Abstract

Background: Biofilms, when formed on the surfaces of water pipes, can be responsible for a wide range of water quality and operational problems. We sought to assess the bacterial and free-living protozoa (FLP) diversity, in relation to the presence of Legionnaire's disease-causing bacteria Legionella pneumophila (L. pneumophila) in 45 biofilms of hot water distribution system pipes of apartment buildings in Riga, the capital city of Latvia.Results: 16S rRNA amplicon sequencing (metataxonomics) revealed that each biofilm contained 224 rather evenly distributed bacterial genera and that most common and most abundant were two genera, completely opposites in terms of their oxygen requirements: the obligately anaerobic Thermodesulfovibrio and the strictly aerobic Phenylobacterium. Water temperature and north-south axis (i.e., different primary water sources) displayed the most significant effect on the inter-sample variations, allowing us to re-construct three sub-networks (modules) of co-occurring genera, one involving (potentially FLP-derived) Legionella spp. Pangenome-based functional profile predictions suggested that all three may be dominated by pathways related to the development and maintenance of biofilms, including quorum sensing and nutrient transport, as well as the utilization of various energy sources, such as carbon and nitrogen. In our 18S rRNA amplicon sequencing data, potential hosts of L. pneumophila were detected in 11 out of 12 biofilm samples analyzed, however, in many cases, their relative abundance was very low (<1%). By validating our findings using culture-based methods, we detected L. pneumophila (serogroups 2, 3, 6 and 9) in nine (20%) biofilms, whereas FLP (mostly Acanthamoeba, Vahlkampfidae and Vermamoeba spp.) were present in six (~13%) biofilms. In two biofilms, L. pneumophila and its potential hosts were detected simultaneously, using culture-based methods.Conclusions: Overall, our study sheds light on the community diversity of hot water biofilms and predicts how several environmental factors, such as water temperature and source might shape it.

Highlights

  • Biofilms, when formed on the surfaces of water pipes, can be responsible for a wide range of water quality and operational problems

  • Biofilms represent a potential reservoir for pathogens, including several genera that belong to the bacterial phyla Proteobacteria, Actinobacteria, Firmicutes, Verrucomicrobia, Nitrospirae and Bacteroidetes (Liu et al, 2013), whereof Firmicutes are the dominant gram-positive bacteria (Wolf et al, 2004) and Proteobacteriare present the major phylum of gram-negative bacteria (Vaz-Moreira et al, 2017)

  • Whether Legionella spp persist freely within biofilms growing to high numbers or within various free-living protozoan (FLP) hosts is still controversial and may be dynamic influenced by many poorly understood factors, such as water temperature, availability of bacterial prey or presence of potential FLP hosts, as well as the concentration of Legionella spp, as, when reaching high concentrations in their hosts, Legionella may be released in different forms, including free cells, cells within biofilm fragments, or secreted vesicles

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Summary

Introduction

Biofilms, when formed on the surfaces of water pipes, can be responsible for a wide range of water quality and operational problems. Domestic water pipe biofilms can be responsible for a wide range of water quality and operational problems, as microorganisms in biofilms tend to display more resistance to antibiotics and disinfecting agents. Stagnant water with excess nutrients, lack of chemical disinfectants, as well as certain surface materials are some of the conditions that promote the growth of and influence the complex interactions within biofilms, including their associated FLP, and Legionella spp. Stagnant water with excess nutrients, lack of chemical disinfectants, as well as certain surface materials are some of the conditions that promote the growth of and influence the complex interactions within biofilms, including their associated FLP, and Legionella spp. (Academies, 2019)

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