Isolation of Legionella species from Noyu (unattended natural hot springs in mountains and fields) samples in Japan

Sodium levels and grazing pressure shape natural communities of the intracellular pathogen Legionella

BACKGROUND

Legionella are parasites of freshwater protozoa, responsible for Legionellosis. Legionella can be found in a variety of aquatic environments, including rivers, lakes, and springs, as well as in engineered water systems where they can potentially lead to human disease outbarks. Legionella are considered to be predominantly freshwater organisms with a limited ability to proliferate in saline environments. Exposure of Legionella to high sodium concentrations inhibits growth and virulence of laboratory strains, particularly under elevated temperatures. Nonetheless, Legionella have been identified in some saline environments where they likely interact with various protozoan hosts. In this work, we examine how these selection pressures, sodium and grazing, help shape Legionella ecology within natural environments. Utilizing Legionella-specific primers targeting a variable region of the Legionella 16S rRNA gene, we characterized Legionella abundance, diversity, and community composition in natural spring clusters of varying sodium concentrations, focusing on high sodium concentrations and elevated temperatures.

RESULTS

We observed the highest abundance of Legionella in spring clusters of high salinity, particularly in combination with elevated temperatures. Legionella abundance was strongly related to sodium concentrations. The Legionella community structure in saline environments was characterized by relatively low diversity, compared to spring clusters of lower salinity. The community composition in high salinity was characterized by few dominant Legionella genotypes, not related to previously described species. Protozoan microbial community structure and composition patterns resembled those of Legionella, suggesting a common response to similar selection pressures. We examined Legionella co-occurrence with potential protozoan hosts and found associations with Ciliophora and Amoebozoa representatives.

CONCLUSIONS

Our results indicate that selection forces in saline environments favor a small yet dominant group of Legionella species that are not closely related to known species. These novel environmental genotypes interact with various protozoan hosts, under environmental conditions of high salinity. Our findings suggest that alternative survival mechanisms are utilized by these species, representing mechanisms distinct from those of well-studied laboratory strains. Our study demonstrate how salinity can shape communities of opportunistic pathogens and their hosts, in natural environments, shedding light on evolutionary forces acting within these complex environments. Video Abstract.

Legionella Occurrence beyond Cooling Towers and Premise Plumbing

Legionella is an environmental pathogen that is responsible for respiratory disease and is a common causative agent of water-related outbreaks. Due to their ability to survive in a broad range of environments, transmission of legionellosis is possible from a variety of sources. Unfortunately, a disproportionate amount of research that is devoted to studying the occurrence of Legionella in environmental reservoirs is aimed toward cooling towers and premise plumbing. As confirmed transmission of Legionella has been linked to many other sources, an over-emphasis on the most common sources may be detrimental to increasing understanding of the spread of legionellosis. This review aims to address this issue by cataloguing studies which have examined the occurrence of Legionella in less commonly investigated environments. By summarizing and discussing reports of Legionella in fresh water, ground water, saltwater, and distribution system drinking water, future environmental and public health researchers will have a resource to aid in investigating these pathogens in relevant sources.

Investigations on Contamination of Environmental Water Samples by Legionella using Real-Time Quantitative PCR Combined with Amoebic Co-Culturing

We analyzed the contamination of environmental water samples with Legionella spp. using a conventional culture method, real-time quantitative PCR (qPCR), and real-time qPCR combined with an amoebic co-culture method. Samples (n = 110) were collected from 19 cooling towers, 31 amenity water facilities, and 60 river water sources of tap water in Japan. Legionella was detected in only three samples (3/110, 2.7%) using the culture method. The rate of Legionella detection using amoebic co-culture followed by qPCR was 74.5%, while that using qPCR without amoebic co-culture was 75.5%. A higher than 10-fold bacterial count was observed in 19 samples (19/110, 17.3%) using real-time qPCR subsequent to amoebic co-culture, compared with identical samples analyzed without co-culture. Of these 19 samples, 13 were identified as Legionella spp., including L. pneumophila and L. anisa, and the non-culturable species were identified as L. lytica and L. rowbothamii. This study showed that the detection of Legionella spp., even in those samples where they were not detected by the culture method, was possible using real-time qPCR and an amoebic co-culture method. In addition, this analytical test combination is a useful tool to detect viable and virulent Legionella spp..