. In the table below describe one mechanism of different types of microbes accom
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Question
. In the table below describe one mechanism of different types of microbes accomplish each step. I completed one box to give you a sense of the kinds of answers I am looking for. (JUST TYPED)
Bacteria
Viruses
Establish
Infection
EXAMPLE:
Bacteria have pili with adhesins that bind to receptors on host cells. This help bacteria adhere to the surface of host cells.
Invade
The Host
Avoid
Host Defenses
Damage
To the Host
Bacteria
Viruses
Establish
Infection
EXAMPLE:
Bacteria have pili with adhesins that bind to receptors on host cells. This help bacteria adhere to the surface of host cells.
Invade
The Host
Avoid
Host Defenses
Damage
To the Host
Explanation / Answer
Bacteria
Establish infection
Bacterial pathogens interact with the host to establish infection. This occurs by a) expressing a wide range of molecules that bind host cell targets and facilitate host responses b) secreting enzymes such as proteases, glycanases that digest host extracellular matrix proteins and polysaccharides c) by invading the host tissue cells and gain access to the intracellular environment. This occurs by phagocytosis. d) adhesins which facilitate binding of the pathogen to host surfaces. The degree to which these various mechanisms play a part in the pathogenesis of an infection depends on the bacterial species or strain, the site of pathogen entry, the immune status of the host and other similar factors. Attachment of bacteria to host surfaces is facilitated by pili or fimbrae. These are hair-like organelles protruding from the surface of bacteria. Attachment of bacteria to host surfaces helps in its colonization or leads to biofilm formation. Aggregate of microorganisms adhere to each other on a surface and this adherence occurs when the cells get embedded within matrix of extracellular polymeric substance (EPS).
Invade host
Once adhered to a host surface, a bacterial pathogen may further invade host tissues. This occurs mainly by phagocytosis. This invasion can be of two types: extracellular and intracellular. In case of extracellular invasion, microbe breaks down the barriers of a tissue to invade the host. the pathogen remains outside of host cells. Enzymes help in the degradation of host cell molecules. Example- hyaluronidase breaks the proteoglycans in connective tissue, streptokinase and staphylokinase dissolve the fibrin clots, lipase degrades oils, nuclease cause breakdown of RNA and DNA, haemolysins makes holes in the host cells. elastase enzyme secreted by Pseudomonas aeruginosa degrades extracellular molecules. Once the pathogen gets access to host tissues it proliferates, disseminates to other sites in the body, express toxins and initiate inflammatory responses. In case of intracellular invasion microbe penetrates the cells of a host tissue and survives within host cells. Gram negative, Gram positive, bacteria have shown the ability to enter host cells. Both phagocytic and non-phagocytic cell types are prone to invasion.
Avoid infection
The epithelial surfaces of the body are the first defenses against infection. Many microorganisms do succeed in crossing the epithelial surfaces and can be successfdully removed by innate immune mechanisms present in underlying tissues. Epithelial cells are held together by tight junctions, which effectively form a seal against the external environment. The intestinal mucus (composed of glycoproteins named mucins, digestive enzymes, antimicrobial peptides and immunoglobulins layer) limits the invasion by bacteria. If a microorganism crosses epithelial barrier it is immediately recognized by the mononuclear phagocytes, or macrophages present in the tissues. These are found in connective tissue, gastrointestinal tract, lung, spleen. The neutrophils or polymorphonuclear neutrophilic leukocytes (PMNs or polys) are another type of phagocytes, Both have key role in innate immunity as they recognize, ingest, and destroy pathogens. Phagocytosis is a process by which pathogen gets surrounded by phagocyte membrane followed by acidification of the vesicle known as phagosome. Granules such as lysosomes conytaining enzymes, proteins also help in antimicrobial response. The lysosomal contents destroy the pathogen.
Damage to host
The release of toxins mainly exotoxins or endotoxins causes damage to host. Exotoxins are released into the surrounding environment. The release of toxins results in the disruption of metabolic pathways in the host. These include damage to cell membranes, disruption in protein synthesis and activation of immune response. Bacteria Listeria monocytogenes, targets cholesterol by producing listeriolysin O a pore-forming toxin protein. This exotoxin affects intracellular processes and creates pores within the cell membranes of the host.
Viruses
Establish infection
Some viruses infect the cell and kill them by producing infectious particles. These are called cytopathic viruses. Others infect cells and produce infectious particles but do not cause death of host cell. These are referred as noncytopathic viruses. Some viruses infect the host cell. Infection in the host can be acute or persistent. A latent or of persistent viral infection is the phase in which, viruses initiate infection, but do not proliferate inside the host. The first step is the binding of the virus to the host target cell. Binding is facilitated by binding of a viral surface protein to a specific receptor on the host cell surface. Example -E. coli surface protein allow binding of bacteriophage lambda to bind to the bacterium. Viruses that infect animal cells use cell-surface receptor molecules such as acetylcholine receptor or cell adhesion protein N-CAM. Some viruses require a secondary co-receptor besides a primary receptor for the attachment and entry into host cells. Example in HIV, primary receptor is CD4 protein which is involved in immune recognition and is found on the surface of many T cells and macrophages and the presence of co-receptor, CCR5 (a receptor for -chemokines) or CXCR4 (a receptor for -chemokines) is also required.
Invade the host
.
Viruses invade the host cells by membrane fusion, pore formation, or membrane disruption. they can fuse directly to the plasma membrane (receptor-mediated fusion) or get trapped into an endosome. After fusion with the plasma membrane, virus binds to a protein in the cell membrane. The binding induces a conformational change in the viral fusion protein leading to fusion. The acidic conditions inside the endosome induce fusion. The viral genome passes through a fusion pore into cytosol, and initiate the infection.
Avoid defense
Viruses have evolved strategies to escape from the immune system. Viruses can avoid detection by both innate and adaptive immune responses. Viruses can express proteins that directly interfere with host processes or mimic host proteins and compete to bind specific receptors. Viruses can avoid modifyg the ligands for the receptors and hewnce can escape the detection by recognition receptors, T cell receptors and antibodies. Interferons, cytokines and chemokines are mimicked or blocked by viral proteins to prevent the efficient development of an immune response. They can repress the replication for some time so that remain dormant inside the cell, remaining undetectable.
Damage to host
After attachment to the host cell surface, they enter the host cell, release its nucleic acid genome from its protein coat or lipid envelope. The nucleic acid gets complexed with viral proteins. the enveloped viruses enter the host cell by fusing either with the plasma membrane or with the endosomal membrane following endocytosis. The fusion occurs via the process like SNARE-mediated fusion of vesicles. The acidic environment in the endosome brings the conformational change in the viral surface protein. The uncoating of the viral nucleic acid via pumping H+ into endosome and its fusion with virus leads to the initiation of the infection. The genomes of virus encodes proteins that can block immune response.
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