Abstract

Free-living amoebae (FLA) can be found both in natural aquatic environments and in artificial, man-made aquatic environments. For a long time, (naked) FLA were considered to be harmless protozoa of soil and water. However, research since the 1960s has demonstrated that FLA can be pathogenic to humans and animals with nearly 100% morbidity from some strains. As etiological agents of the so-called Acanthamoebiasis, the Acanthamoebae can trigger several specific diseases (or symptoms) in humans. The amoebiasis of the central nervous system is called granulomatous amebic encephalitis (GAE), when Acanthamoebae are the etiological agents, showing subacute to chronic progress. GAE differs clinically from the primary amoebic meningoencephalitis (PAM(E)), which is caused by Naegleria fowleri. The Acanthamoeba keratitis is not necessarily associated with an immune suppression, but rather with a trauma, exposure to contaminated water or, particularly, the improper handling of contact lenses, which promotes infection. The clinical picture of GAE by Balamuthia mandrillaris is characterized by headache and neck stiffness. The infection is chronic and the time between infection and appearance of neurological symptoms may range from one month to about two years. The prevalence of FLA in water networks is associated with biofilms, where the amoebae live within a biocoenosis sympatric with other microorganisms. These biofilms serve as feeding grounds for the FLA and provide protection to a certain degree while the FLA adhere to the surfaces. In such a biocoenosis there are multiple interactions between FLA and other microorganisms: In addition to their role as pathogens, FLA are known to serve as host of and vehicles for diverse intracellular organisms (bacteria, viruses, eucaryonts), some of them being natural human pathogens. They act as reservoir or vehicle for various microorganisms such as various Legionella sp., Listeria monocytogenes, Pseudomonas aeruginosa and some Mycobacterium sp. In the cyst-stage of the FLA these intracellular organisms (endocytobionts) are protected to a high degree against any adverse environment (FLA as “Trojan horse”). This host-endocytobiont relationship may further lead to health risks in terms of the development of pathogenicity/virulence and antibiotic resistance (FLA as “Trainings ground”).

Highlights

  • Naked Free‐living amoebae (FLA) are protozoa that are ubiquitously distributed in soil‐ and aquatic habitats and complete their entire life cycle without a host

  • Some Acanthamoeba strains, and the two FLA‐species Balamuthia mandrillaris and Naegleria fowleri are well known as pathogens for many years

  • granulomatous amebic encephalitis (GAE) cases caused by Balamuthia mandrillaris are rare

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Summary

Introduction

Naked Free‐living amoebae (FLA) are protozoa that are ubiquitously distributed in soil‐ and aquatic habitats and complete their entire life cycle without a host They inhabit biocoenoses in natural and artificial humid environments. The biofilms occur in water supply lines/plumbings in industry, private houses, and even in healthcare areas [2] Within these biofilms FLA and other microorganisms are protected to a certain degree against harsh conditions or countermeasures, e.g., disinfection procedures [3]. FLA are not obligatory parasites, they do not need a host to survive or proliferate, but several species or strains can live parasitically in humans and animals [4] They cause different diseases with different clinical presentations which may affect the central nervous system, the eyes, or the skin of humans. In addition to their relevance as human pathogens, a range of different FLA (and amoebal stages of slime moulds) are known to act as reservoirs and carriers of other pathogenic microorganisms, hereby protecting them and interacting with them to a high degree [5]

FLA as Human pathogens
Acanthamoebae
Naegleria fowleri
Balamuthia mandrillaris
Other FLA with Pathogenic Capability Discovered to Date
FLA as Reservoirs for Human Pathogens

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