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Saturday, January 30, 2010 @ 8:01 PM
Entry04: Virus-Host Interaction Animal Viruses 1. Attachment Attachment is the binding of attachment sites on the viral surface with receptor sites on the host cell cytoplasmic membrane. For a virus to infect a host cell, that cell must have receptors for the virus on its surface and also be capable of supporting viral replication. These host cell receptors are normal surface molecules involved in routine cellular function, but since a portion of a molecule on the viral surface resembles the chemical shape of the body's molecule that would normally bind to the receptor, the virus is able to attach to the host cell's surface. 2. Penetration • Enveloped viruses Enveloped viruses enter the host cell in one of two ways: • In some cases, the viral envelope may fuse with the host cell cytoplasmic membrane and the nucleocapsid is released into the cytoplasm • Usually they enter by endocytosis, whereby the host cell cytoplasmic membrane invaginates and pinches off, placing the virus in an endocytic vesicle • Naked viruses Naked viruses enter the cell in one of two ways: • In some cases, interaction between the viral capsid and the host cell cytoplasmic membrane causes a rearrangement of capsid proteins allowing the viral nucleic acid to pass through the membrane into the cytoplasm • Most naked viruses enter by receptor-mediated endocytosis whereby the host cell cytoplasmic membrane invaginates and pinches off, placing the virus in an endocytic vesicle 3. Uncoating Before viruses can replicate within the infected host cell, the viral genome needs to released from the remainder of the virus. This process is sometimes referred to as uncoating. In the case of most viruses with an RNA genome, the viral RNA genome is released from the capsid and enters the cytoplasm of the host cell 4. Replication and expression The viral genome directs the host cell's metabolic machinery (ribosomes, tRNA, nutrients, energy, enzymes, etc.) to synthesize viral enzymes and viral parts. The viral genome has to both replicate itself and become transcribed into viral mRNA molecules. The viral mRNA can then be translated by the host cell's ribosomes into viral structural components and enzymes need for replication and assembly of the virus. 5. Maturation During maturation, the capsid is assembled around the viral genome • Maturation of an enveloped virus • Maturation of a naked virus 6. Release a. Naked viruses Naked viruses are predominantly released by host cell lysis. While some viruses are cytolytic and lyse the host cell more or less directly, in many cases it is the body's immune defenses that lyse the infected cell. b. Enveloped viruses With enveloped viruses, the host cell may or may not be lysed. The viruses obtain their envelopes from host cell membranes by budding. As mentioned above, prior to budding, viral proteins and glycoproteins are incorporated into the host cell's membranes. During budding the host cell membrane with incorporated viral proteins and glycoproteins evaginates and pinches off to form the viral envelope. Budding occurs either at the outer cytoplasmic membrane, the nuclear membrane, or at the membranes of the Golgi apparatus Two types of Replication Cycle Lytic Cycle One of the two cycles of viral reproduction (the other being the lysogenic cycle), which is usually considered as the main method of viralo reproduction because it ends in the lysis of the infected cell releasing the progeny viruses that will in turn spread and infect other cells. Lysogenic Cycle One of the ways of virus reproduction. ( other one:lyticcycle )Bacteriophage's nucleic acid fuses together with hosts, nucleic acid, so that genetic information of the virus is transmitted through daughter cells. Animation Link Viral replication cycle in which the virus's nucleic acid is integrated into the host cells chromosome; a provirus is formed and replicated each time the host cell reproduces; the host cell is not killed until the lytic cycle is activated. VIRUS GROWTH PHASES: 1. ECLIPSE PHASE - Phase during which the virion has entered the cell and before progeny virus are made. NO INFECTIOUS VIRUS is present during this phase. - Period in which virus gains control of host synthetic machinery and produce components required to assemble into virus. - Defined as the period between addition of virus and the appearance of assembled virus progeny inside the cell. 2. LATENT PHASE - Period following the eclipse phase from the time of disappearance of the infecting virus to the appearance of infectious virus in the surroundings. - Viruses are internal and must be released to be assayed because animal viruses may not have a classic burst of viruses but rather release virus slowly over a long period of time. Thus, it is possible to find large internal pools of virus - Virus yield is determined on the number of virus released per cell. Viral Infections: It should be noted that even when we consider viral diseases that replication and death of cells occurs cell by cell. Virus-host interaction: Virus can enter into our body via: - Skin - Mucous membrane - Placenta - Parenteral route Skin: formidable barrier to most viruses and only after this barrier is breached will viruses be able to infect the host. Some pathogens can enter via hair follicles, sweat glands, cuts and bruises. Mucous membrane: a thin, moist warm and cells within the mucous membrane are living. Examples of mucous membrane includes: respiratory, gastrointestinal, urinary, reproductive and conjunctiva (eyes) Placenta: Pathogens can cross placenta and infect the foetus. Thus, the foetus may have birth defections, the baby is prematurely born or the mother had gone through spontaneous abortion. Parenteral route: Pathogens also may deposit via punctures (nail, thorn), bites, cuts, wounds or through surgery. When viruses enter the body, it must leave the body too. Most of the portals of entries are the same as exits. The viruses leave the infected body so as to infect other healthy bodies. Some example of portals of exit includes: secretion (tears, saliva, vaginal, semen) and excretion (urine, faeces). Virus human relationship: When one of the organism benefit and the other is unaffected, then it’s commensalism. When one of the organism benefit and the other is harmed, then it’s parasitism. When both the organisms are benefitted, then it’s mutualism. Microflora is also known as microbiota. In a healthy individual, the internal tissues (blood, brain, muscle, etc.) are normally free from microbes. However, the surface tissues (skin and mucous membrane), are colonized by various microbial species, typically bacteria, fungi and rarely protists. The normal flora of humans consists of a few eukaryotic fungi and protists, but bacteria are the most numerous and obvious microbial components of the normal flora. 1. Virulence Factor 2. Stages of Infectious Disease 3. Sources of Pathogen 4. Modes of Transmission Virulence Factor Virulence factor is the ability of a microorganism to cause disease. Virulence may also be used to indicate the degree of pathogenicity. Virulence involved a complex interplay between the parasite and the host and is often multifactorial. Various host factors, including age, sex, nutritional status, genetic constitution, and the status of the immune system, affect the outcome of the parasite-host interaction. 1. Bacterial Virulence • Adhesion To cause infection, many bacteria must first adhere to a mucosal surface. To accomplish this, bacteria have evolved attachment mechanisms, such as pili (fimbriae), that recognize and attach the bacteria to cells. Colonization factors (as they are often called) are produced by numerous bacterial pathogens and constitute an important part of the pathogenic mechanism of these bacteria. • Invasion Factor Mechanisms that enable a bacterium to invade eukaryotic cells facilitate entry at mucosal surfaces. Some of these invasive bacteria (such as Rickettsia and Chlamydia species) are obligate intracellular pathogens, but most are facultative intracellular pathogens. The specific bacterial surface factors that mediate invasion are not known in most instances, and often, multiple gene products are involved. • Capsules and other surface components Bacteria have evolved numerous structural and metabolic virulence factors that enhance their survival rate in the host. Capsule formation has long been recognized as a protective mechanism for bacteria. Encapsulated strains of many bacteria (e.g., pneumococci) are more virulent and more resistant to phagocytosis and intracellular killing than are nonencapsulated strains. • Endotoxins Endotoxin is comprised of toxic lipopolysaccharide components of the outer membrane of Gram-negative bacteria. Endotoxin exerts profound biologic effects on the host and may be lethal. The term endotoxin was coined in 1893 by Pfeiffer to distinguish the class of toxic substances released after lysis of bacteria from the toxic substances (exotoxins) secreted by bacteria Basic structure of endotoxin (lipopolysaccharide) from Gram-negative bacteria 2. Viral Virulence Viral virulence factors determine whether infection occurs and how severe the resulting viral disease symptoms are. Viruses often require receptor proteins on host cells to which they specifically bind. Typically, these host cell proteins are endocytosed and the bound virus then enters the host cell. Five stages of Infectious Disease • Incubation The time between the exposure to an infectious disease and its development. This period may last from a few minutes to a few days, weeks, months or even years. • Prodormal An early sympton (or set of symptoms) that might indicate the start of a disease before specific symptoms occur. • Illness Most severe signs or symptons • Decline This is the period during which symptons (and signs) decrease as the infection is brought further under control. • Convalescence This is the time during which the host repairs the damage wrought by the infection. Individuals are not necessarily, depending on disease/pathogen, no-longer contagious. The body returns to normal health. Sources of Infectious Disease Animal reservoirs Zoonoses are diseases that can be transmitted from animals to humans. Sources of infection: • Direct contact with animals • Direct contact with the animal feaces • Ingesting the infected animal • Via animal vector Human reservoirs Direct or indirect contact with infected carriers for example contact with an ill person or their secretions, or objects touched by them. Nonliving reservoirs These infecting agents may also be transmitted through liquids, food, body fluids, contaminated objects, airborne inhalation, or through vector-borne spread. Videos! Labels: #entry |