring of known bacterial species. Overall, these observations led to a hypothesized growth model for pathogenic Legionella spp. in PP by suggesting uptake of Legionella spp. by FLA only under certain conditions (such as during food scarcity), followed by intracellular growth of the bacterium, lysis of the FLA host, release of the newly replicated bacterial cells to the water and re-initiating the cycle if conditions permit. Difference in water-biofilm bacterial community compositions (based on metagenomics analysis) on Cu and PVC pipe material at room temperature (RT) and 40 °C indicated that the temperature was a stronger determinant of water-biofilm bacterial community compositions than pipe material. Nonetheless, Cu pipe material supported higher colonization of Legionella, whereas PVC supported more mycobacteria, both of which are opportunistic water-based pathogens. Finally, a probiotic, biological control option for Legionella was tested and found promising. The probiome selector (naturally-developed water biofilms on virgin PVC granules) affected the downstream biofilm bacterial community and reduced the colonization of Legionella in Cu biofilms. Further optimization of the probiome (identified through vigorous microbiome screening and process optimization) is still required, but has the potential to be an alternative sustainable strategy to control Legionella spp. (instead of high-temperature or heavy chemical disinfection) in high Legionella-risk built aquatic environments. Moreover, the water-biofilm community profiles indicated that microbial community mapping could be useful to identify the vulnerable or compromised sites of a PP, and in combination with risk assessment and Hazard Analysis Critical Control Point (HACCP)-based water management approaches, it could ensure better water and public health safety.

  • Subjects / Keywords
  • Graduation date
    Spring 2021
  • Type of Item
    Thesis
  • Degree
    Doctor of Philosophy
  • DOI
    https://doi.org/10.7939/r3-c5fr-b796
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.