Analysis of the Legionella longbeachae Genome and Transcriptome Uncovers Unique Strategies to Cause Legionnaires' Disease

Legionella pneumophila and L. longbeachae are two species of a large genus of bacteria that are ubiquitous in nature. L. pneumophila is mainly found in natural and artificial water circuits while L. longbeachae is mainly present in soil. Under the appropriate conditions both species are human pathogens, capable of causing a severe form of pneumonia termed Legionnaires' disease. Here we report the sequencing and analysis of four L. longbeachae genomes, one complete genome sequence of L. longbeachae strain NSW150 serogroup (Sg) 1, and three draft genome sequences another belonging to Sg1 and two to Sg2. The genome organization and gene content of the four L. longbeachae genomes are highly conserved, indicating strong pressure for niche adaptation. Analysis and comparison of L. longbeachae strain NSW150 with L. pneumophila revealed common but also unexpected features specific to this pathogen. The interaction with host cells shows distinct features from L. pneumophila, as L. longbeachae possesses a unique repertoire of putative Dot/Icm type IV secretion system substrates, eukaryotic-like and eukaryotic domain proteins, and encodes additional secretion systems. However, analysis of the ability of a dotA mutant of L. longbeachae NSW150 to replicate in the Acanthamoeba castellanii and in a mouse lung infection model showed that the Dot/Icm type IV secretion system is also essential for the virulence of L. longbeachae. In contrast to L. pneumophila, L. longbeachae does not encode flagella, thereby providing a possible explanation for differences in mouse susceptibility to infection between the two pathogens. Furthermore, transcriptome analysis revealed that L. longbeachae has a less pronounced biphasic life cycle as compared to L. pneumophila, and genome analysis and electron microscopy suggested that L. longbeachae is encapsulated. These species-specific differences may account for the different environmental niches and disease epidemiology of these two Legionella species.
Author Summary Legionella longbeachae, found in potting soil, and L. pneumophila, present in aquatic environments, are opportunistic human pathogens that cause Legionnaires' disease, a severe and often fatal pneumonia. The analysis and comparison of the genome sequences of four L. longbeachae genomes together with the study of its gene expression program and virulence pattern in different infection models provides important new insight on the organism's lifestyle and virulence strategies. L. longbeachae harbors a unique repertoire of secreted substrates, many of which encode eukaryotic like domains that may help the pathogen to subvert host functions and cause disease. Curiously, L. longbeachae may also be able to interact with plants. Several proteins present mainly in plants and phytopathogenic bacteria and several enzymes that might confer the ability to degrade plant material were identified in its genome. Interestingly, L. longbeachae encodes a chemotaxis system but no flagella, in contrast L. pneumophila encodes flagella but no chemotaxis system. It will be an interesting aspect of future research to understand these peculiarities. Finally, the genome sequence and analysis reported here will aid in understanding how L. longbeachae causes disease and will open new possibilities to develop tools for rapid identification and risk prediction of L. longbeachae infection.