We have also observed that MIFs are significantly
more infectious in human pneumocyte cells compared with SPFs. These results strongly suggest a potential role of ciliates in increasing the risk of legionellosis. Legionella pneumophila, a ubiquitous gram-negative freshwater bacteria, is an intracellular pathogen of freshwater amoeba that, when aerosolized, can cause DNA Damage inhibitor a severe pneumonia known as legionellosis or Legionnaires’ disease in susceptible individuals (Fields et al., 2002). Legionellosis is considered an environmental disease because person-to-person transmission does not occur. Therefore, transmission of legionellosis is primarily linked to man-made devices (e.g. cooling towers, whirlpool
spas) that produce aerosols from warm water contaminated with Legionella. The relationship between L. pneumophila and protozoa has been described as very important BAY 80-6946 order for two main reasons: (i) protozoa provide protection against environmental stresses (Barbaree et al., 1986) and (ii) protozoa, particularly amoeba, provide the principal natural haven for Legionella replication (Rowbotham, 1980; Borella et al., 2005). In this respect, it is known that L. pneumophila multiplies inside free-living amoebae and could be released as free bacterial cells or as groups of cells enclosed in vesicles (for recent reviews see Borella et al., 2005; Bichai et al., 2008). The role of vesicles as complex infectious particles has been hypothesized to be important in the transmission of L. pneumophila and legionellosis (Rowbotham, 1983). Tetrahymena spp. are ciliated protozoa that, depending on the incubation temperature, can support the growth of Legionella (Fields et al., 1984; Barbaree et al., 1986; Berk et al., 2008). In the species Tetrahymena tropicalis, L. pneumophila is efficiently ingested but does not replicate inside food vacuoles, in spite of resisting Chlormezanone digestion.
Consequently, live L. pneumophila resides transiently (1–2 h) in the food vacuoles before being expelled in the form of pellets. Legionella pellets are clusters of up to 100–200 L. pneumophila cells kept together by outer membrane fragments derived from a few digested legionellae reflecting massive ingestion by Tetrahymena, and perhaps a ciliate-derived material from the lumen of food vacuoles (Berk et al., 2008). In addition, the surviving L. pneumophila cells present in the pellets expelled by T. tropicalis have all the morphological characteristics of mature intracellular forms (MIFs) (Faulkner et al., 2008), initially described in HeLa cells (Garduno et al., 2002). In a previous study, we observed that passage of L. pneumophila in free-living amoebae produces legionellae able to survive numerous adverse conditions such as starvation and antibiotic presence (Bouyer et al., 2007). The aim of this study was to determine whether passage of L.