Biological characteristics and pathogenicity of Acanthamoeba

Abstract

Acanthamoeba is an opportunistic protozoa, which exists widely in nature and is mainly distributed in soil and water. Acanthamoeba usually exists in two forms, trophozoites and cysts. The trophozoite stage is one of growth and reproduction while the cyst stage is characterized by cellular quiescence, commonly resulting in human infection, and the lack of effective monotherapy after initial infection leads to chronic disease. Acanthamoeba can infect several human body tissues such as the skin, cornea, conjunctiva, respiratory tract, and reproductive tract, especially when the tissue barriers are damaged. Furthermore, serious infections can cause Acanthamoeba keratitis, granulomatous amoebic encephalitis, skin, and lung infections. With an increasing number of Acanthamoeba infections in recent years, the pathogenicity of Acanthamoeba is becoming more relevant to mainstream clinical care. This review article will describe the etiological characteristics of Acanthamoeba infection in detail from the aspects of biological characteristic, classification, disease, and pathogenic mechanism in order to provide scientific basis for the diagnosis, treatment, and prevention of Acanthamoeba infection.

Keywords: Acanthamoeba; biological characteristics; classification; disease; pathogenesis.

FIGURE 1

Number of publications related to Acanthamoeba. Increasing scientific interest in the field of Acanthamoeba as determined by published articles cataloged in PubMed over time.

FIGURE 2

The structure of Acanthamoeba. (A) Scanning electron micrograph of an Acanthamoeba trophozoite showing many spinous pseudopods around the whole surface of the cell. Acanth, acanthopod. (B) Transmission electron micrograph of the trophozoite stage of Acanthamoeba. Nu, nucleus; V, vacuoles; M, mitochondria; PM, plasma membrane. (C) Transmission electron micrograph of an Acanthamoeba cyst. OL, outer layer; IL, inner layer. These pictures are from the work of Siddiqui et al. (2012).

FIGURE 3

Phylogenetic relationship of 49 various genotypes or subtypes of Acanthamoeba T1-T23 based on “complete” 18S rRNA gene sequence. The tree was constructed using the Neighbor-Joining algorithm in MEGA 4.

FIGURE 4

Host-Parasite interaction molecular signaling pathways. Adhesion through spinous pseudopodia and adhesin (such as MBP) is the basis for Acanthamoeba to establish infection. Once adhesion is completed, the intracellular signal transduction process is activated and further triggers cascade effects such as phagocytosis of target cells, secretion of protease, and apoptosis, resulting in direct pathological damage. Phagocytosis: There are three fully studied pathways involved in this process. The RhoA pathway, which leads to stress fiber formation; Rac1 activation, which triggers plate foot formation; and Cdc42 activation, which promotes filamentous foot formation. Apoptosis: There are at least three pathways related to apoptosis after infection of Acanthamoeba. Ecto-ATPases driven cell apoptosis; Interference with the expression of important genes that regulate cell cycle; and the phosphatidylinositol 3-kinase (PI3K) mediated apoptosis pathway. Lysis: Acanthamoeba secretes a variety of proteases involved in cell lysis. A serine protease (MIP133) has been identified as a key component in the pathogenesis of Acanthamoeba.