Abstract
The Legionella genus comprises more than 60 species. In particular, Legionella pneumophila is known to cause severe illnesses in humans. Legionellaceae are ubiquitous inhabitants of aquatic environments. Some Legionellaceae are motile and their motility is important to move around in habitats. Motility can be considered as a potential virulence factor as already shown for various human pathogens. The genes of the flagellar system, regulator and structural genes, are structured in hierarchical levels described as the flagellar regulon. Their expression is modulated by various environmental factors. For L. pneumophila it was shown that the expression of genes of the flagellar regulon is modulated by the actual growth phase and temperature. Especially, flagellated Legionella are known to express genes during the transmissive phase of growth that are involved in the expression of virulence traits. It has been demonstrated that the alternative sigma-28 factor is part of the link between virulence expression and motility. In the following review, the structure of the flagellar regulon of L. pneumophila is discussed and compared to other flagellar systems of different Legionella species. Recently, it has been described that Legionella micdadei and Legionella fallonii contain a second putative partial flagellar system. Hence, the report will focus on flagellated and non-flagellated Legionella strains, phylogenetic relationships, the role and function of the alternative sigma factor (FliA) and its anti-sigma-28 factor (FlgM).
Highlights
AND OVERVIEWThe Legionellaceae family consists of a single genus: Legionella that comprises more than 60 species so far (Gomez-Valero et al, 2009; Bajrai et al, 2016; Khodr et al, 2016)
The review aimed to summarize knowledge gained about flagella and the flagellar regulon of different Legionella species
The majority of Legionella species exhibit genes encoding for a functional flagellum and they are flagellated (Elliott and Johnson, 1981; Bornstein et al, 1991; Bangsborg et al, 1995; Heuner et al, 1995)
Summary
The Legionellaceae family consists of a single genus: Legionella that comprises more than 60 species so far (Gomez-Valero et al, 2009; Bajrai et al, 2016; Khodr et al, 2016). New species are identified continuously (i.e., Legionella drancourtii, Legionella gresilensis, and Legionella beliardensis), extending the list of known Legionella species (Lo Presti et al, 2001; La Scola et al, 2004; Gomez-Valero et al, 2009; Rizzardi et al, 2015; Bajrai et al, 2016; Khodr et al, 2016). More than 20 pathogenic Legionella species are known today that differ in their ability to infect hosts and to cause severe to mild diseases in humans (Rizzardi et al, 2015). Human pathogens known to cause the Legionnaires’ disease—an atypical pneumonia—are for instance Legionella pneumophila, Legionella micdadei, and Legionella longbeachae (Yu et al, 2002; Whiley and Bentham, 2011). Legionella known to cause the Pontiac fever—a mild flu-like disease—are for instance Legionella feelei, L. micdadei and Legionella anisa, and L. pneumophila. Often Legionella strains of the same species and same serogroup cause one of the mentioned diseases (Swanson and Hammer, 2000; Fields et al, 2002; Wang et al, 2015)
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