FREE-LIVING AMOEBA: INTERACTIONS WITH ENVIRONMENTAL ...

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172 Harfendosymbiosis occurred in 14 of 57 axenically grown Acanthamoeba isolates; theendosymbionts comprised Gram negative rods and cocci. Michel et al (1994) gaveanother example for bacterial obligate parasites in Acanthamoeba. Jeon (1983)described a symbiosis with a dependency of the amoeba on the presence ofharboured bacteria; prokaryotic xenosomes, from initially virulent for the hostcell, become vital for the latter.Amoeba can use bacterial metabolism to protect themselves against xenobioticpollutants. Our group showed a nonobligate or transitory endocytobiosis, whereamoeba acquired Hg-resistance by using harboured resistant bacteria (Hagneréand Harf, 1993) (Fig. 1).On the other hand bacteria use amoeba as host; bacterial multiplication in amoebais dependent on an intracellular environment and not on soluble nutrients secretedby amoeba (King et al., 1991). Bacteria also use amoeba as dispersal vector andprotection system against fluctuating environmental parameters (Harf andMonteil, 1988). Several authors have shown that bacteria grown in free-livingamoeba are less susceptible to chlorination and biocides (Barker et al., 1992;Kilvington and Price, 1990; King et al., 1988). According to King et al. (1988),resistance to digestion by predatory protozoa might be an evolutionary precursorof bacterial pathogenicity and a survival mechanism for bacteria in aquaticenvironment.PATHOGENIC BACTERIAAs the cell-cycle of Acanthamoeba consists of growth and replication by binaryfission under optimal growth conditions and encystment under environmental stress,harboured bacteria are thus disseminated and are protected from chemical inactivationin cysts. Jadin (1973), isolating Mycobacteria from free-living amoeba, foresaw theirpart as hosts for virus and bacteria. Free-living amoeba were incriminated in theecology and epidemiology of legionellosis (Fields, 1991; Rowbotham, 1984; Tyndalland Domingue, 1982). They are also likely to be natural hosts for well-knownpathogens like Vibrio cholerae and Listeria monocytogenes, and nosocomial newpathogens, i.e. Pseudomonas sp., Xanthomonas maltophilia, Flavobacterium sp..LegionellapneumophilaLegionella appear to have the capacity to survive as intracellular parasites offree-living protozoa as well as intracellular pathogens in humans. Legionellosisconsists of two distinct clinical syndromes, Legionnaire's disease and Pontiac fever.Legionnaire's disease is characterized by pneumonia while Pontiac fever is a self-
Endocytobiosis & Cell Res. 10, 167-183 (1993/94) 173limited, non-pneumonic, respiratory illness. In the pneumonic form of legionellosis,the bacteria multiply intracellularly in the alveolar macrophages of an infected oftenimmunodepressed host. This ability to multiply intracellularly is considered thebacterium's primary virulence factor.Inhalation of aerosols containing Legionella is presumed to be the primarymeans of acquiring legionellosis (Fields, 1991). Known sources for transmission ofLegionella via aerosols have been identified: showers, respiratory care equipment,air-conditioning cooling-towers, whirlpool baths, ultrasonic mist machines, lakes andrivers (Breiman, 1993). Legionella are ubiquitous in freshwater environment andwere isolated from a number of aquatic habitats. The contrast between the ubiquity ofthese bacteria and the fastidiousness of their recovery from environmental samples,added to the fact that their growth in the environment in the absence of protozoa hasnot been documented, led to the assumption that protozoa are their primary means ofproliferation in nature (Fields, 1993; Rowbotham, 1980). Rowbotham (1980) was thefirst to report phagocytosis of L. pneumophila by amoeba and suggested that avacuole or an amoeba, full of legionellae might be the infective particle for man ratherthan free legionellae themselves.Several studies confirmed the intracellular growth of Legionella in variousprotozoa (Anand et al., 1984; Fields et al., 1984; Tyndall and Domingue, 1982). FiveCATEGORYAmoebaORGANISMAcanthamoeba castellaniiA. polyphagaA. palestinensisA. royrebaA. culbertsoniNaegleria gruberiN. fowleriN. lovaniensisN. jadiniHartmannella vermiformisH. cantabrigiensisVahlkampfia jugosaEchinamoeba exundansCiliated protozoa(Fields, 1993)TetrahymenaT. voraxpyriformisTable 1: Protozoa supporting the growth of Legionella
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