PATHOGENESIS OF BACTERIAL DISEASES

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1 PATHOGENESIS OF BACTERIAL DISEASES Learning Objectives; i. Define the portal of entry, pathogenicity, and virulence ii. Explain how adherence, capsules, cell wall components and enzymes contribute to pathogenicity iii. Compare the effects of Hemolysins, Leukocidins, coagulase, kinases, Hyaluronidase and Collagenase iv. Contrast the nature and effects of exotoxins and endotoxins v. Outline the mechanism of action of cytotoxins (diphtherotoxin), enterotoxins (choleragen) and neurotoxins (tetanus toxin), and lipid A Contact-Infection-Disease: The Host Parasite relationship. The human body is constantly in contact with microbes. Some are pathogens that may cause an infection whereas others are resident (normal) flora. The resident flora are a huge and rich mixed population of microorganisms residing on body surfaces exposed to the environment including the skin, mucous membranes, parts of the gastrointestinal tract, urinary tract, reproductive tract, and upper respiratory tract, and are important to the host in the following ways; o One protective effect of the normal flora is competitive exclusion of pathogens, such as preventing adherence of invading organisms to the host by covering binding sites that might otherwise be used for attachment. o Lactobacillus species growing in the vagina produce lactic acid as a fermentation end product, resulting in an acidic ph that inhibits the growth of many potential disease-causing organisms. Disruption of normal flora, which occurs when antibiotics are used, can predispose a person to infections, e.g. antibiotic-associated colitis, caused by the growth of toxin-producing strains of Clostridium difficile in the intestine, and vulvovaginitis, caused by excessive growth of Candida albicans in the vagina. Anatomical sites lying within body cavity (organs) and fluids (blood, urine) do not harbor flora. System Examples of normal flora in a given body part/ system respiratory Staphylococcus, Corynebacterium, Haemophilus influenzae, Strep. Pneumoniae, Step. viridans Skin Staphylococcus, Corynebacterium, Propionibacterium genera, Begumisa MG lecture notes series Page 1

2 Factors affecting the Course of Infection and Disease 1. Pathogenicity and Virulence a. Pathogenicity is a property/ability of microorganisms to cause infection and disease b. Virulence is the precise factors used by the microbe to invade, and damage host tissues (helps define the degree of pathogenicity) c. Pathogenicity varies with a microbe s ability to invade or harm host tissues, and with the condition of host defenses d. A true /primary pathogen produces virulence factors that allow it to readily evade host defenses and to harm host tissues. True pathogens can infect normal, healthy hosts with intact defenses e. An opportunistic pathogen is not highly virulent but can cause disease in persons whose defenses are compromised by predisposing conditions such as age, genetic defects, medical procedures, and underlying organic disease. 2. Mechanism of Infection and Disease a. The portal of entry is the route by which microbes enter the tissues; primarily via skin, alimentary tract, respiratory tract (pneumonia, tuberculosis), urinogenital tract (sexually transmitted diseases), or placenta. b. Pathogens that come from outside the body are exogenous; those that originate from normal flora are endogenous c. The size of the infectious dose is of great importance in pathogenesis d. In the process of adhesion, a microbe attaches to the host cell by means of fimbriae, flagella, capsules or receptors that position it for invasion 3. Virulence Factors a. Exoenzymes digest epithelial tissues, disrupt tissues, and permit invasion of pathogens b. Toxigenicity is a microbe s capacity to produce toxins at site of multiplication which affect cellular targets. c. Antiphagocytic factors include Leukocidins (white blood cell poisons) and capsules Begumisa MG lecture notes series Page 2

3 Steps for infection by pathogenic bacteria include; 1. Maintain a reservoir; a reservoir is a place to live before and after causing an infection. The most common reservoirs for human pathogens are other humans, animals and the environment. 2. Initially be transported to the host. The most obvious means is direct contact from host to host (coughing, sneezing, body contact ). Soil, air, water and food are indirect vehicles that harbor and transmit bacteria to hosts. Most pathogens enter through the mucous membranes of the gastrointestinal (food & water), respiratory (drops of moisture & dust particles) and genitourinary system. Some other pathogens can enter through the parenteral route and the skin (Clostridium tetani, Clostridium perfringens, Yersinia pestis) Bacterial portal of entry Route Examples Ingestion Salmonella spp, Shigella spp, Yersinia enterocolitia, Enterotoxigenic E.coli, Vibrio spp., Campylobacter spp, Clostridium botulinum, Bacillus cereus, Listeria spp., Brucella spp Inhalation Mycobacterium spp., Nocardia spp., Mycoplasma pneumoniae, Legionella, Bordetella, Chlamydia psittaci, Chlamydia pneumoniae, Streptococcus species, Trauma Clostridium tetani Needlestick Staphylococcus aureus, Pseudomonas species Arthropod bite Rickettsia spp, Ehrlichia spp, Coxiella spp, Yersinia pestis Sexual Neisseria gonorrhoeae, Chlamydia trachomatis, Treponema pallidum transmission 3. Adhere to, and/ or colonize the host: Colonization depends on the ability of bacteria to compete successfully with the host s normal microbiota for essential nutrients and space. Specialized structures (adhesins) that allow bacteria to compete for surface attachment sites are also necessary for colonization. Bacterial pathogens and some other nonpathogens adhere with a high degree of specificity to particular tissues. Adherence factors called adhesins or ligands are one reason for this specificity. Adhesins are specialized molecules or structures on the bacteria s cell surface that bind to complementary receptor sites on the host cell surface. The presence or absence of complementary cell receptors in various host tissues determines which tissues are colonized (tissue tropism). They are one type of virulence factors that contribute to pathogenicity. If adhesins, receptors or both are altered to interfere with adherence, it s possible to prevent or control infection. Begumisa MG lecture notes series Page 3

4 Bacterial adherence factors (adhesins) that play a role in infections Adherence factor Fimbriae Glycocalyx or capsule Slime layer Pili Teichoic and lipoteichoic acids Waxy coat Description Filamentous structures that help attach bacteria to other bacteria or solid surfaces e.g. E.coli, Vibrio cholerae, Neisseria gonorrhea & Candida albicans. The M protein of Strep. pyogenes mediates attachment of the bacterium to epithelial cells of the host and helps the bacterium resist phagocytosis by white blood cells A layer of exopolysaccharide fibers with a distinct outer margin that surrounds many cells. It inhibits phagocytosis & aids in adherence; when the layer is well organized and not easily washed off it is called a capsule. e.g. Streptococcus pneumoniae, Klebsiella pneumoniae, Haemophilus influenzae, Bacillus anthracis and Yersinia pestis. A bacterial film that is less compact than a capsule and is removed easily Filamentous structures that bind prokaryotes together for the transfer of genetic material. cell wall components in gram positive bacteria that aid in adhesion Increase the virulence of Mycobacterium tuberculosis by resisting digestion by phagocytes. M.tuberculosis can even multiply inside phagocytes. 4. Multiply (grow) or complete its life cycle on or in the host or the host s cells: those areas of the body that provide the most favorable conditions (nutrients, temperature, ph) within the host will harbor the pathogen and allow it to grow and multiply to produce an infection. Some bacteria can actively grow and multiply in the blood plasma. The presence of viable bacteria in the bloodstream is called bacteremia. The presence of bacteria or their toxins in blood is often termed septicemia. 5. Initially evade the host defense mechanisms: Pathogens often actively penetrate the host s mucous membranes and epithelial after attachment to the epithelial surface. This may be accomplished through production of lytic substances and extracellular enzymes that alter the host tissues by; a. Attacking the ground substance and basement membranes of integuments and intestinal linings. b. Degrading carbohydrate-protein complexes between cells or on the cell s surface.(the glycocalyx) c. Disrupting the cell surface One way the pathogen accomplishes evasion of the host defense (invasiveness) is by producing special products and/ or enzymes that promote spreading (shown in table below). These products are virulence factors. Once the circulatory system is reached, the bacteria have access to all organs and systems of the host. Begumisa MG lecture notes series Page 4

5 Bacterial products (virulence factors) involved in pathogen dissemination in host. Product Organism Involved Mechanism of Action Coagulase Staphylococcus aureus Coagulates (clots) the fibrinogen in plasma. The clot protects the pathogen from phagocytosis & isolates it from other host defenses Collagenase Clostridium spp Breaks down collagen that forms the frame work of connective tissues, allows the pathogen to spread. Deoxyribonuclease Group A Blowers viscosity of exudates, giving the pathogen more mobility Streptococci, Staphylococci, & Clostridium perfringens Elastase & alkaline protease Pseudomonas aeruginosa Cleaves laminin associated with basement membranes Hemolysins Hyaluronidase Hydrogen peroxide & ammonia Immunoglobulin A protease Staphylococci, Streptococci, E.coli, Clostridium perfringens Groups A,B,C and G streptococci, Staphylococci, Clostridia Mycoplasma spp Streptococcus pneumoniae Lyse erythrocytes causing anemia and weakened host defenses, make iron available for microbial growth. Hydrolyzes hyaluronic acid, a constituent of the intercellular ground substance that cements cells together and renders the intercellular spaces amenable to passage by the pathogen Are produced as metabolic wastes. These are toxic and damage epithelia in respiratory and urinogenital systems Cleaves immunoglobulin A (IgA) into Fab & Fc fragments Lecithinase Clostridium spp Destroys lecithin (phosphatidycholine) component of plasma membranes, allowing pathogen to spread. Leukocidins Staphylococci, Streptococci, Pneumococci Porins Salmonella typhimurium Protein A Staphylococcus aureus Pyrogenic exotoxin B (cysteine protease) Streptokinase (fibrinolysin, staphylokinase) Group A Streptococci, Strep. pyogenes Group A,C and G streptococci, staphylococci Pore forming exotoxins that kill leukocytes, cause degranulation of lysosomes within leukocytes which decreases host resistance Inhibit leukocyte phagocytosis by inactivating the adenylate cyclase system Located on cell wall. Immunoglobulin G (IgG) binds to protein A by its Fc end thereby preventing complement from interacting with bound IgG Degrades proteins A protein that binds to plasminogen and activates the production of plasmin, thus digesting fibrin clots; this allows the pathogen to move from the clotted area Begumisa MG lecture notes series Page 5

6 Urease Helicobacter pylori An enzyme that hydrolyses urea present in gastric juices to ammonia, neutralizing the acidity in the stomach thus creating an alkaline microenvironment. 6. Possess the ability to damage the host (a combination of invasiveness and virulence). If the pathogen overcomes the host defense, then it can damage host cells in three basic ways; a. Causing direct damage in the immediate vicinity of the invasion b. Producing toxins, transported by blood & lymph, that damage sites far removed from the original site of invasion. Toxins are poisonous substances that are produced by certain microorganisms and are frequently the primary factor that contributes to the pathogenic properties of those microbes. The capacity of microorganisms to produce toxins is called toxigenicity, causing serious and sometimes lethal, effects. Some toxins produce fever, shock, cardiovascular disturbances and diarrhea. Toxins are of two types, exotoxins and endotoxins. c. By inducing hypersensitivity reactions. ENDOTOXINS: Endotoxins are proteins that can be produced by gram-positive and gram-negative bacteria. These include cytolytic enzymes and receptor-binding proteins that alter the function or kill the cell. In many cases, the toxin gene is encoded on a plasmid (tetanus toxin, LT and ST toxins of Enterotoxigenic E.coli) or a lysogenic phage (Corynebacterium diphtheriae & C. botulinum). Exotoxins may be grouped into three principle types, based on their mode of action; Cytotoxins, which kill host cells or affect their functions Neurotoxins which interfere with normal nerve impulse transmission Enterotoxins which affect cells lining the gastrointestinal tract Many toxins are dimeric with A & B subunits (A-B toxins). The B portion of the A-B toxins binds to a specific cell surface receptor, and then the A subunit is transferred into the interior of the cell, where cell injury is induced. The biochemical targets of A-B toxins include ribosomes, transport mechanisms, and intracellular signaling (cyclic adenosine monophosphate production, G protein function); with effects ranging from diarrhea to loss of neural function to death. Examples of A-B toxins include; anthrax toxins, Bordetella adenylate toxin, Botulinism toxin, Cholera toxin, Diphtheria toxin, heat labile enterotoxin of E.coli, Pertussis toxin, Pseudomonas exotoxin A, Shiga toxin, shiga-like toxins of E.coli, and Tetanus toxin. Begumisa MG lecture notes series Page 6

7 Diseases Caused by Exotoxins Disease Bacterium Mechanism Botulinism Clostridium botulinum Neurotoxin prevents transmission of nerve impulses, flaccid paralysis results Tetanus Clostridium tetani Neurotoxin blocks nerve impulse that permits relaxation of one skeletal muscle while the opposing muscle is contracting Gas gangrene & food poisoning Diphtheria Scalded skin syndrome, food poisoning, and toxic shock syndrome Clostridium perfringens & other species of clostridium Corynebacterium diphtheriae One exotoxin (cytotoxin) causes massive RBC destruction (hemolysis), another exotoxin (enterotoxins) is related to food poisoning & causes diarrhea Cytotoxins inhibits protein synthesis especially in heart, nerve & kidney cells Staphylococcus aureus One exotoxin causes skin layers to separate & slough off (scalded skin); another exotoxin (enterotoxins) produces diarrhea and vomiting; still another exotoxin produces symptoms associated with Toxic shock syndrome Cholera Vibrio cholera Enterotoxins induces diarrhea Scarlet fever Streptococcus Cytotoxins cause vasodilation that results into the characteristic pyogenes rash Traveler s diarrhea Enterotoxigenic E. coli & Shigella spp Enterotoxin causes excessive secretion of ions and water; diarrhea results. ENDOTOXINS The outer membrane of gram-negative bacteria consists of lipoproteins, phospholipids and lipopolysaccharides (LPS). The lipid portion of LPS, called lipid A is the endotoxin; thus endotoxins are lipopolysaccharides. All endotoxins produce the same signs and symptoms regardless of the species of microbe, although not to the same degree. Responses by host to endotoxin include fever, weakness, generalized aches, and in some cases shock. The fever (Pyrogenic response) is believed to occur when gram-negative bacteria are ingested by phagocytes, degraded in vacuoles, and LPS of bacteria cell wall are released. Endotoxins binds to specific receptors on macrophages, B cells and other immune cells stimulating to production and release of cytokines such as interleukin- 1 (IL-1), tumor necrosis factor-α (TNF-α), and interleukin-6 (IL-6), which are carried via blood to the hypothalamus. IL-1 induces the hypothalamus to release proteins called prostaglandins that reset the thermostat in the hypothalamus at a higher temperature, the result being fever. Begumisa MG lecture notes series Page 7

8 Comparison of Exotoxins and Endotoxins Property Exotoxins Endotoxins Bacterial source Mostly from gram-positive bacteria Almost exclusively from gram-negative bacteria plus a few gram-positive bacteria Relation to microbe Metabolic product of growing cell Presence from LPS of outer membrane of cell wall & released only with lysis of cell Chemistry Protein or short peptide Lipid portion (lipid A) of LPS of outer membrane Heat stability Unstable; can easily be destroyed at o C (except staphylococcal enterotoxins) Stable, can withstand autoclaving (121oC for one hour. Toxicity high Low Immunology Can be converted to toxoids, neutralized Not easily neutralized by antitoxin, therefore by antitoxins effective toxoids can t be made Pharmacology Specific for a particular cell structure or function in the host (mainly cytotoxins, neurotoxins & enterotoxins) Lethal dose small Considerably large Representative disease Gas gangrene, tetanus, botulinism, diphtheria, scarlet fever, cholera, traveler s diarrhea General such as fever, weakness, aches, and shock; all produce the same effects Typhoid fever, urinary tract infections, and meningococcal meningitis 7. Leave the host and return to the reservoir or enter a new host (portal of exit). The last determinant of a successful bacteria pathogen is its ability to leave the host and enter either a new host or a reservoir. Most bacteria employ passive escape mechanisms. Passive escape occurs when a pathogen or its progeny leave the host in feces, urine, droplets, saliva or desquamated cells. Koch s postulates; is this the etiologic agent? The microbe in the initial and second isolations and, the disease in the patient and experimental animal must be identical for the postulates to be satisfied. Once established, these postulates were rapidly put to test and helped determine the causative agents of tuberculosis, diphtheria and plague. Today, most infectious diseases have been directly linked to a known infectious agent. Every decade, new diseases challenge the scientific community and require application of the postulates. Prominent examples are AIDS, Toxic shock syndrome, Lyme disease and Legionnaires disease. Begumisa MG lecture notes series Page 8

Bacterial Pathogenesis. Assistant professor Dr. Hayfaa Alhadithi

Bacterial Pathogenesis. Assistant professor Dr. Hayfaa Alhadithi Bacterial Pathogenesis by Assistant professor Dr. Hayfaa Alhadithi OBJECTIVES 1- List and define steps of bacterial pathogenesis from it s entry till the appearance of disease. 2- Discuss the important