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Pathogenesis of Bacterial Infections by Dr. Riyadh Saleh Hamoud Ass. Prof. of Molecular biology and Medical Microbiology Faculty of medicine and health sciences –Aden University PhD, Alexandria university - Egypt ...

Pathogenesis of Bacterial Infections by Dr. Riyadh Saleh Hamoud Ass. Prof. of Molecular biology and Medical Microbiology Faculty of medicine and health sciences –Aden University PhD, Alexandria university - Egypt  Mechanism of Bacterial Pathogenesis  Ability of bacteria to produce disease or tissue injury is often referred to as two closely related but not synonymous term ‘pathogenesis’ and ‘virulence’.  P ™athogenesis: is generally employed to refer to the ability of a microbial species to produce disease  Virulence: is used more specifically to describe the relative degree of pathogenesis (tissue damage), which may vary between different strains of the same organism depending upon the expression of the virulence factors  Virulence: is a relative term; different strains of same species may exhibit varying degrees of virulence. Some strains are highly virulent; while some strains are low and some are a virulent (vaccine strains).  The virulence of a strain may undergo spontaneous or induced variation.  ™ Exaltation: Enhancement of virulence is known as exaltation, which can be induced experimentally by serial passage into susceptible hosts  Attenuation: It refers to the reduction of virulence, which can be achieved by passage through unfavorable hosts, repeated cultures in artificial media, growth in high temperature or in the presence of weak antiseptics, desiccation or prolonged storage in culture.  Microbial pathogenicity depends upon the sum total of several factors 1. Route of transmission of infection 2. Infective dose of the organism: Minimum inoculum size that is capable of initiating an infection. 3. Evasion of the local defenses 4. Adhesion: By Fimbriae or pili, other adhesins, biofilm formation 5. Invasion: Virulence factors that help in invasion include: Mechanism of Bacterial Pathogenesis  Route of transmission  Route of transmission of infection plays a Mode of transmission of bacterial infections crucial role in the pathogenesis of certain bacteria o Some bacteria, such as streptococci, can initiate infection whatever be the route of entry o Others can survive and multiply only when introduced by the optimal routes. o Vibrio cholerae are infective orally but are unable to cause infection when introduced subcutaneously o This difference is probably related to the modes by which different bacteria are able to initiate tissue damage and establish themselves.  Infective Dose Infective dose of the bacteria is referred to as the minimum inoculum size that is capable of initiating an infection. Infective dose plays a major role in determining whether the disease is going to set in or not. Low infective dose: Certain organisms require a relatively small inoculum to initiate infection Shigella: Very low (as low as 10 bacilli), Escherichia coli O157: H7 (