Pythium insidiosum

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Classification

Higher order taxa

Eukaryota; Stramenopiles; Oomycetes; Pythiales; Pythiaceae; Pythium

Species

NCBI: [1]

Pythium insidiosum

Description and Significance

Pythium insidiosum is a pathogenic species of water mold (oomycete) that infects humans and animals, primarily horses, cattle, dogs, and cats [2]. This microorganism is usually found in wet, swampy areas with dense vegetation [2]. Exposure to this organism occurs via contact with the water in which it lives as well as plants it is living on. Infection by this pathogen leads to a disease called Pythiosis, which is characterized by large lesions on the skin and in tissues such as the gastrointestinal tract; it also has a high mortality rate [3]. The issue of high mortality makes this pathogen a public health concern because infection advances quickly once it occurs; thus, early detection is crucial. P. insidiosum is well adapted to invading and thriving in its mammalian and plant host’s tissues; its life cycle and motile zoospores increase its pathogenicity as well as the rate at which infection progresses. To address this issue, substantial research has been conducted on early detection methods using polymerase chain reaction, antigen identification, and other molecular and serological assay techniques [2] [4] [5]. Scientists have information about the organism itself in terms of its morphology and pathogenicity, yet more information is needed for early detection of this organism and rapid identification in a host, as well as genetic and structural information regarding this organism’s genome. P. insidiosum currently kills large amounts of livestock and food crops in tropical developing nations, leading to high rates of infection and death in humans living in countries such as Thailand where agricultural land is swamp-like [5] [6], and affects domesticated pets in southern parts of the United States [6]. These events are presenting both economic and public health concerns that call for immediate attention.

The Organism

Genome Structure

The genome of P. insidiosum has not been sequenced [6]. Genomes of oomycete clades vary widely in size, some containing 19 to 21 million base pairs while others contain between 220-280 million base pairs, many of which make up non-coding DNA [7]. However, genomic sequencing of other species in the genus Pythium (to which this organism belongs) has revealed that some organisms in this genus have genomes that contain around 43 million base pairs with about 15,000 protein-coding genes with only 7 percent of the genome being repetitive DNA sequences [7]. As an oomycete, P. insidiosum is eukaryotic and its DNA is packed into chromosomes in a membrane-bound nucleus. Additionally, this organism is capable of carrying out horizontal gene transfer with some fungal species [8].

Cell Structure

This organism forms hyphae, which is a common structure of fungi; however this organism is not a true fungus for several reasons [9]. First, the cell wall does not contain chitin, but instead is composed of cellulose and β-glucans [9]. Second, the plasma membrane of the cells lacks ergosterol, which is a compound commonly found in fungi that helps convert alcohols into a usable form of vitamin D2 under ultraviolet light [2]. Third, this organism’s cells are diploid in its vegetative state, while fungi are usually haploid in their vegetative state [10]. In fact, this organism, as an oomycete, is more closely related to kelp, specifically brown algae of the phylum Phaeophyta [8]. As for cell morphology and components, the cells of this organism are eukaryotic and have a round and spherical shape that includes many filamentous projections, vacuoles, a cell wall, and membrane bound organelles such as mitochondria, endoplasmic reticulum, and others like those found in other eukaryotes [6]. Broad hyphae that are arranged at right angles projecting off of the main cell body are a distinguishing morphological characteristic of this organism [9].

Life Cycle and Zoosporogenesis

Pythium insidiosum is capable of reproducing asexually in plant and animal tissues as well as in aquatic environments, by producing sporangia and zoospores [6]. These structures are produced quickly in warm, moist conditions. Research indicates that it only takes about 35 minutes for the zoospores to be released once the sporangia are produced. During its asexual cycle, P. insidiosum creates sporangia at the end of its hyphae, and the protoplasm containing cytoplasmic fluids, ions, and organelles flows to the hyphal tip; the vesicle at the tip increases in size until the protoplasm stops flowing [6]. At this point, the base of the vesicle where it connects to the hyphal tip forms a cleavage and then forms biflagellate zoospores inside the vesicle [10]. The motile zoospores mechanically break the vesicle’s wall and are released into the surrounding environment [10]. They are globose to ovoid in shape, have two flagella, and swim around from anywhere between 10-60 minutes before encystment, depending on the environment [2] [10]. They exhibit chemotaxis toward a favorable nutrient or substrate in an environment, and show more significant chemotaxis toward mammalian and plant tissue as well as animal hair; it is even more pronounced when the tissue is damaged [10]. Over time, zoospores slow down and settle in a particular place, where their two flagella detach, they encyst on a surface such as a piece of tissue, adhere to the surface by secreting a glycoprotein, and develop a germ tube that elongates into a filament like those of the original mature organism [10]. In environments containing plant and animal tissues, the zoospores move more directly and encyst more quickly, most often directly onto the tissues. Movement is much more random and encystment and germ tube production occur much more slowly in environments that only contain a control substance like water without nutrients or useful substrates [10]. This phenomenon shows how preferred these tissues are for this organism, which gives evidence about its pathogenicity and its ability to infect larger hosts quickly as a result of this strong exhibited preference.

Metabolic Processes

P. insidiosum uses its filamentous hyphae for feeding and nutrient uptake from the surrounding environment [6]. Because it lives in swampy ecosystems, it often decomposes decaying organic matter for nutrients. P. insidiosum also conducts aerobic respiration [6]. During decomposition, it secretes different enzymes and compounds into the surrounding environment and uses water and its hyphae to get nutrients to flow back to the organism to be digested [6]. However, the specific details of this mechanism are not yet described or understood. Metabolic activity is stimulated by the presence of magnesium, potassium, and calcium ions [6]. Additionally, P. insidiosum has a high affinity for iron, which can expedite or stimulate metabolic processes [12]. This activity is especially useful when P. insidiosum invades a host and depletes the resources available there, because iron can be readily available in host physiological environments [13].






Edited by [Lindsay Smith], student of Jennifer Talbot for BI 311 General Microbiology, 2014, Boston University.