FreeLiving Amoebae as Vectors of Human Pathogens

Authors: Patrick Scheid

Publish Date: 2015

Volume: , Issue: , Pages: 1-9

PDF Link

Abstract

Those microorganisms adapted to an intracellular life within FLA benefit from the suitable growth conditions the protection against adverse environmental conditions and may benefit from distribution with the help of their hosts or vehicles This phenomenon has led to the term “FLA as the Trojan horse of the microbial world” Barker et al 1992 Khan 2009Numerous attempts to define the relationships between the intracellular microorganisms and their hosts the FLA have not yet been adequate Terms such as symbionts endosymbionts or endobionts are not uniquely defined and differ between language areas Proposed terms such as “ARB” amoebaresistant bacteria do not include in their scope such organisms as intracellular fungi viruses or other eukaryonts Greub and Raoult 2004 Therefore we propose to name these intracellular microorganisms being associated with FLA as “endocytobionts” eg Scheid et al 2008 2010 It is expedient to call these organisms “endocytobionts” at least during the initial steps of the studies Hence we do not need to go into potential characteristics of these relationships such as parasitism phoresy zoochory or mutualism at an early stage of the study These relationships can also be variable depending on the temperature Legionella and Acanthamoeba as an example or other environmental conditions Endocytobionts are defined as more or less autarkic phenomena being divergent from the host genome It is therefore of minor relevance whether they emerge facultatively or obligatorily Kleinig and Sitte 1984 Endocytobiology deals with the studies and the classification of intracellular endocytobiosis In contrast symbiosis is defined as the coexistence for mutual benefit of adapted organisms symbionts of different species There exist many examples of symbiosis for example the relationship between fungi and algae lichens For most of the relationships between FLA and intracellularly replicating organisms the term “symbiosis” does not apply at all because in most of the cases the amoebae are harmed by some means In 1883 Schimper created the endosymbiont theory Schimper 1883 Basically this theory states that autoreduplicative plastids and mitochondria are derived from ancestral protozoa These protozoa were incorporated as cytosymbionts within the cell at early stages of evolution Gradually they were domesticated as organelles Cyanobacteria are seen therefore as an ancestral part of the plastids while mitochondria descend from Proteobacteria An ancestral eukaryotic cell phagocytized these bacteria without being able to lyse them or use them as food Subsequently they developed into endosymbionts Schwemmler et al 1984 Such an evolutionary process shows the adaption of endocytobionts to their hosts Greub and Raoult 2004 In particular parasites played an important role in the evolutionary process through the “arms race” with their hosts Thus endosymbiosis could have contributed to the complexity of parasitic protozoa early in the history of evolution Certain dinoflagellates are known to incorporate phototrophic and eukaryotic protozoa as endocytobionts leading to permanent endocytobiosis As some of these endocytobionts within FLA are of human pathogenic relevance the FLA therefore serve as hosts vehicles or vectors in addition to their role as potential human pathogens Scheid 2007 Scheid et al 2008 As we do not see the need to extend the term “symbiont” artificially the term “endocytobiont” is a suitable term to outline these intracellularly surviving microorganismsFLA usually uptake other microorganisms as predators by phagocytosis and food cup building and lyse them subsequently intracellularly in food vacuoles phagosomes Some of these potential prey organisms resist or escape their fate that awaits them by the exposure to oxygen radicals the acidic environment and the lysosomal enzymes in the phagosomes In 1956 an intracellular bacterium Sarcobium lyticum later known as Legionella lytica has been described in an Acanthamoeba strain isolated from soil which showed not to be culturable extracellularly in subsequent investigations and therefore was proved obligate intracellularly Drozanski 1956 1991 The most “famous” relationship in this sense is the intracellular proliferation of Legionella bacteria in acanthamoebae Rowbotham has carried out cocultivation assays of acanthamoebae and legionellae in 1980 successfully These legionellae were located intracellularly and proliferated at 35 °C in the FLA FLA serve therefore as hosts or reservoirs and vehicles of Legionella and have also a protective function Rowbotham 1980 Similar findings count for Chlamydiae which are important human obligate intracellular pathogenic microorganisms as well Michel et al 2004 2005 Three Chlamydia species are known to cause various diseases in humans so farIn addition to bacteria also viruses fungi and protozoa were found intracellularly and a proliferation in the FLA or a distribution through these FLA at least was demonstrated for overview see Greub and Raoult 2004 Scheid 2014 During the last 10 years publications on the association of viruses with FLA increased Some of these viruses were called giant viruses mainly due to their size and gene repertoire

Keywords: