Pathogenicity and Virulence Lecture Notes PDF

Pathogenicity and virulence Pathogen: any agent capable of producing disease. Pathogenicity: the ability of parasite to gain entrance to a host and produce physiological or anatomical changes (disease). Virulence: the degree of pathogenicity, or ability of an organism to cause infection. Infection:...

Pathogenicity and virulence Pathogen: any agent capable of producing disease. Pathogenicity: the ability of parasite to gain entrance to a host and produce physiological or anatomical changes (disease). Virulence: the degree of pathogenicity, or ability of an organism to cause infection. Infection: interaction between two living things, the host and the parasite, with a competition for superiority. If the parasite is successful, disease results. If the host is successful, disease does not occur. Infestation differs from infection in that it indicates the presence of animal parasites in or on the body of the host. Parasitism: eating beside another The type of antagonism that exists when one organism lives at the expense of another, its host. Saprophytic organisms: live on dead or decaying organic matter. Resistance: the ability of an animal or plant toward off infection. Resistane is dependent on many factors: Physical or metabolic and inherent in the host species or in an individual (natural resistance factors) Lack of resistance is called susceptibility. Factors influencing virulence: 1- Toxic factors: Some microorganisms produce poisonous substances of high molecular weight (toxins). Toxins May be: exotoxins endotoxins Excreted into the retained within the cell surrounding medium The capacity of a microorganism to produce toxin and the potency of the toxin are important factors in the ability of the organism to cause disease. Exotoxins are diffusible proteins. Lose their toxicity when heated or treated with acids. There is evidence that their toxicity is due to the spatial configuration of the amino acids in the molecules. When these arrangements are altered, the toxicity is lost and the resulting substances are called toxoids. Gram negative bacteria, do not elaborate a soluble toxin from the living intact cells but produce an endotoxin that is liberated when the cells disintegrate. -Endotoxin which located in the cell wall, are complex substances containing phospholipid, carbohydrate and protein The difference between exotoxins and endotoxins Endotoxins are: 1- relatively heat stable. 2- do not form toxoids 3- less toxic. 2- Enzymatic and other factors: a- Hyaluronidase: (spreading factor) It affects the ability of the pathogen to penetrate the tissues of the host by hydrolyzing the hyaluronic acid (tissue cement in living cells). It is an adaptive enzyme produced only in the presence of a specific substrate which is hyaluronic acid. This enzyme is produced by several of the cocci, some of the clostridia and some of other bacteria. b- Lecithinase: This enzyme destroys various tissue cells and is active in lysis of red blood corpuscles. It is secreted by Clostridium perfringes and its virulence is referred in part to the production of this enzyme. Function of this enzyme : lysis and degradation of lecithin (one of the phospholipids) Lecithin is essential component of all the cellular membranes and its destruction cause many disturbance in the function of these cellular membranes such as respiration and permeability. C- Collagenase: It is an enzyme that destroys collagen, an albumin-like substance which is found in muscle, bone, and cartilage and upon which tissue cells are built. It is also secreted by clostridium perfringens. D- Coagulase: It is an enzyme which acts with an activator in plasma to coagulate fibrinogen in the serum of certain enzymes, and thus fibrin coats the cell walls of the bacteria and so protect them against the action of phagocytosis. The enzyme is produced by some virulent staphylococci. E- Leucocidin: Is a substance produced by some staphylococci, streptococci. Can kill leucocytes (white blood cells) in vitro. F- Hemolysins: These are substances which liberate hemoglobin from red blood cells. Produced by several kinds of bacteria. Virulence of the bacteria is enhanced by their ability to produce hemolysins. 3- Capsular materials: The virulence of pathogen is influenced by the presence or absence of a non-toxic polysaccharide material in the capsule surrounding the cells. This is demonstrated by the pneumococci. Factors Influencing infection: 1- Tissue affinity: Some microorganisms have a particular affinity for certain cells and tissue, which may injure and destroy. Their interference with the normal processes of the cells affects the hole organism, and disease results. e.g. typhoid bacillus chooses the lymphoid tissue of the intestinal wall Poliomyelitis virus has an affinity for nerve cells. 2- Hypersensitivity: Animal tissues sometimes become abnormal sensitive to compounds of bacterial cells or their metabolic products. These hypersensitivity is an important factor in the ability of some microorganisms to produce pathological changes in the host. Hypersensitivity to some microorganisms may result in a chronic disease. 3- Infective dosage: The number of organisms required to cause disease in a given host. Infective dose varies with the species of host and the variety or strain of microorganisms. It was found that the number of organisms required to cause disease is inversely proportional to their virulence. When extremely virulent microorganisms enter a susceptible host through the proper portal, very few are required to established infection. Less virulent strains of the same organism entering by the same portal must be present in greater numbers to establish infection. 4- Portal of entry : Not only pathogens enter the body in adequate number to produce disease, but many of them must enter through certain route called the portal of entry which differs for the various organisms depending on their ability to attack certain organs or parts of the body. e.g. alimentary tract is the portal of entry of typhoid, dysentery and cholera Respiratory tract is the portal of entry of tuberculosis and diphtheria organisms. This selectivity as to portal of entry is due to certain natural barriers. The intact skin and mucous membranes are excellent mechanical barriers to most pathogens. 5- communicability: is dependent on two important factors: - The escape of the pathogen from its host - Its entrance into a susceptible person. - The method of transmission of pathogenic microorganisms are classified into: - 1- contact 2- airborne 3- food and water-borne 4- insect-borne organisms.