Role of Acanthamoeba in the Environment
The amebae which belongs to the genus Acanthamoeba belong to the category of free living amebae. This organisms are ubiquitous by nature and are seen to be found in the habitats which includes water, soil, and dust along with the domestic tap water, air-conditioning units, dialysis units and also in the contact lenses or the lens cases. They are also present as contaminants in the tissue culture (Lorenzo-Morales, Khan and Walochnik 2015). The studies have shown that the life cycle of Acanthamoeba species comprises of two stages that includes the active feeding stage followed by the dividing trophozoite along with the dormant cyst. This amebae is responsible for causing diseases like amoebic keratitis (AK), granulomatous amoebic encephalitis and pneumonitis especially in the immune-compromised hosts. These free living amoebas or FLA have showed resistance to the adversities of the environment along with the germicides which successfully played important roles in the control of the population of the microbial communities (Magnet et al. 2015). This is mainly because of the predatory behaviour along with the microbicidal activity. In the case of Acanthamoeba, these have been playing the role of excellent reservoirs for the microorganisms which are amoeba-resistant. The trophozoite and cyst stage of the Acanthamoeba sp involves the biochemically active stage, having the cell dimensions between 25 and 40 μm. This in turn is seen to divide by mitosis and has major organelles. It also possesses the characteristics of eukaryotic cells as was reported by the studies (Guimaraes et al. 2016). The trophozoites showed vegetative growth, where feeding was through phagocytosis of bacteria along with yeasts and algae that are present in the environment. Here the digestion occurs within phagolysosomes and through pinocytosis, along with liquid nutrients absorption (Omaña-Molina et al. 2013).
While there has been several investigations on the role of the Acanthamoeba, it has been reported that there is adhesion of the Acanthamoeba to the host cells which is turn is responsible for the initiation of the secondary processes which includes phagocytosis along with the secretion of the toxins. The process of phagocytosis is actin dependent which in turn involves the polymerization of monomeric G-actin into filamentous F-actin (Denoncourt, Paquet and Charette 2014). Some ecological significance is obtained from the Acanthamoeba phagocytosis that is involved in the uptake of the particles of food such as the plasmids, bacteria and the fungal cells. This Acanthamoeba has the capability to form food cups which are also known as amoebastomes that are formed during incubations. They are often seen to have a role in the pathogenesis of Acanthamoeba. However according to the studies, there have been mention of the fact that cytochalasin D is responsible for the inhibition of the Acanthamoeba-mediated host cell death (Landell et al. 2013). The Acanthamoeba has the capability to phagocytose that is an intracellular signalling-dependent process in which there is involvement of the Genistein as well as sodium orthovanadate which is able to enhance phagocytosis of Acanthamoeba. There is also an indication that a tyrosine kinase-induced actin polymerization signal is a significant step in the phagocytosis of Acanthamoeba. Reports have also shown that Rho GTPases act as the primary regulators in case of the actin cytoskeletal which are able to link the signals to the cytoskeleton that are external in nature (Magnet et al. 2014).
Bronowski, James and Winstanley (2014) in a study showed that acanthameoba was infected as well as lysed by the bacteria. These also harboured in the bacteria as endosymbionts as stated in the year 1975 (Whiley et al. 2013). In the later years, there were studies conducted that have shown that Acanthamoeba was acting as the reservoirs for pathogenic facultative mycobacteria. This organism Acanthamoeba which have shown to harbour the virulent Legionella spp which is known to be associated with the disease Legionnaires (Landell et al. 2013). Similarly the fact Acanthamoeba is able to consume bacteria from the environment is well established, which allows the interaction of the Acanthamoeba and bacteria which are highly complex along with the dependence one virulent property of bacteria and the conditions of the environment.
Often there is development of convoluted interactions may be beneficial to Acanthamoeba which end up in the establishment of symbiotic relationships. The Acanthamoeba is also seen to interact with various Gram-positive and Gram-negative bacteria which adds to the complexity of the organism. These microorganisms also act as the bacterial predators which is used as the source of food for the Acanthamoeba. Bacteria are most of time consumed by phagocytosis. This in turn is followed by their lysis in phagolysosomes (Qvarnstrom, Nerad and Visvesvara 2013). These Acanthamoeba is able to take up both gram positive and gram negative bacteria, however the preference is on the gram negative bacteria which is used up as the food source. The virulent properties of the microorganism is proportional to the ability of Acanthamoeba to consume bacteria and also on the conditions of the environment. Magnet (2014) in a study had provided examples which revealed that in the absence of nutrients, the strains of Escherichia coli K1 which are virulent in nature, invade Acanthamoeba and remain viable in an intracellular way. However when the nutrients are available, the K1 escapes from the Acanthamoeba, which then grows exponentially and plays a part in the lysis of the host amoebae (Risler, Coupat-Goutaland, and Pélandakis, 2013).
Studies had shown that microorganisms act as the bacterial reservoirs, similarly the microorganism Acanthamoeba act as a bacterial reservoir. This is because majority of the bacteria is seen to be involved in the human pathogens. Some of the microorganism to which the Acanthamoeba act as a bacterial reservoir includes Legionella pneumophila, Escherichia coli O157, Coxiella burnetii, Pseudomonas aeruginosa, Vibrio cholera, Helicobacter pylori, Simkania negevensis, Listeria monocytogenes and finally Mycobacterium avium (Corsaro et al. 2015). This bacterial reservoir property of Acanthamoeba is essential as these bacterial pathogens have the ability to survive intracellularly and can also multiply within them. This therefore allows the bacteria to transmit through the environment. These evades the host defences or the chemotherapeutic drugs and this reproduce in numbers that are sufficient to produce disease (Ovrutsky et al. 2013). During the presence of the favourable conditions, there is increase in the bacterial densities which is able to lyse their host amoebae along with the infection of the new amoebae or the production of the disease. In cases of long term, the Acanthamoeba–bacteria interactions which can be considered to be parasitic so that the result in the death of the amoeba.
Lass et al., (2017) also argued that there was high colonization of Mycobacterium avium in case of mice, while it was inoculated that too in the presence of amoebae. Thus the capacity of Acanthamoeba to resist the situation which are harsh like that of extreme temperatures, pH and osmolarity that are especially prevalent during the cyst stage that recommended the usefulness as being bacterial vectors. It has also been stated in several studies that the Acanthamoeba cysts in particular are notoriously resistant to chlorine that is used in the cleaning of the water systems (Guimaraes et al. 2016). Therefore this is able to pose a great challenge in the process of the eradication of the bacterial pathogens from the public water supplies, mainly in the case of developing countries. Additionally these is presence of Acanthamoeba–bacteria interactions that also plays a role in the bacterial virulence. Studies have stated several examples in this respect which includes growth of the L. pneumophila, within which the Acanthamoeba exhibited increase of motility in addition to virulence and drug resistance in comparison to the axenically grown Legionella (Magnet et al. 2015).
Several studies conducted recently showed that the FLAs like Acanthamoeba sp. is responsible for the contamination of water. This is significant in terms of emerging problems of epidemics. These are potential human pathogens but also they host and protect other microorganisms. Thus, some pathogenic bacteria, fungi and viruses have developed strategies to escape the killing or the process of inactivation mechanisms associated with that of Acanthamoeba that uses the amoeba as a reservoir for their own benefit or as a “Trojan horse” (Lorenzo-Morales, Khan, and Walochnik 2015). This is in the case of co-infection of amoeba harbouring pathogenic microorganisms. Once it is adapted to the intracellular portion of amoebae and these pathogens are protected from biocides and environmental conditions which makes their survival along with dissemination more effective. This is because there is extensive association with different pathogens, A. castellanii has been used as an amoeboid model in order to study host-parasite interactions (Guimaraes et al. 2016).
In the year of 1973, the acanthamoeba keratitis was recognized first in UK. Later it was declared that Acanthamoeba keratitis is not a disease which is notable. It was therefore the true incidence of the infection in the UK or on a worldwide basis that is was difficult to ascertain. Studies by Co?kun et al. (2013) have shown that due to the resistance to the Acanthamoeba cysts from most of the antimicrobial agents, there is development of acanthamoeba keratitis that has been reported to be one of the difficult ocular infections. The maximum risk is developed in regards to the contact lens wearers who are most susceptible to this infection that has been reported to account for 90%. The Acanthamoeba cysts that is responsible for the causing of this disease is able to endure extremes of temperature along with disinfection and desiccation. Therefore as stated by Maciver et al. (2013), the presence of this organism in the environment is seen to be a constant challenge especially to the contact lens wearers. However most of the time it also becomes difficult to determine the source of this infection in a precise way.
Previously studies had reported that Acanthamoebae were harmless soil and water bacteria organisms, however later some studies proved that the contaminants from Acanthamoebae were responsible for causing some cytopathic effects in the culture of the monkey kidney cells (Geisen et al. 2014). According to Bradbury, French and Blizzard (2014), it was reported that there are contaminants found in the tissue culture for which Acanthamoebae was responsible. Recent studies have shown that rate of Acanthamoeba contamination in case of the contact lens storage cases in Korea was 10.6-15.7% (Lass et al. 2017). Due to the poor hygiene practices, which includes rinsing and storing of lenses in non-sterile saline or the use of tap water where the primary risk factors include lens case contamination.
Reports have also shown that this microorganism is responsible for other diseases as well like AGE. This is a rare infection but when caused most of the time turns to be fatal. The pathophysiology of this disease involves the central nervous system, which in turn leads to the induction of the responses of the pro-inflammation followed by the invasion of the blood–brain barrier along with the connective tissue. There is also an evident neuronal damage caused that leads to the brain dysfunction. Another such is the cutaneous acanthamebiasis, which is seen to involve the e nasopharyngeal and is most of time common cutaneous infections. Omaña-Molina et al. (2013) mentions in a study that the initial binding of the Acanthamoeba to the surface of the cells that are also the host cells that is involved in interfering with the signalling pathways. From the previous studies it has been seen that there is increase in the cytosolic levels of Ca21 that is in response to Acanthamoeba metabolites (Denoncourt, Paquet, and Charette 2014). These are dependent on trans membrane influx of extracellular Ca21.
Bronowski, James and Winstanley (2014) stated that there are changes in the levels of the intracellular Ca21 that can exert effects on the structure of the cytoskeletal. This is in turn able to induce changes in the morphology along with the alteration in the permeability in the plasma membrane. This therefore finally leads to the death of the target cell within minutes. There is understanding of the complex pathway of the intracellular signalling which is crucial for the identification of the targets for the therapeutic intervention. According to Landell (2013), there is the presence of 10,000 signalling molecules that is present in the single host cell at any time. They in turn interact in response to the Acanthamoeba which leads to the functional outcome and projects to be a challenge to the environment. In most of the events there is involvement of the both transcriptional and the post transcriptional level. Tyrosine is most of the time required to be present as one of the established proteins along with that or serine or threorine phosphorylations for intracellular signalling. However recent studies till date has been able to report that Acanthamoeba involves in upregulation or downregulation of the expression of a number of genes that are important for regulating the cell cycle. According to the studies by Magnet et al. (2014), there has been mention of the fact that Acanthamoeba upregulates the expression of genes including GADD45A and p130 Rb (Qvarnstrom, Nerad and Visvesvara 2013). These genes are in turn associated with that of the cell cycle arrest and also in the inhibition of the expression of the other genes including that of cyclins F, G1 and cyclin-dependent kinase-6. These in turn plays an important part in the encoding of the proteins that are significant of the progression of the cell cycle. This is further supported by the dephosphorylation of retinoblastoma protein (pRb) (Ovrutsky et al. 2013).
According to recent studies, the microorganism Acanthamoeba acts as a Trojan horse. This is one of the most important needs of the bacterial pathogens in order to protect themselves from the harsh environments that is faced during the transmission from host body to another. On invading the host tissue which might involve the nasal mucosa or the gut mucosa to produce disease or lung epithelial cells, there might be inclusion of the resistance to the innate defences along with which there are also the biological barriers. At the end of this, the microorganism Acanthamoeba may act as a ‘Trojan horse’ for bacteria. This term Trojan horse is often used to refer to the presence of the bacteria inside of the Acanthamoeba as opposed to ‘carrier’. Hence there might be some kind of attachment or adsorption present on the surface. Studies by Co?kun et al. (2013), had shown that Burkholderia cepacia is able to remain viable within Acanthamoeba but does not multiply. Bradbury, French and Blizzard (2014) also argued that this property as has been discussed above, has been also observed with other bacterial pathogens. Most of these findings recommended that Acanthamoeba is able to facilitate the bacterial transmission and is able to provide protection against that of the human immune system.
References
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