Mechanism of bacterial pathogenesis
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This document discusses the mechanisms of bacterial pathogenesis. It defines key terms like commensal, opportunistic pathogen, pathogen, virulence, infection, and portal of entry. It describes bacterial virulence factors such as capsules, adhesins, invasiveness, exoenzymes, and toxins. It also discusses concepts like infecting dose, clinical manifestations, and the evolution of infection.
Mechanism of bacterial pathogenesis
- 1. B Y M S . J I G I S H A P A N C H O L I H E A D D E P T . O F B I O C H E M I S T R Y & M I C R O B I O L O G Y I N D I A N I N S T I T U T E O F A Y U R V E D I C P H A R M A C E U T I C A L S C I E N C E S G U J A R A T A Y U R V E D U N I V E R S I T Y J A M N A G A R Mechanism of Bacterial Pathogenesis
- 2. Introduction
- 3. Introduction Humans and animals have abundant bacterial flora that are usually harmless. A general balance exists between host and his environment. The host- parasite interaction is influenced by host factors as well as the infecting microbe. The outcome of this interaction can range from no demonstrable effect to death. Majority of the free living organisms are saprophytes.
- 4. Definitions
- 5. Commensal Commensals live in complete harmony with the animal body (host) without causing any harm to it. The commensals constitute the normal bacterial flora of the body, such as S. epidermidis of skin and E. coli of gastrointestinal tract. They subsist on secretions, food residues or waste products of the body. They serve important functions in their hosts, like production of vitamins, protection of host from colonisation with pathogenic organism etc.
- 6. Opportunistic Pathogen These are the commensals or saprophytes which can produce disease when the body resistance is lowered. Example: Candida albicans – a causal agent of opportunistic oral and genital infections in humans Staphylococcus aureus- occurs as commensal on human skin but may cause Staph infections
- 7. Pathogen A micro organism capable of producing disease is called as pathogen. Pathogenicity is the ability of a class of bacteria (Pathogen) to produce disease. Examples: Bacillus anthracis causes anthrax Clostridium tetani causes tetanus Salmonella typhi causes typhoid
- 8. Virulence Virulence is the measure of the degree of pathogenicity and depends on invasiveness and toxigenecity of the organism. The ability of a bacteria to cause disease is described in terms of the number of infecting bacteria, the route of entry into the body, the effects of host defense mechanisms and intrinsic characteristics of the bacteria called as virulence factors.
- 9. Infection Infection may be defined as lodgement and multiplication of an infectious agent in the body. All infections do not invariably result in disease. Some infections may remain asymptomatic and others may lead to development of signs and symptoms (disease) after break down of host- parasite relationship in favour of the parasite.
- 10. Portal of Entry The infectious agent enters the body by one of the following routes: Oral Respiratory Genitourinary Conjunctiva Cutaneous
- 11. Incubation Period The time interval between the entry of the infectious agent and the onset of clinical manifestations of disease is called incubation period. The infective agent, after reaching the selective tissue, undergoes multiplication during this period. The period may be as short as minutes to as long as thirty years in the case of variant Creutzfeldt- Jakob disease
- 12. Clinical Manifestation An infection that does not cause any detectable manifestation is known as in apparent or subclinical infection. A person with symptomless infection is called as carrier. In some diseases, either because of inadequate treatment or immune response, the infectious agent us not eliminated, leading to carrier state. Carriers may be temporary or chronic.
- 13. Temporary carrier state lasts for less than six months, whereas the chronic carrier is the one who excretes the disease producing organisms for indefinite period of several years or throughout life. Examples: Cholera, polio, diphtheria, meningitis, typhoid etc. Following infection, some pathogens may remain in latent or dormant form in host tissues and proliferate when host resistance is lowered producing clinical disease. This is called as Latent infection. Example: HIV
- 14. Primary infection is referred to the first or initial infection caused by an infecting organism. Subsequent infections by the same parasite in the host is called as re infection. When the body resistance of a patient is lowered by a pre existing infectious disease or during some treatment, a new micro organism sets up an infection which is called as secondary infection. Example: A vaginal yeast infection after taking antibiotics to remove bacterial infection.
- 15. When a new infection from patient or healthy staff carrier is set up in an already diseased person, it is called as cross infection. Cross infections acquired in hospitals are called as hospital acquired or nosocomial infections. Infection where the typical or characteristic manifestations of a particular disease is lacking, it is called as atypical infection. Eg: Atypical mycobacterial infections in HIV patients An iatrogenic infection is defined as physician induced infection resulting from drug therapy or diagnostic procedure, eg. Hepatitis B following blood transfusion.
- 16. Evolution of Infection From the portal entry the parasites may directly invade the tissue or may pass through lymphatic channels into blood stream for dissemination into internal organs. In majority of the instances, the parasites get destroyed by the host defense mechanisms. When the host resistance fails, an acute inflammatory response is developed which is characterized by vascular dilatation, marked exudation of plasma and accumulation of leucocytes with the formation of pus. The pus forming bacteria are termed pyogenic, eg. S. aureus, S. pyogenes.
- 18. Capsule Capsule is one of the most important virulence factors. They surround many bacterial cells which shield the bacteria from immune and phagocytic response. They are made up of either carbohydrates or proteins
- 19. Capsule inhibits phagocytosis by means of several ways: 1. Capsule prevents interaction between antibody and C3 bound to outer membrane of bacteria and their respective receptors on phagocytic cells. Complement activation is inhibited by the capsule.
- 20. Adhesins The pathogenesis of many bacteria depend on the ability to adhere to mucosal cells as a first step. With adhesion factors, many bacteria adhere to epithelial or endothelial cell linings of bladder, intestine and blood vessels. Types: Pili Lipopolysaccharide M proteins
- 21. Pili: in most cases pili serve as adhesion factor. Many of the adhesin proteins are present at the tip of pili and binds tightly to the target tissue. Examples of bacteria producing pili as adhesion factor: E.coli Neisseria gonorrhoeae Lipopolysaccharides: They are present in the outer membrane of gram –ve bacteria.
- 22. M protein: the expression of M proteins on the cell membrane of bacteria mediates their adherence to epithelial cells as seen in S. pyogenes that causes pharyngitis, which appears to be directly related to the production of M proteins.
- 23. Invasiveness Invasiveness of bacteria appears to be multifactorial and complex process. Invasive bacteria either destroy the barrier or penetrate into the cells of the barrier. Examples: 1. Shigella: a series of invasive proteins are involved: a. Proteins called as “invasion plasmid antigens (IPA)” expressed on outer membrane permit the bacterium to bind to the luminal surface of mucosal M cells of intestine then endocytosis occurs which is followed by rapid exit of the bacterium from the endosome into the cytoplasm.
- 24. b. Another protein, known as “intracellular spread protein (ICS)” expressed on the outer membrane of the bacterium, allows to interact with the host cell integrins. Following interaction, the bacterium travels towards the cell membrane which forms a protrusion and fuses with cell membranes of adjacent cells, thereby enabling Shigella to diffuse from cell to cell.
- 25. 2. Enteropathogenic Yersinia: they secrete an invasion protein which promotes binding of bacteria to the host cells, which in turn stimulates the cell to invaginate and take in the bacteria. 3. Neisseria gonorrhoea: with its pili adhere to mucosal cells. The bacteria also contains an enzyme that dissolves mucosal cell lining and thereby enables the organism to penetrate submucosal tissues.
- 26. Exoenzymes Many bacteria release enzymes that can damage host tissue in a variety of mechanisms: 1. Enzymes that break down collagen (eg.: collagenase, hyaluronidase) and fibrin (eg: fibrinolysins) allows better penetration of microbes into tissues. 2. Enzymes that break down cellular material (Eg: proteases, lecithinases) are associated with many Clostridia 3. Enzymes that modify and inactivate antibiotics (eg. Beta lactamase) hydrolyses the beta lactum ring of the antibiotics. The beta lactamase for penicillin and cephalosporin are called penicillinase and cephalosporinase respectively.
- 28. Toxins Bacterial toxins directly harm tissue or trigger destructive biological activities. They are classified into two categories: Exotoxins and Endotoxins
- 29. Exotoxins They are proteins produced and released extra cellularly from the bacterial cell to cause toxicity. They are produced by both Gram +ve and Gram –ve bacteria. In many cases the toxin is encoded by the plasmid, eg: LT and ST toxins of E.coli, tetanus toxin of Cl. tetani or a lysogenic phage, eg: C. diphtheriae, Cl. botulinum
- 31. Most exotoxins have two structural domains, “A” subunit (active domain)that causes cell cytotoxicity and “B” domain (binding domain) subunit that binds the A domain to specific cellular receptors. “A “subunit is transferred to the interior of the cell, where the cell injury is induced
- 32. Functional types of Exotoxins Enterotoxins: Affect gastrointestinal tract and include cholera toxin, E.coli toxin Neurotoxins: Affect nervous system and include botulinum toxin, tetanus toxin Cytotoxin: Affect cells in a variety of tissues and include diphtheria toxin, pseudomonas toxin A
- 33. Identification of exotoxins Ileal loop test: Ligated loops of ileum are inoculated with enterotoxin producing bacteria (V. cholerae, Enterotoxigenic E.coli), accumulation of fluid occur in the lumen in the organism is toxigenic. Cell culture test: to monolayers of sensitive cells (eg: Vero cells) bacteria free culture filtrate is added and incubated. The monolayer will be disrupted if the filtrate contains toxin (eg Vero cytotoxin of E.coli)
- 34. Serological test (Immunoassay): Since the enterotoxin are immunogenic, antisera can be used to detect the presence of toxin in specimens (eg: faecal material) or in culture media Molecular method: for detection of DNA of toxin producing organism in specimens are now being developed.
- 35. Endotoxins They are lipopolysaccharides (LPS) present on Gram –ve bacterial cell wall. The lipid A portion of LPS is responsible for endotoxin activity. They are the integral part of the bacteria and are released as bacteria lyse. They are primarily responsible for sepsis and septic shock. Endotoxin mediated toxicity include: Fever, activation of complement, thrombocytopenia, disseminated intra vascular coagulation, decreased peripheral circulation and perfusion of organs, shock and death
- 37. Infecting Dose Adequate number of bacteria is needed for successful infections The dose of infection depends on virulence of bacteria, organisms of high virulence can produce severe infection in small numbers Bacterial virulence is the sum total od invasiveness to tissues, capacity of multiplication in tissues and production of toxin Minimum lethal dose (MLD) is the minimum number or weight of organisms or toxin in micrograms needed to kill a particular species of animal when administered by a certain route.
- 38. Minimum infecting dose (MID) is the minimum number of bacteria required to produce clinical evidence of infection in a susceptible animal when administered under standard condition. LD 50 and ID 50 Organisms like S. typhi can infect in small doses while S. typhimurium require large number of organisms to make infection
- 39. Enhancement in virulence is called exaltation which can be achieved by serial passage in susceptible hosts. Reduction of virulence is termed are attenuation which can be attained by serial passage of the organism through un favourable hosts, prolonged storage, growth under high temperature, repeated cultures in artificial media and in presence of weak antiseptic substances.