Acanthamoeba spp.: "Opportunistic Pathogens"

Members of the genus Acanthamoeba are free-living amebae that are major predators of bacteria. They serve as important laboratory models for understanding the cellular biology and biochemistry of locomotion, phagocytosis, and cell differentiation. Some species of Acanthamoeba can be opportunistic human pathogens, causing amebic meningoencephalitis and keratitis. There also is reason to think that they may be symbionts of plants. Human keratitis caused by Acanthamoeba spp. has been clearly linked to contact lens wear. Once transmitted by the lens to the surface of the cornea, acanthamebas are able to attach to the surface by using junctional complexes formed between trophozoites and surface epithelial cells, as well as by manipulation of the acanthopodia between cell junctions (possibly seeking bacteria). Penetration of the cornea beyond the surface squamous layer may involve both elaboration of enzymes and active phagocytosis. Acanthamoeba spp. are now considered to be an important group of opportunistic protozoan pathogens, but recognition of this role has evolved but slowly during the (approximately) 100 years since their discovery (Ma et al., 1990; Martinez, 1985). The first observations of the free-living amebas were made by Von Rosenhof (1755). The amebas he observed were large enough so that his observations were made without the aid of a microscope. His illustrations of Die Kleine Proteus were used by Linneaus (1758), who described the amebas as Volvox chaos. Not until microscopical techniques became available were the smaller flattened forms and limax (monopodial cylindrical) amebas discovered. Dujardin (1841) was among the first to observe them. He found numerous examples in 1 Past-Presidential address presented 28 December 1990 at the Annual Meeting of the American Microscopical Society in San Antonio, Texas. 2 I thank Jerry Y. Niederkorn, James P. McCulley, Robert Silvany, YuGuang He, Joel Dougherty, and Mary Beth Moore, of the Department of Ophthalmology, University of Texas Southwestern Medical School, for their hospitality in their laboratories during my sabbatical leave. I also thank Tom Sawyer and Don Munson for their comments and encouragement. Publication costs, in part, are being met by a grant from the Spencer-Tolles Fund of the American Microscopical Society. TRANS. AM. MICROSC. Soc., 110(4): 289-299. 1991. ? Copyright, 1991, by the American Microscopical Society, Inc. This content downloaded from 207.46.13.16 on Thu, 16 Jun 2016 06:14:25 UTC All use subject to http://about.jstor.org/terms TRANS. AM. MICROSC. SOC. water samples that he collected from the river Seine, and he was the first scientist to apply the name Amoeba limax. That small amebas also were recovered from plants may be noteworthy. Musgrave & Clegg (1904, as cited by Franchini, 1922a), working in the Philippines, described Ameiba which occurred on the surface of lettuce plants having a milky sap. These researchers cultivated the amebas (probably a mixed culture that contained some Entamoeba histolytica) and fed them to monkeys, two of whom developed an amebic dysentery. Wells (1911) also contributed significantly to early literature on amebas by isolating cysts from dust in the air, demonstrating that they could be transferred in this manner. Some of the small amebas were placed in the genus Hartmannella by Alexeieff (1912a,b), who used H. hyalina Dangeard, 1900 as the type species. He included the small amebas that divided by conventional mitosis in this genus but recognized other genera as well. Aware of the earlier report (probably resulting from an error in identification and culturing) by Musgrave and Clegg, Franchini (1922a) examined the latex of many plants in France, including those belonging to the Asclepiadaceae (Milkweed family); Apocynaceae (Dogbane family), Compositae (Composite family) Tribe Cichorieae (Cichory and Lettuce tribe), Euphorbiaceae (Spurge Family), and the Utricaceae (Nettle Family). He isolated numerous amebas that he compared to the ones found earlier by Musgrave and Clegg. Following Musgrave and Clegg, Franchini (1922a) named the species of ameba amiba lactucae n. sp. Franchini was one among many individuals who believed that he had discovered the potential of free-living amebas to become opportunistic pathogens. He noted that when amebas were cultured in a medium containing horse erythrocytes they were able to phagocytize them (Franchini, 1922b). Not long after, Castellani (1930) reported amebas growing in cultures of yeast, and, later in the same year, Douglas (1930) named this ameba H. castellanii. Volkonsky (1931) recognized the variety of amebas in the genus Hartmannella and subdivided them into three genera: Hartmannella, Acanthamoeba, and Glaeseria. The acanthamebas possessed a specialized organelle called an acanthopodium that appears to be an adaptation for feeding on bacteria in the soil. The acanthopodia can be manipulated around soil particles to ferret out bacteria. Volkonsky's work in taxonomy was challenged by several workers leading to confusion that has persisted to the present time [see Visvesvara & Balamuth (1975) for a review]. The genus Acanthamoeba was described in 1931 and the genus Hartmannella in 1912; however, some of the literature prior to 1931 referred to those amebas, now recognized as members of the genus Acanthamoeba, as belonging to the genus Amoeba, simply referred to as limax amebas. Acanthamoeba polyphaga, for example, was named Amoeba polyphaga when described in 1913. Pussard (1966) and Page (1967a,b) recognized the unique cyst structure and the specialized pseudopodia termed acanthopodia, and correctly reestablished the genus Acanthamoeba. Nevertheless, there continue to be many reports in 290 This content downloaded from 207.46.13.16 on Thu, 16 Jun 2016 06:14:25 UTC All use subject to http://about.jstor.org/terms VOL. 110, NO. 4, OCTOBER 1991 the literature referring incorrectly to various species of Hartmannella (and not Acanthamoeba) that cause disease in humans. Finally, A. castellanii has been claimed to be isolated from the latex of subterranean portions of Euphorbia marginata (Euphorbiaceae) by Troll (1966). He also reported isolating a bacillus, Bacillus megaterium, from the latex of all parts of the plant. Probably, the acanthamebas may have been feeding on the bacteria. He also found trophozoites of A. castellanii in the alimentary canal of many larvae of a beetle, Tetraopes tetraophthalmus (Coleoptera: Cerambycidae). The beetle may have served as a vector for the transfer of acanthamebas between plants. Cysts of A. castellani also were isolated from the soil around the plants. Troll's observations have not been repeated, and he has not published more on this subject. Quite unlikely is the possibility that the amebas examined by Mesgrave and Clegg, Franchini, Castellani, Douglas, and Troll actually represent the same species. The small size of the limax amebas made them difficult to study until modern microscopical techniques, including scanning and transmission electron microscopy, were developed. Using such instruments, development of better standardized culture conditions, and cloning procedures allowed these species of amebas to be reassigned to several genera, including the genera Hartmannella, Acanthamoeba, and Mayorella. Page (1967a,b) carefully redefined the genus Acanthamoeba, and Sawyer & Griffin (1975) proposed the family Acanthamoebidae. This family currently embraces the genera Acanthamoeba, Comandonia, and Protacanthamoeba. The genus Comandonia contains only a single species and Protacanthamoeba contains only two. These genera can be distinguished from each other by mainly ultrastructural characteristics. The genus Acanthamoeba contains as many as 18 different species (Pussard & Pons, 1977), which are arranged into three groups based on morphology of cysts (Table I). Species of Acanthamoeba have long been recognized as important laboratory models (friends) by scientists concerned with cellular biology. Whereas the largest amebas have been used in studies of nuclear transfers, they never have been cultured axenically. By contrast, the smallest amebas are cultured easily, and their rapid growth in chemically defined media or broth, as well as their relatively large size, make the acanthamebas ideal models in many investigations. Moreover, the smaller amebas display active motility and can be readily induced to phagocytize a variety of objects. They also can be easily induced to undergo synchronous cell differentiation, another useful characteristic for many studies (see Byers, 1979).