Non-Mammalian Model Organisms - Assignment Example

Today, health is a major concern. It is not only human health that requires our attention but plant health and animal health are also equally important because of the economical value that they are associated with. Diseases caused by various pathogens are therefore a threat to the overall health of living organisms and therefore in order to improve health and facilitate better management and control of diseases it is necessary to gain an in-depth understanding of the diseases caused by various pathogens. This is done by studying the interactions between the affected host and the pathogenic agent. Studying these interactions at the molecular level helps in research and diagnosis as well. Since humans cannot be directly be studied for such interactions, they seek the help of other genetically similar organisms. Model organisms have thereby emerged as handy tools in such studies primarily because they have simple genetic makeup that resemble that of humans and the interactions can be studied within laboratory conditions. Various model organisms are employed today to study host-pathogen interaction dynamics. Introduction Host – pathogen refers to interaction between pathogenic organisms such as bacteria or virus and their host. Today, model organisms become an important tools for clinical research and study (Hunter, 2008). The best way to study the interactions in both vivo and vitro conditions are non-mammalian model organisms, because they are easy to handle in the laboratory, also they have a low life span compared to mammalian. These organisms allow us to study molecular and genetic levels of interactions of the pathogens and hosts. Zebrafish Zebrafish, known as Danio rerio, it is a freshwater fish and it is an important model organism for research today. It is a freshwater fish and belongs to family Cyprinidae and order Cypriniformes. zebrafish is important model organisms among others orginisum especially cancer research (Fietsma and Cuppen, p.2008). Zebrafish suitable to study genetic and developmental biology because The genome of zebrafish is linked to human genome such as 70% of the human genes have one zebrafish orthologue (Howe et al, 2013). Figure: Comparison of Human and Zebrafish protein-coding genes and their orthology relationships (Howe et al, 2013). Structure and life cycle The zebrafish female is larger than the male. It grows to about 1.5 inches in length. The female produces between 300 eggs -500 eggs. Zebrafish has 4 main stages in its lifecycle : embryo, larva, juvenile and adult. Zebrafish as a model for Host-pathogen Interaction zebrafish model helpe to study Host-pathogen interactions when the study was hard to do in higher organisms. Study the interactions in higher animals is harder beacuse it gave limited control when it was infected (Medina and Royo, 2013). One important pathogen studied using Zebrafish model is Streptococcus iniae. The bacterium is responsible for causing meningoencephalitis. Neely et al studied the infection of zebrafishes with S.iniae and studied the host-pathogen interactions within the organism (2002). RopB gene is an important regulator and activator of transcriptional genes, RopB gene was examined and it played an important role in the virulence. In order to study this, the researchers used mutants of zebrafish which could not express ropB gene and that mutants showed lesser virulence. Many cutaneous infections are caused by this agent and the results of this study shows the infection enters through the skin. Advantages and Disadvantages of using zebrafish Zebrafish is heavily used by biologists because of the transparency of the embryo. Moreover, the fish is small and easy to keep under laboratory conditions. The life span is small and this allow us to study a several generations within a short period of time. Zebrafish is a vertebrate so it is an ideal model for research on the vertebral system. Zebrafish genome has been sequenced and hence conduction of genetic analysis is also possible. On the other hand Zebrafish has some disadvantages.it is a new model organism very little literature uses the fish in the research. Also, behavioural study of the fish has not been studied. Caenorhabditis elegans Caenorhabditis elegans is another model organism used in biological research. C.elegans belongs to phylum Nematoda, Order Rhabditida and Family Rhabditidae. It is a small worm and it is a free-living that is non-parasitic and non-pathogenic, so it is an ideal as a model organism. It is a growing almost 1 mm in length and transparent roundworm. Bristol is the most common used strain of C.elegans,It was the first multi cellular organism have been sequenced. it is used for a range of studies such as aging studies, neurodegenerative disorders and cancer research. Structure and Life cycle Caenorhabditis elegans is a member of the phylum nematode. It has an unsegmented and cylindrical body shape. C.elgans have two sexes a hermaphrodite (XO) and a male (XX). There are 3 main stages of C.elgans - egg, larva and adult. The larval 1 (L1) stage is observed before 8 hourse of fertilization. The worm will be mature in about 1.5-2 days after reach larval stages and then adult stage. The average of life cycle is 3 days (Epstein and Shales, 1995). C.elegans for studying Host-Pathogen interactions C.elegans is the best tool to study interactions of host and pathogens in-vivo. Pseudomonas aeruginosa is one of several pathogens cause infection in both human and C.elegans.it is Gram negative bacteria which is a part of human microbiota.The bacteria enter and colonize in the intestine and synthesized outer membrane vesicles and this cause abnormalities in the intestines. Irazoqui et al, studied infection of P.aeruginosa within the organism (2010). The study showed after three different time points the accumulation of the pathogen increased within the tissue, and around 48 hours the intracellular invasion happened and the bacteria was found in other tissue.therefor the pathogen have ability to cross barriers to other tissues.in addition, genetic study was done to see the changes when in the interaction between the host and pathogen happened.the result showed some genes that produced are poisons such as ShK.Shk gene is up regulated during the infection and leading to extra production of the toxins in the body.Moreover , the result showed that the pathogen induced production of antimicrobial and detoxifying proteins with RNAi technology.(Irazoqui et al, 2010). Advantages and Disadvantages of using C.elegans There is several advantages of using C.elegans as a model organism. Firstly, they are non-parasitic and non-pathogenic so handled without any problems,also is a very simple organism and can rear easily within the laboratory conditions. C.elegans have a small life span about 3 days,so heredity and genetic studies can be ease. C.elegans has a simple structure which could be genetically manipulated. The most important advantageous factor of C.elegans genes can be easily knocked down using RNAi technology. However, there are many disadvantages but the most important one is C. elegans cannot survive at 37°C. Escherichia coli E.coli was discovered in the years 1885 by Theodor Escherich. Escherichia coli is a gram negative and free living bacteria which is found to exist in the large intestine of warm blooded animals like humans. This bacterium is known to be useful to humans because it helps in metabolic processes but many strains of E.coli are also known to cause infections such as diarrhoea. Structure and life cycle E.coli is rod shaped and has a very simple structure. The bacteria consist of a cell wall containing of a periplasmic space and a peptidoglycan layer between outer and inner cytoplasmic membranes of the cell wall. The outer walls are made of lipopolysaccarides. Figure 2: Rod shaped bacteria E.coli has only one chromosome which is circular in shape and some may also have circular plasmid. E.coli was among the first organisms to be sequenced by researchers. The chromosome of the model organism has about 4600 kb and 70% of the chromosome is composed of single genes. Normally, bacteria reproduce through binary fission and conjugation (mostly during unfavourable time). E.coli for Host pathogen interactions Being one of the most useful and well studied models E.coli has been used to study host and pathogen interactions. Jain and Srivastava used E.coli model to understand the interaction between host and viral pathogens. RNA viruses cause many diseases in humans- mild ones such as cold and cough and even deadly ones like HIV-AIDS, Ebola etc. For the study, the researchers used the MS2 strain of bacteriophage to infect E.coli.MS2 was selected over others because it had common characteristics with that of eukaryotic viruses and was also much easier to handle. Studying this model and using the concept of “flux balance analysis” alongside doing experiments the researchers established that viral infection hampered bacterial metabolism. Growth of the bacteria’s cells and other mechanisms of the cells was also stopped on being infected by the virus. Furthermore the researchers also saw an increase in the activity of the pentose phosphate pathway which is the pathway which helped the virus biosynthesize and grow (Jain and Srivastava, 2009). This study has helped to understand the effects on the metabolic activities on being infected by a virus. The effects can also be extended to organisms such as humans in order to understand the possible effects on being infected and designing better drugs to treat such infections. Advantages and disadvantages of using E.coli as a model organism E.coli is an extremely useful model organism because we have extensive scientific literature based on it and this extensive knowledge is useful for testing new analytical technologies (Lee and Lee, 2003). The simple structure and the ease with which it can be handled make it easy to be genetically manipulated and bioengineered. Fruit Fly Drosophila melanogaster is a fruit fly which been used a model organism for more than 100 years. It belongs to Order Diptera and Family Drosophilidae. D.melanogaster is the most organisms used in research, because the genes can manipulate easily which can be an ideal organism to study genetics and developmental biology. The genome of the fly sequenced completely. 13,600 genes are encoded in the genome, which lesser than C.elegans (Adams et al, 2000). The length of the genome is about 165 million base pairs. The first illustration of Drosophila (Curtis, 1833). Structure and Lifecycle The length of adult fruit fly is between 4-5 mm. The life cycle of Drosophila ranges between 8-10 days during which it completes embryo, larva, pupa and adult stages of development. After fertilization the embryo gives rise to the lst instar larvae. This larva feeds voraciously and crosses the 2ndand 3 rd instar stages after which it pupates. Pupation takes place for about 72 hours after which the adult fly with functional sexual organs emerges. Fruit fly study for Host-Pathogen Interactions Listeria monocytogenes is one of pathogens studied within the Drosophila body system. Listeria monocytogenes is a bacterial pathogen capable to infect the host and replicating causing infection. The pathogen infects the fruit fly causing lethal infections and death for drosophila.,also the pathogen required genetic assistance for intracellular growth and the most important gene was actA gene. Mutant pathogen that lacked with this gene was more pathogenic than others. Act A gene involved in Two signalling pathways Tool and Imd pathways and they are responsible for the response in the host on microbial infection (Mansfield et al, 2003).. Advantages and disadvantages of using fruit fly as a model organism Drosophila is one of the most used model organism in biological sciences. Drosophila has several advantages. It is easily to handle and rear under laboratory conditions also cost less and cheap than other model organisms, Moreover ,it has a very short generation time and high survivability and fecundity. Arabidopsis thaliana Arabidopsis thaliana is an angiosperm. It belongs to mustard family, Brassicaceae. This small flowering plant has emerged rapidly a model organism especially in plant biology. It was originally used to study plant genetics. It is native of Europe but is found in other parts of the world as well. Previously viewed as a model organism solely for plant based research, this plant model organism is also becoming a key organism for understanding molecular mechanisms of various human diseases. After the entire genome of the plant had been sequenced it was seen that most of the genes that are implicated during human diseases is also present in the plant genome (Xu and Moller, 2011). It was about 1.6 billion ago that humans and Arabidopsis diverged however even though they appear to be vastly distant, high percentage of similarity is seen in the protein functions and cellular processes of both he organisms. Thus, the plant is widely used today in human health researches as well. Structure and Life cycle This is an annual plant that grows to about 20-25 cms in height. Rosette formation of leaves is seen at the base of the plant. The leaves maybe purplish or greenish in colour and covered with trichomes which are small hair like structures. The flowers are arranged in corymb. The entire life cycle spans about 6 weeks. The flowers naturally self pollinate. Seeds are produced which then germinate and give rise to the plant. Arabidopsis thaliana for studying Host-Pathogen interactions Arabidopsis thaliana has been sued to study host-pathogen interactions especially using those pathogens that cause disease in the plant system and are both agriculturally and economically harmful. One of the most important pathogens studied is Pseudomonas. Plants are capable of resisting pathogens and there are several mechanisms within the plant systems that serves this particular purpose. The gene, RPP13 is a resistance gene and it has been studied most of all. Attack by pathogens initiates action of RPP13 –mediated resistance. Allen et al studied the cloning of avirulence gene ATR13 that is primarily responsible for stimulating RPP13 gene. The gene was seen to exhibit high levels of protein polymorphism when they interact with pathogens (2004). The study showed that the host-pathogen interaction in this model organism enhanced co-evolution. Diversifying selection assisted in evolution of the RPP13 gene during the interaction (Allen et al,2004). Lee et al in 2013 studied the mechanism of Arabidopsis host resistance against Pseudomonas syringae. It was seen that the heterotrimeric G proteins present in the plants play an important role in such resistance against disease causing pathogens. G proteins are produced in all eukaryotic organisms and help in the resistance against pathogenic microbes. Lee et al studied the interaction between the host and the pathogen. They also studied the role of the G proteins in these interactions. They induced mutations within the model organism by genetic engineering. G protein consists of three different subunits since all the three subunits i.e. Gα, Gβ and Gγ are involved in host-specific and non-host specific interaction. Mutants model organism containing mutant form of G protein when inoculated with the disease causing microbe showed greater susceptibility to the disease. The genetically altered models showed almost a 10-15 times rise in susceptibility when compared to the susceptibility showed by the wild type i.e. the type which is not genetically engineered (Lee et al, 2013). The researchers also established that during interaction the plant stomata also assisted against evading bacterial pathogens. This is part of the plant’s inborn immunity which is present from the very first. Ps.syringe produced PAMAPs such as flaggelin, lipopolysaccharide and other products that caused closing of the stomatal opening in the epidermis of the Arabidopsis leaves. Lee et al showed that mutant Gα and Gβ - defiecient mutants stooped the closing of the stomata. Gene for gene was also observed in the interaction study. Mutants and wild type were given very minute amount of pathogen- P.syringe. However no disease development was seen in the wild type while in spite of expression of the resistance gene RPM1 each of the mutant variants of the model plant showed different degrees of development of disease. Again, a 10-15 times rise in disease susceptibility was seen in the mutant type indicating that gene-for-gene resistance was non-functional in the mutant variants. Some research studies have also indicated that host-pathogen interaction is partially mediated by the secondary metabolism products that are produced by the plants. Some of these products help to tolerate the attack of the pathogen or even resist attacks by pathogens. Arabidopsis is known to contain a number of secondary metabolites most of which have shown antimicrobial activity. Pathogenic Botrytis cinera and its interaction with Arabdiopsis have revealed that not all strains of the pathogen are able to cause disease in the plant. Botrytis cinera shows different responses to a chemical called camelexin. Only those pathogen strains that show sensitivity towards the chemical are able to cause large lesions in those Arabdiopsis that lack the chemical but the same strain are unable to cause lesions in the wild type of the model organism. This was basically owing to the production of secondary metabolites (Kleibenstein et al, 2005). Advantages and disadvantages of using Arabidopsis thaliana as a model organism Arabidopsis thaliana is considered an ideal model organism in plant biology. It is small in size and is easy to maintain in the laboratory. Almost 115 Mb of the 125 Mb of the genome has been entirely sequenced. The small genome size makes it easier to study. The short life span of the plant makes it easier to study it through more than one generation. It also produces high number of seeds through self fertilization. Another important advantage was the development and utilization of mutant screens which helped in studying of induced mutations in the organism (Koornneef and Meinke,2010).However there exist a few disadvantages of using the model organism. Genetic variation exists within the population of the plants and hence this might interfere with certain studies especially in genetic studies. Conclusion The progress in the field of research studies is heavily based on model organisms. The use of model organisms in combination with technological advancements has made studies at the genetic and molecular level simpler than they were. Host-pathogen interactions have become very important since they allow researchers to understand the basic molecular mechanisms that underlie these interactions. This also allows them to predict the resistibility or susceptibility of the host and understand the ability of the pathogen to cause an infection or disease. There exist a number of non-mammalian model organisms however zebrafish, C.elegans, Drosophila and Arabidopsis are mostly used particularly to study the interface between the hosts and the pathogens. Each of these models have contributed significantly to host-pathogen interaction studies and helped in developing strategies to either increase the susceptibility of the host or decrease the virulence of the pathogenic agents. The success of each study establishes the fact that model organisms are reliable and efficient for understanding the primary mechanisms of various disorders and in the future can be employed to understand more diseases in both humans as well as in other organisms enabling better management, precaution and treatment measures. References Adams., M.D.(2000). The genome sequence of Drosophila melanogaster. 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