Increasing importance of Balamuthia mandrillaris

Abstract

Balamuthia mandrillaris is an emerging protozoan parasite, an agent of granulomatous amoebic encephalitis involving the central nervous system, with a case fatality rate of >98%. This review presents our current understanding of Balamuthia infections, their pathogenesis and pathophysiology, and molecular mechanisms associated with the disease, as well as virulence traits of Balamuthia that may be potential targets for therapeutic interventions and/or for the development of preventative measures.

FIG. 1.

(A) Traditional classification scheme for free-living amoebae, based largely on morphological characteristics. (B) Present classification scheme for protozoan pathogens, based largely on their genetic relatedness. It is noteworthy that some taxonomists have placed free-living amoebae (Naegleria and Acanthamoeba) within the kingdom Euglenozoa, based on 18S rRNA sequences.

FIG. 2.

Life cycle of Balamuthia mandrillaris. The infective form of B. mandrillaris, also known as trophozoites, exhibits distinct morphological characters under a phase-contrast microscope. Under harsh conditions, trophozoites differentiate into cysts. Although cysts are tripartite (when observed under an electron microscope), under an optical microscope only two layers are observed. Bars, 10 μM.

FIG. 3.

Transmission electron micrographs of a Balamuthia mandrillaris cyst. The wall is made up of a thin, wavy ectocyst, a fibrous mesocyst, and a thick, round endocyst. The cytoplasm is filled with numerous pinocytotic vacuoles and/or vesicles as well as mitochondria. In the lower panel, chromosomes are seen in the nucleoplasm.

FIG. 4.

Balamuthia mandrillaris-induced pRB dephosphorylation in HBMEC, using Western blotting (WB) assays. The HBMEC were grown in 60-mm dishes and incubated with B. mandrillaris (approximately 2 × 10⁶ amoeba) for up to 60 min (A) or up to 3 h (B). Proteins were immunoprecipitated (IP) with anti-phospho-pRb antibodies and immunoblotted with anti-pRb antibody. In controls, proteins were immunoprecipitated and immunoblotted with anti-pRb antibody. Note that B. mandrillaris induced pRb dephosphorylation in HBMEC in a time-dependent manner.

FIG. 5.

Balamuthia mandrillaris-induced perturbation of the tight junction barrier. Balamuthia mandrillaris amoebae were incubated with confluent HBMEC monolayers for various times, and cells were lysed with RIPA lysis buffer. Equal amounts of cell lysates were used for Western blotting (WB) assays using anti-ZO-1 (A), anti-occludin (B), and anti-claudin (C) antibodies. Note that B. mandrillaris induced a loss of ZO-1 and occludin within 40 min but had no effect on claudin-1. Results are representative of three independent experiments.