Pathogenicity of Bacteria PDF

PATHOGENESIS OF BACTERIAL INFECTION Defininition of terms Adherence (adhesion, attachment): The process by which bacteria stick to the surfaces of host cells. After bacteria have entered the body, adherence is a major initial step in the infection process. The terms adh...

PATHOGENESIS OF BACTERIAL INFECTION Defininition of terms Adherence (adhesion, attachment): The process by which bacteria stick to the surfaces of host cells. After bacteria have entered the body, adherence is a major initial step in the infection process. The terms adherence, adhesion, and attachment are often used interchangeably. Carrier: A person or animal with asymptomatic infection that can be transmitted to another susceptible person or animal. Inactivation: Loss of pathogenicity Infection: Multiplication of an infectious agent within the body. Multiplication of the bacteria that are part of the normal flora of the gastrointestinal tract, skin, and so on is generally not considered an infection; on the other hand, multiplication of pathogenic bacteria (eg, Salmonella species) —even if the person is asymptomatic—is deemed an infection. Invasion: The process whereby bacteria, animal parasites, fungi, and viruses enter host cells or tissues and spread in the body. Microbiota: Microbial flora harbored by normal, healthy individuals. Nonpathogen: A microorganism that does not cause disease; may be part of the normal microbiota. Opportunistic pathogen: An agent capable of causing disease only when the host’s resistance is impaired (ie, when the patient is “immunocompromised”). Pathogen: A microorganism capable of causing disease. Pathogenicity: The ability of an infectious agent to cause disease. Resolution: Microorganism cannot be isolated. Superantigens: Protein toxins that activate the immune system by binding to major histocompatibility complex (MHC) molecules and T-cell receptors (TCR) and stimulate large numbers of T cells to produce massive quantities of cytokines. Toxigenicity: The ability of a microorganism to produce a toxin that contributes to the development of disease. Virulence: The quantitative ability of an agent to cause disease. Virulent agents cause disease when introduced into the host in small numbers. Virulence involves adherence, persistence, invasion, and toxigenicity. Identifying Bacteria that Cause Disease - Microbiota Do not produce disease Ensures the survival, growth, and propagation of both the bacteria and the host. Some bacteria that causes disease are cultured commonly with the normal flora. Some pathogens are present but latent, and the host is a carrier. Koch’s Postulate 1. The microorganism should be found in all cases of the disease in question, and its distribution in the body should be in accordance with the lesions observed. 2. The microorganism should be grown in pure culture in vitro (or outside the body of the host) for several generations. 3. When such a pure culture is inoculated into susceptible animal species, the typical disease must result. 4. The microorganism must again be isolated from the lesions of such experimentally produced disease. Molecular Koch’s Postulate 1. The phenotype should be significantly associated with pathogenic strains of a species. 2. Specific inactivation of the gene should lead to a measurable decrease in pathogenicity or virulence. 3. Reversion or replacement of the mutated gene with the wild-type gene should lead to restoration of pathogenicity or virulence. Molecular Guidelines for Establishing Microbial Disease Causation 1. The nucleic acid sequence of a putative pathogen causing the disease should be present preferentially in anatomic sites where pathology is evident. 2. The nucleic acid sequence of a putative pathogen should be absent from most healthy control participants. If detected, it should be in a lower prevalence as compared with patients with disease. 3. The copy number of a pathogen-associated nucleic acid sequence should decrease or become undetectable with resolution of disease, and should increase with relapse or recurrence of disease. 4. The presence of a pathogen-associated nucleic acid sequence in healthy subjects should help predict the subsequent development of disease. 5. The nature of the pathogen should be consistent with the known biologic characteristics of closely related organisms and the nature of the disease. The significance of a detected microbial sequence is increased when microbial genotype predicts microbial morphology, pathology, clinical features of disease, and host response.  Whipple disease (Tropheryma whipplei) Bacillary angiomatosis (Bartonella henselae) Human monocytic ehrlichiosis (Ehrlichia chaffeensis) Hanta virus pulmonary syndrome (Sin Nombre virus) Kaposi sarcoma (Human herpesvirus 8) Limitations of Koch’s Postulate 1. Some microorganism cannot be grown in vitro, but can infect animals Treponema pallidum (syphillis) Mycobacterium leprae (leprosy) 2. Some microorganism can be readily cultured in vitro, but no animal mode of infection. Neisseria gonorrhoeae (gonorrhea) 3. It does not include the following concepts: a. The asymptomatic carrier state (Koch’s only included the diseased organisms b. The biologic spectrum of disease c. Epidemiologic and immunologic elements of disease d. Prevention of disease by elimination of putative cause as element of causation. e. Multiple causation (heart failure, chronic kidney disease, and other complications). f. One syndrome having different causes in different settings. g. Reactivation of latent agents as cause of disease. f. Immunologic processes as cause of disease. ( immunocompromised state) Koch’s postulates have been partially satisfied by showing bacterial pathogenicity in an in vitro model of infection rather than in an animal model. Escherichia coli induces diarrhea by its interaction with host cells in tissue culture. Polymerase Chain Reaction Used to amplify microorganism-specific nucleic acid sequences from host tissues or fluids. Classification of Bacteria Based on Principle of Koch’s Postulate 1. Pathogens: Their presence is abnormal Mycobacterium tuberculosis (TB) Yersinia pestis (plague) Escherichia coli (UTI) 2. Non-pathogens 3. Opportunistic Pathogens: Only cause disease in immunosuppressed and debiliated persons. Pesudomonas species Stenotrophomonas maltophila Yeasts and molds Transmission of Infection I 1. Disease in humans exists primarily in animals and incidentally infect humans Salmonella species Campylobacter species 2. Transmission by the fleas to human is inadvertent Y. pestis (plague) has a well-established life cycle in rodents. Rodent fleas. 3. Transmission to humans by products Bacillus anthracis (anthrax) is found in raw hair from an infected animal. I 4. Transmission to humans by ingestion Clostridium perfringens (gastroentiritis) Clostridium botulism (botulism)  5. Soil contamination Clostridium perfringens ( gas gangrene) Clostridium tetani (tetanus) 6. Spores Protect the organisms’ nucleic acid from harsh environmental factors (ultraviolet light, desiccation, chemical detergents, and pH extremes). Ensure survival in external environments (foods ingested by humans) Germinate into the vegetative, metabolically active form of the pathogen after ingestion. 7. Food & Water contamation Vibrio cholera (diarrhea) Escherichia coli (diarrhea) 8. Respiratory disease Mycobacterium tuberculosis (tuberculosis) 9. One person. to another on hands Staphylococcus areus found in the anterior nares 10. Nosocomial infections Transmitted from one patient to another on the hands of hospital personnel. Portals of Entry of Pathogenic Bacteria Sites where mucous membranes meets the skin Respiratory tract (upper & lower airway) Gastrointestinal tract (primarily mouth) Genital tracts Urinary tracts They contain IgA as a primary defense against infection The Infectious Process 1. Establishment of a primary site infection by adherence to the host cells. 2. Multiplication through tissues or via kympathic system to the bloodstream. Bacteremia Allows bacteria to spread widely in the body and permits them to reach tissues particularly suitable for I their multiplication. Pneumococcal pneumonia 1. Pneumococci located in the nasopharynx are aspirated into the lungs (absent gag & cough reflexes) 2. Infection develops in the terminal air spaces of lungs in persons without antibodies. 3. Multiplication of pneumococci and resultant inflammation lead to pneumonia. 4. Pneumococci enter the lympathics of the lungs and move to bloodstream. 5. When bacteremia occurs, it can lead to secondary site of infection (cerebrospinal fluid, heart valves, and joint spaces) leading to meningitis, septic arthritis, endocarditis)  Vibrio cholerae Causes disease by ingestion. Production of cholera toxin results in flow of chloride and water into the lumen of the gut, causing diarrhea and electrolyte imbalance. Genomics and Bacterial Pathogenicity Bacteria are haploid They limit their genetic interaction to avoid changes in their chromosomes. Changes in their chromosomes may disrupt their adaptation and surival in a specific environment. Disruption of their environment leads to the displacement of bacteria from their usual site, resulting in infection. Primary Mechanisms for Genetic Exchange 1. Natural transformation DNA from one organism is released into the environment and is taken up by a different organism. E. coli has a resistant gene against antibiotic and Pseudomonas do not. Incomplete exposure of E. coli to antibiotic may not completely kill the pathogen. The content of E. coli including the gene is released to the environment and may be taken up by Pseudomonas aeruginosa. 2. Genetic Transmission Plasmids Transposons Bacteriophages Extra Highly mobile Carrries and chromosomal segment of DNA injects phage pieces of DNA in moving from one DNA into are capable of part of DNA to bacteria which replicating. another. recombines It carries majority Transposition from with the of the antibiotic prophage (DNA) to bacterial resistant gene. chromosome. chromosome. This can result in recombination between extrachromosomal DNA and the chromosome. Illegitimate or Non Homologous Recombination. Pathogenicity Islands (PAI) Large group of genes located on the chromosome. Not capable of self-replication Encode different virulence factors Originate from gene transfer from foreign species. Determines different pathogenic strains of particular species Escherichia coli, Salmonella, Shigella, Streptococcus pneumoniae Major properties Contains one or more virulence genes Present in the genomes of pathogenic members of species, but absent in nonpathogenic members. Large chromosomal regions Different guanine + cytosine content Commonly associated with tRNA Found with parts of the genome associated with mobile genetic elements (transposons) Possess genetic instabiity Often present mosaic structures with components acquired at different times. Escherichia coli & Shigella spp: Mucosal protection is invaded Bacillus anthracis: Difficult to kill because of spores Clostridium botulinum: Botulinum toxin is injected into the muscles that cause wrinkles. This induces the muscle relaxation to help reduce appearance of wrinkles. Corynebacterium diphtheriae: Destruction of the pharynx. Regulation of Bacterial Virulence Factors I Pathogenic bacteria (and other pathogens) have adapted both to saprophytic or free-living states, environments outside of the body, and to the human host. Environmental signals often control the expression of the virulence genes (temperature, iron availability, osmolality, growth phase, pH, and specific ions (eg, Ca2+) or nutrient factors. Corynebacterium diphtheriae: Its toxin gene is produced only by strains lysogenized by the phages which is greatly enhanced when it is grown in a medium with low iron. Bordetella pertusis: Virulence is enhanced when grown at 37°C and suppressed at lower temperature, presence of high concentrations of MgSO4, or nicotinic acid. Vibrio Cholerae: Toxin expression is higher at a pH of 6.0 than at a pH of 8.5 and higher also at 30°C than at 37°C. Yersinia pestis: Produces a plasmid-encoded proteins which is expressed maximally at 35-37°C, and minimally at 20-28°C. Yersinia enterocolitica: Motile at 25°C but not at 37°C. Listeria monocytogenes: Motile 25°C but not/minimally motile at 37°C. -  Bacterial Virulence Factors Adherence Only one step in the infectious process, is followed by development of microcolonies and subsequent steps in the pathogenesis of infection. The more hydrophobic the bacterial cell surface, the greater the adherence to the host cell. Adherence Factors Pili (thick rodlike appendages) and fimbriae (shorter hairlike structures) Help mediate adherence of the bacteria to host cell surfaces. Type 1 Pili of E. coli Adherence can be blocked in vitro by addition of d-mannose to the medium. E. coli causing Urinary Tract Infection d-mannose- mediated adherence is absent. Contains P-pili which attach to a portion of the P blood group antigen. the minimal recognition structure is the disaccharide α-d-galactopyranosyl-(1–4)- β-d- galactopyranoside (GAL–GAL binding adhesion) E. coli causing Diarrhea Contains pilus (fimbriae-mediated) adherence to the intestinal epithelial cells. Group A Streptococci (Streptococcus pyogenes) The fimbrae contains lipoteichoic acid, protein F, and protein M. Lipoteichoic acid and protein F adherence to buccal epithelial cells via fibronectin (host cell). M protein acts as an antiphagocytic molecule and is a major virulence factor. Invasion of Host Cells and Tissues Central to the infectious process Invades tissues through the junctions between epithelial cells (Salmonella) Invades specific types of host’s epithelial cells and may enter the tissue (Yersinia species, N, gonorrhoeae, Chlamydia trachomatis) Invasion Entry of bacteria into host cells, implying an active role for the organisms and a passive role for the host cells. Toxin production and other virulence properties are generally independent of the ability of bacteria to invade cells and tissues C diphtheriae is able to invade the epithelium of the nasopharynx and cause symptomatic sore throat even when the strains are nontoxigenic. Shigella and Yersiniae enterocolitica have the same adherence-invasion process. 1. In vivo the Shigellae adhere to integrins on the surface of M cells in Peyer’s patches. 2. M cells present them to macrophages in the submucosa. 3. Shigella inside the M cells and macrophages activates the apoptosis 4. The shigellae spread to adjacent mucosal cells by actin polymerization that propels the bacteria. Shigella adheres to HeLa cells in vitro. Invasion plasmid antigens (IpA-D) aids in the process. Listeriae monocytogenes Adhere to and invade the intestinal mucosa, reach the bloodstream, and disseminate. The host cell engulfs the bacteria with the aid of internalins. It requires actin polymerization to propel the bacteria. Legionella pneumophilia Infects pulmonary macrophages and causes pneumonia. Adherence of the legionellae to the macrophage induces formation of a long, thin pseudopod that then coils around the bacteria, forming a vesicle (coiling phagocytosis). The vesicle remains intact, phagolysosome fusion is inhibited, and the bacteria multiply within the vesicle. Neisseria gonorrhoeae Uses pili and opacity associated proteins (Opa) as primary and secondary adhesins to host cells. Pili and Opa together enhance the invasion of cells cultured in vitro. In uterine (fallopian) tube organ cultures, the gonococci adhere to the microvilli of nonciliated cells and appear to induce engulfment by these cells Toxins Exotoxin: Most often excreted from the cell. It accumulate inside the cell and are either injected directly into the host or are released by cell lysis. Endotoxin: Lipid molecules that are components of the bacterial cell membrane. A. Exotoxins Produced by gram-positive and gram-negative bacteria. Contains A and B subunits B subunit: mediates adherence of the toxin complex to a host cell and aids entrance of the exotoxin into the host cell. A subunit: provides the toxic activity. Its high antigenicity induces more antibodies formation. Toxoids Vaccines developed for some exotoxin-mediated disease. They are modified so they are no longer toxic Corynebacterium diphtheriae Gram-positive rod that can grow on the mucous membranes of the upper respiratory tract or in minor skin wounds. Strains carry a lysogenic, temperate corynebacteriophage (β-phage or ω-phage) produce diphtheria toxin, causing diphtheria. Diphtheria toxin Very potent Fragment A: Inhibits peptide chain elongation factor EF-2 by catalyzing a reaction that attaches an adenosine diphosphate–ribosyl group to EF-2, yielding an inactive adenosine diphosphate–ribose–EF-2 complex. Fragment B: Binds to specific host cell receptors and facilitates the entry of fragment A into the cytoplasm. Clostridium tetani Anaerobic gram-positive rod that causes tetanus Contaminates wounds The spores germinate in the anaerobic environment of the devitalized tissue. Infection often is minor and not clinically apparent. Produces tetanospasmin. Cleaved by a bacterial protease into two peptides.  Tetanospasmin It binds to the presynaptic membrane receptors of motor neurons. It migrates to the retrograde axonal transport system to the cell bodies of these neurons to the spinal cord and brainstem. It diffuses to terminals of GABA and Glycine secreting neurons. It degrades synaptobrevin. Required for docking the neurotransmitter vesicle on the synaptic membrane. Release of the inhibitory glycine and GABA is blocked, and the motor neurons are not inhibited. Spastic paralysis results. Globulin: Tetanus vaccine used by immunocompromised people. Toxoid: Vaccine used by immunologically normal people Clostridium botulinum Causes botulism Anaerobic, gram positive spore-forming organism is found in soil or water and may grow in foods. Heat labile and is destroyed by sufficient heating. Some serologic types of toxin (A, B, E, & F) are associated with human disease. Botulinum Absorbed from the gut and binds to receptors of presynaptic membranes of motor neurons of the peripheral nervous system and cranial nerves. Proteolysis, by the light chain of botulinum toxin, inhibits the release of acetylcholine at the synapse It results to lack of muscle contraction and flaccid paralysis. Clostridium perfringens The spores are introduced into wounds by soil or feces contamination. In the presence of necrotizing tissue, spores germinate, and vegetative cells can produce several different toxins, resulting to gas gangrene. Alpha toxin It is a lecithinase that damages cell membranes by splitting lecithin to phorylcholine and diglyceride. Theta toxin It has nectrotizing effect DNAses & Collagenases Produced by clostridiae as well. Staphylococcus aureus Grows on mucous membranes or in wounds. Toxic Shock Syndrome Toxin-1 (TSST-1) Super antigen and stimulates T-cells to produce IL-2 and TNF. It causes Toxic Shock Sydrome Characterized by shock, high fever, and a diffuse red rash that later desquamates. Group A β-hemolytic streptococci Pyrogenic exotoxin A Super antigen (acts similar to TSST-1) Results in scarlet fever (similar to streptococcal erythrogemic toxin). B. Exotoxins Associated with Diarrheal and Food Poisoning Enterotoxins Exotoxins associated with diarrheal diseases Vibrio cholerae It penetrates the intestinal mucosa and attaches to microvilli. Serotype O1 (and O139) can produce toxins consisting of two subunits. Subunit B Has five identical peptides and rapidly binds the toxin to cell membrane ganglioside molecules. Subunit A1&A2 Enters the cell membrane, increases adenylate cyclase activity, and increases concentration of cAMP, resulting to a secretion of electrolytes to lumen (with impairment of sodium and chloride absorption and loss of bicarbonate) Life threatening massive diarrhea (20-30 L/day) occurs and acidosis develops. Treatment is electrolyte and fluid replacement. Staphylococcus aureus Produce enterotoxins while growing in meat, dairy products (not properly refrigerated). Most common cause of food poisoning. Staphylococcal enterotoxin Superantigen Ingested, absorbed in the gut, stimulates vagus nerve receptors. The stimulus is transmitted to the vomiting center in the CNS, which occurs within hours. Other enterotoxins Y. enterocolitica Vibrio parahaemolyticus Aeromonas species C. Lipopolysaccharides of Gram-Negative Bacteria: Endotoxin LPS are often liberated when the bacterial lyse Heat-stable Contains lipooligosaccharides, LOS, and lipopolysaccharides. Can be extracted with phenol-water Pathophysiologic effects of LPS LPS in the bloodstream is initially bound to circulating proteins. It interacts with receptors on macrophages, neutrophils and other cells of the reticuloendothelial system. Proinflammatory cytokines released (IL-1, IL-6, IL-8, & TNF-α) Complement and coagulation cascades are activated. Characterized by fever, leukopenia, and hypoglycemia; hypotension and shock; intravascular coagulation; and death from massive organ dysfunction. Injection of IL-1 produces fever after 30 minutes (repeated injection produces the same response) Injection of LPS causes Fever after 60-90 minutes The time needed for the body to release IL-1 Leukopenia Secondary leukocytosis occurs later. The early leukopenia coincides with the onset of fever caused by liberation of IL-1 Hypoglycemia Due to the increase activity of glycolysis Anaphylatoxins, chemotactic responses, membrane damage and a Complement cascades drop in serum levels of C3, C5–C9. Factor XIII (Hageman factor) The first step of the intrinsic clotting system involving the conversion of fribrinogen to fibrin. Plasminogen to plasmin Attack fibrin with the formation of fibrin split products. Platelets adherence & small blood vessel Cause of schemic or hemorrhagic necrosis in various organs. occlusion Arteriolar and venular constriction, peripheral vascular dilatation, increased vascular permeability, decrease in venous return, lowered Hypotension cardiac output, stagnation in the microcirculation, peripheral vasoconstriction, shock, impaired organ perfusion, disseminated intravascular coagulation (DIC) Limulus Test Used to assess endotoxin levels D. Peptidoglycan of Gram-Positive Bacteria Made up of cross-linked macromolecules that surround the bacterial cells. May also lead to shock More cell wall-associated peptidoglycan Invariably less potent than LPS. Enzymes Intrinsically toxic but do play important roles in the infec- tious process. A. Tissue-Degrading Enzymes Enzymes from C. perfringens, S. aureus, and group A streptococci C. perfringens Lecithinase Collagenase: degrades collagen, the major protein of fibrous connective tissue, and promotes spread of infection in tissue. S. Aureus Coagulase: works in conjunction with blood factors to coagulate plasma. : contributes to the formation of fibrin walls which helps them persist in tissues : causes deposition of fibrin on the surfaces, which may help protect them from phagocytosis or from destruction within phagocytic cells. Hyaluronidase: hydrolyze hyaluronic acid, a constituent of the ground substance of connective tissue. Group A Streptococcus Streptokinase: fibrinoysin, activates a proteolytic enzyme of plasma. : able to dissolve coagulated plasma and probably aids in the rapid spread of streptococci through tissues.  : used to treat acute MI to dissolve fibrin clots Streptolysin O : hemolytic for red blood cells from many animals : can be oxidized and inactivated, but it is reactivated by reducing agents. : Antigenic Streptolysin S : Not antigenic Other enzymes: Cytolysins Hemolysins: dissolves red blood cells (E. coli, S. aureus, & Staphylococci, & Clostridium) Leukocidins: kill tissue cells or leukocytes B. IgA1 Proteases Allows pathogens to inactivate the primary antibody on mucosal surfaces and thereby eliminate protection of the host by the antibody. Split IgA1 at specific proline–threonine or proline–serine bonds in the hinge region and inactivate its antibody activity. Important virulence factor of the pathogens (N gonorrhoeae, N meningitidis, H influenzae, and S pneumoniae, Prevotella melaninogenica) Antiphagocytic Factors Some pathogens evade phagocytosis or leukocyte microbicidal mechanisms Surface structures show much antigenic heterogeneity S. aureus has surface protein A which binds to Fc portion of IgG. S. pneumoniae & N. meningitidis have polysaccharide capsules. S pyogenes has M protein N gonorrohoeae has pili Capnocytophaga & Bordetella species produce soluble factors or toxins that inhibit chemotaxis by leukocytes Intracellular Pathogenicity They live within neutrophils, monocytes, or macrophages. Mechanisms: Avoid entry into phagolysosomes and live within the cytosol of the phagocyte. Prevent phagosome–lysosome fusion and live within the phagosome. Resistant to lysosomal enzymes and survive within the phagolysosome. Antigenic Heterogeneity The surface structures of bacteria – considerable antigenic heterogeneity. Antigens are used as part of a serologic classification system for the bacteria. Salmonellae – based on the types of the O (LPS (lipopolysaccharide) side chain) and H (flagellar) antigens. E. coli O types – more than 150 types E. coli K (capsule) types – more than 100 types Streptococcal M proteins – high incidence of poststreptococcal glomerulonephritis Neisseria meningitidis – capsular polysaccharide types A and C Type III Secretion System Complex protein secretion system employed by many Gram-negative pathogenic bacteria Transport bacterial effector proteins across three membrane barriers into eukaryotic host cytoplasm The effector proteins delivered by TTSS are capable of modulating and interfering with the host cellular processes Plague Typhoid fever Bacterial dysentery Composed of more than 20 structural proteins, effector proteins, and chaperones.