Free-Living Amoebae and Their Multiple Impacts on Environmental Health

Abstract

Free living amoebae (FLA) pose a considerable risk to environmental health and therefore have public health significance. From diverse environmental habitats in soil and water they can easily reach and adversely affect humans or animals. There are many cases providing evidence that Acanthamoebae, Naegleria fowleri, Balamuthia mandrillaris and other FLA are facultative human pathogenic microorganisms. The non-fatal and occasional fatal outcomes show paradigmatically how readily these pathogenic FLA can affect humans. A specific threat with respect to environmental health arises from the findings that FLA often serve as vectors of pathogenic microorganisms moving from the environment to humans. The so called “endocytobionts” can be transported by FLA, among them being bacteria, viruses, protozoa or fungi of public health relevance. Some of these endocytobionts are even able to proliferate within the FLA. In addition to the capability of FLA to transport these endocytobionts (phoresis), this possible intracellular lifestyle leads to an enhanced protection of these microorganisms, especially when the endocytobionts hide inside cyst-forming amoebae. The relationship between FLA and their endocytobionts has an influence on evolutionary processes and the development of (human) pathogenicity and virulence. Changes to the environment arising from nature and human activity are affecting patterns of occurrence and significance for FLA deriving diseases. Environmental and climatic changes and global warming may have an influence on the FLA abundance, which may lead to an increase of infectious diseases associated with FLA or their endocytobionts. The food-borne association of FLA as direct human pathogens and vectors of other pathogenic microorganisms is described as well. Within the environment the interactions of FLA and bacteria, viruses, and fungi are of particular interest, some of the latter being amoebophagous fungi either as parasites (endoparasitic fungi) or as predators (predaceous fungi).