Free-living amoebae (FLA) are found in soil and water habitats throughout the world. These amoebae ingest bacteria, yeast, and other organisms as a food source. Unlike “true” parasites, pathogenic FLA can complete their life cycles in the environment without entering a human or animal host. Of the many FLA that are found in the environment, four genera of FLA have been associated with human disease. One species of Naegleria
, N. fowleri,
one species of Balamuthia, B. mandrillaris
, and several species of Acanthamoeba
can cause fatal central nervous system (CNS) infections. In addition to a CNS infection, Acanthamoeba
can cause cutaneous lesions and Amoebic Keratitis, a sight-threatening infection of the cornea [1
]. Human infections with these amoebae have been reported from all over the world [3
]. More recently, a newly recognized pathogen was detected in brain tissue of a patient with CNS symptoms who survived the infection. Although the amoebae was not isolated from the patient, it was identified by light and electron microscopy as a species of Sappinia
sp. has not been shown to be lethal in humans or experimental animals [9
].shows the morphology of trophozoites of N. fowleri
(A), Acanthamoeba spp.
(B), and B. mandrillaris
(C), by scanning electron microscopy.
Fine morphology of trophozoites of N. fowleri, (a) Acanthamoeba spp., (b) and B. mandrillaris, (c) by Scanning Electron Microscopy. Bars represent 5 μm.
Pathogenic FLA can be isolated from freshwater lakes, thermally polluted waters, sediment, thermal springs, swimming pools, soil, air conditioning vents, air, and the domestic water supply [3
]. In addition to causing human disease, FLA also can harbor intracellular pathogenic bacteria such as Legionella pneumophila
and may serve as vectors of bacterial infections in humans [15
]. A number of clinical FLA isolates from corneal and cutaneous lesions have been shown to harbor bacterial endosymbionts or pathogens [16
]. Laboratory studies, also, have shown that a number of pathogenic bacteria including Mycobacterium avium, Burkholderia spp.
, Escherichia coli O157:H7
, and Vibrio cholerae
, can survive and multiply in FLA [6
]. Intracellular growth of bacteria within amebae has been shown to increase bacterial resistance to antibiotics and to biocides, and to increase bacterial virulence [20
Since the majority of FLA infections are fatal and diagnosed postmortem, it is important to recognize the diseases and to develop more rapid diagnostic methods. The mode and pathogenesis of infection differ for each of the FLA that cause human infections.