Bacterial Pathogenesis PDF

Bacterial Pathogenesis RAKESH SHARDA Department of Veterinary Microbiology NDVSU College of Veterinary Science & A.H., MHOW  MICROORGANISM AND HOST  Saprophytism – Living on dead or decaying organic matter.  Parasitism – Living on or within another living organ...

Bacterial Pathogenesis RAKESH SHARDA Department of Veterinary Microbiology NDVSU College of Veterinary Science & A.H., MHOW  MICROORGANISM AND HOST  Saprophytism – Living on dead or decaying organic matter.  Parasitism – Living on or within another living organism - there are different types of host-parasite relationships.  Commensalism – parasite lives on/in the host without causing any disease.  Symbioism – mutally beneficial relationship  Opportunistic pathogen – The organism is generally harmless, but can cause disease when it gains access to other sites or tissues.  Obligate pathogen – parasite always causes disease.  Infectivity – It is the capacity of the organism to penetrate the tissues of host, to survive the host defenses, and to multiply and disseminate in host.  Pathogenicity – It is the capacity of the microbial species to produce disease.  SOURCES OF INFECTION Sources of infection are animal and inanimate in nature. Animal sources  Normal flora  Animals in incubation period of disease  Animals with overt disease.  Convalescent carrier animals – In these animals shedding of the pathogen occurs for varying periods after clinical recovery. The period may vary from weeks to months.  Contact carrier or subclinical infections – They acquire pathogenic organisms from other animals suffering with infectious disease without contracting the disease themselves. Such animals are called as contact or subclinical carriers. The carrier state may be temporary for a few days or lasting for months. Inanimate sources (fomites) Contaminated utensils, feed and water troughs, vehicles, etc.  TRANSMISSION Disease can be transmitted by direct or indirect contact. Direct contact contact with discharges or aerosols from the animal. Coitus. Vertical transmission from mother to offspring. Indirect contact Organisms excreted by the infected animal are carried in/on various vehicles like feed, water, litter, clothing, footwear, farm house products and by-products, equipments, personnel, logistics, air or dust. Such contaminated objects are called as fomites. Contaminated instruments may also spread the infection.  Routes of Entry  Inhalation  Ingestion  Inoculation through the skin or mucous membrane  Coitus or artificial insemination.  Transplacental / in ovo  Hospital acquired infections - nosocomial infections.  Physician induced infections - iatrogenic infections.  Opportunistic infections compromised people/animal – normal flora Skin – Staphylococcus aureus – S. epidermidis – Propionibacterium acnes Intestine – Bacteroides * high numbers – Enterobacteriaceae * low number – environment – Dermatophytes – nosocomial  PATHOGENICITY PATHOGENICITY: The ability to cause disease virulence factors number of initial organisms immune status The "objective" of bacteria is to multiply rather than to cause disease; it is in the best interest of the bacteria not to damage or kill the host. A disease ensues when the balance between bacterial pathogenicity and host resistance is upset. Pathogenic bacteria can be grouped into three categories on the basis of their invasive properties for eukaryotic cells: Extracellular Obligate Intracellular Facultative Intracellular.  TERMINOLOGIES Bacteraemia  Bacteraemia is the presence of bacteria in blood.  The pathogenic organisms may gain entry in to a blood capillary or venule actively or passively from the initial site of entry.  Once in blood stream the organism can cause localized infection or spread to various parts of the body, e.g. Leptospires reach the kidneys following bacteraemia.  Organisms can directly gain access to the blood by first infecting the lymphoid system. Septicaemia  Septicaemia is the presence of actively multiplying bacteria in the blood.  One of most severe septicaemia is anthrax in which the number of bacteria in blood may often exceed the erythrocytes in blood.  Septicaemic infections often start as localized infections that later become generalized, e.g, streptococcal pharyngitis, bubonic plague. Toxaemia Toxemia is the presence of toxins in blood.  VIRULENCE VIRULENCE: is the measure of the pathogenicity of an organism to cause disease; usually used to describe the difference in disease causing capability between two different strains of the same species. Virulence can be expressed as LD50 (lethal dose for 50% of the inoculated hosts) or ID50 (infectious dose for 50% of the inoculated hosts).  Virulence Factors VIRULENCE FACTORS: The factors produced by a microorganism and induce pathology in a host are called virulence factors. These factors help pathogen to (1) invade the host, (2) cause disease, and (3) evade host defenses. Virulence factors are classified into two categories – 1. Virulence factors that promote bacterial colonization of the host: Adherence Factors Invasion and/or Spreading Factors Compete for iron and other nutrients; Evasion of host immune responses 2. Virulence factors that damage the host. Exotoxins Endotoxins PATHOGENIC ATTRIBUTES OR VIRULENCE FACTORS OF BACTERIA  Genetic Basis for Virulence Virulence factors in bacteria may be encoded on chromosomal DNA, bacteriophage DNA, plasmids, or transposons. For example, the heat-labile enterotoxin (LTI) of E coli is plasmid encoded, the heat-labile toxin (LTII) is encoded on the chromosome, and diphtheria toxin of C.diphtheriae is coded by phage. Virulence factors are acquired by bacteria by vertical or horizontal gene transfer  Bacterial Virulence Factors that Promote Colonization in the Host  Factors for contact with host cells 1. Bacterial motility - e.g., non-motile mutants of Vibrio cholerae are less virulent than the motile wild types. 2. Bacterial enzymes - e.g., Streptococcus pyogenes produces streptokinase that facilitate spread of the bacterium by liquefying the fibrin clot  Factors for adherence with host cells - adhesins Adhesins are proteins found on the cell wall of various bacteria that bind to specific receptor molecules on the surface of host cells and enable the bacterium to adhere intimately to that cell in order to colonize and resist physical removal, e.g. common fimbriae, capsule, biofilm, liptotechoic acid, Fibronectin binding protein (FBP), etc. Adherence factors - Pili (fimbriae) Adherence factors - capsules (biofilms) Adherence factors – FBP (eg. S.pyogenes) F-protein lipoteichoic acid fibronectin  Spreading Factors "Spreading Factors" are a family of bacterial enzymes that affect the physical properties of tissue matrices and intercellular spaces, thereby promoting the spread of the pathogen. Hyaluronidase - depolymerize hyaluronic acid, the interstitial cement substance of connective’ tissue; produced by streptococci, staphylococci, and clostridia. Collagenase - breaks down collagen; produced by Clostridium histolyticum and Clostridium perfringens. Neuraminidase - degrades neuraminic acid (also called sialic acid) present on epithelial cells of the mucosa; produced by Vibrio cholerae, Shigella dysenteriae, P.multocida, and M.haemolytica Streptokinase and Staphylokinase - convert inactive plasminogen to plasmin which digests fibrin. Edema Factor of B.anthracis - adenylate cyclase activity promote bacterial invasion.  Invasins  Some bacteria have mechanisms by virtue of which they initiate phagocytosis in non-phagocytic cells for invasion by:  binding to some receptor on cell, eg. Yersinia pestis  injecting invasins, such as Type III secretion system in bacterial cytoplasm, eg. Salmonella In either case changes in host cell cytoskeleton cause the bacteria to be ingested  Some pathogens can utilize actin fibres intracellularly to move through host cells (transcytosis), eg. Listeria monocytogenes  Invasins may also mediate uptake of bacteria into professional phagocytic cells in a way that bypass normal phagosome formation  The Type III Secretion system in Bacteria The bacteria having the type III secretion system on contact with cells, delivers proteins into the cells which cause polymerization and depolymerization of actin filaments resulting in cytoskeletal rearrangement. Thus the invasins is able to trick the non-phagocytic cell into behaving like a phagocyte and engulf the bacterium into phagosome like vacuole. The bacteria then cause the vacuole membrane to rupture and escape into the cytoplasm Transcytosis  The Ability to Compete for Nutrients and Iron Bacteria compete for nutrients by synthesizing specific transport systems or cell wall components capable of binding limiting substrates and transporting them into the cell.  Siderophores--low MW compounds that chelate iron with very high affinity, eg. E.coli  Direct binding of host transferrin, lactoferrin, ferritin, or heme by bacterial surface receptors, eg. Yersinia species  Exotoxins that lyse host cells (can be used to obtain other nutrients as well), eg. haemolysins Bacterial Virulence Factors that helps in Evasion of Immune Response  Evasion of Innate Immune Responses  Invade or remain confined in regions inaccessible to phagocytes. e.g. the lumen of glands and the skin are not patrolled by phagocytes.  Avoid provoking an inflammatory response.  Hide the antigenic surface of the bacterial cell. eg, S.aureus produces coagulase which clot fibrin on the bacterial surface  Inhibit chemotaxis of phagocytes, e.g. Streptococcal streptolysin, fractions of Mycobacterium tuberculosis and Clostridium ø toxin suppresses neutrophil chemotaxis  Inhibit ingestion by phagocytes, e.g. capsule inhibit recognition and engulfment by phagocytes  Resistance to complement mediated lysis (serum resistance), e.g. capsule, LPS, S-layers, etc. Capsules Blocking the Attachment of Bacteria to Phagocytes Resistance to opsonization/phagocytosis ii. LPS O polysaccharide iii. S-layer iv. Extracellular products: enzymes that inactivate C5a chemoattractant (S. pyogenes), toxins that kill phagocytes (leukotoxins) (Mannheimia haemolytica),inhibit migration, or reduce oxidative burst. Strategies for surviving phagocytosis: I. Escape from phagosome before fusion with lysosome (example: Listeria monocytogenes, mediated by listeriolysin) II. Prevent phagosome-lysosome fusion – eg, Salmonella, Mycobacterium , Legionella and Chlamydia III. Express factors that allow survival in phagolysosome  Evasion of Adaptive Immune Defenses Antigen masking - Some bacteria are able to coat themselves with host proteins such as fibrin, fibronectin, lactoferrin, or transferrin and in this way avoid antibodies. Antigenic switching or phase variation - one way certain bacteria can evade antibodies is by changing the adhesive tips of their pili or vary other surface proteins so that antibodies already made will no longer "fit." Staphylococcus aureus produces protein A while Streptococcus pyogenes produces protein G. which non-specifically binds IgG with very high affinity  Immunoglobulin proteases - Bacteria such as Haemophilus influenzae , Streptococcus pneumoniae , Helicobacter pylori , Shigella flexneri , Neisseria meningitidis , Neisseria gonorrhoeae and Enteropathogenic E. coli produce enzymes that degrade the antibodies found in body secretion (IgA). Staphylococcus aureus Resisting Opsonization via Protein A The Fc portion of the antibody IgG, the portion that would normally binds to Fc receptors on phagocytes, instead binds to protein A on Staphylococcus aureus. In this way the bacterium becomes coated with a protective coat of antibodies that do not allow for opsonization. Bacterial Virulence Factors that damage the Host Virulence factors that damage the host include:  The cell wall components that bind to host cells causing them to synthesize and secrete pro-inflammatory cytokines and chemokines.  Toxins.  Induce autoimmune responses. Bacterial Cell Wall Components that Promote Synthesis and Secretion of Inflammatory Cytokines and Chemokines. LPS of Gram-negative bacteria, and teichoic acids and glycopeptides of Gram-positive bacteria induces cytokine production and secretion These cytokines, such as TNF-alpha, IL-1, interleukin-6 IL-6, IL-8, and platelet-activating factor (PAF) promote inflammation and lead to activation of the complement pathways and the coagulation pathway. At moderate levels, inflammation, products of the complement pathways, and products of the coagulation pathway are essential for body defense. However, these when excessive produced in excessive amounts cause exaggerated inflammatory response which leads to MOSF. In some bacteria, lipoproteins in the outer membrane may also play a role in leading to excessive cytokine production. Harmful Effects of Glycopeptides and Teichoic Acid Released During Gram-Positive Infections BACTERIAL TOXINS  BACTERIAL TOXINS Toxins are of two types: Exotoxin Endotoxin.  ENDOTOXINS  Endotoxins are the the lipopolysaccharides component of the outer membrane of cell wall of the Gram negative bacterial cell  The endotoxins are released into the medium only following the death or the lysis of cells which occurs during late growth stages of culture.  The toxic effects of endotoxins are observed only after they are released in to the medium.  All endotoxins exhibit similar pharmacological effects. They cause pyrexia, blood changes and shock.  LPS is bound by LPS binding proteins (LBP) in plasma, which then binds CD14. This complex binds Toll-like receptor 4 (TLR4) on macrophages and monocytes. Macrophages and monocytes release cytokines (IL-1, IL-6, IL-8, TNF alpha, Platelet Activating Factor), which subsequently trigger prostaglandin and leukotriene release The complement and coagulation cascades are activated. Endotoxic shock occurs when bacterial products reach high enough levels in the blood to trigger exaggerated cytokine release, complement activation,, and coagulation cascade activation in many parts of the body. Circulatory system collapse followed by multiple organ system failure occurs.  Harmful Effects of LPS-Endotoxin a. fever production b. inflammation c. tissue destruction d. respiratory distress e. capillary damage (leading to petechial rash , capillary leakage, and hypovolemia) f. intravascular coagulation g. hypotension h. decreased cardiac output i. irreversible shock j. wasting of the body k. diarrhea (from endotoxin in intestines) l. allow bacteria to cross the blood-brain barrier Harmful Effects of LPS-Endotoxin  Exotoxins  produced by bacteria (both Gram-positive and Gram-negative)  released into the surrounding environment  proteins in nature  usually enzymes  heat stable (high mol. wt.) or heat labile (low mol. wt.)  functions for the bacteria are usually unknown  site of action is more localized and is confined to particular cell types  exotoxins are excellent antigens - elicit specific antibodies called antitoxins Exotoxins v/s Endotoxins  Types of Exotoxins On the basis of mode of action:  super antigens, e.g. Toxic shock syndrome toxin-1 produced by some strains of Staphylococcus aureus  toxins that act on the extracellular matrix of connective tissue, e.g. Clostridium perfringens collagenase  A-B toxins, e.g. botulinum toxin  exotoxins that damage host cell membranes, e.g. botulinum toxin On the basis of site of action: cytotoxins, e.g.diphtheria toxin and erythrogenic toxins neurotoxins, e.g.botulinum toxin and tetanus toxin. enterotoxins, e.g. cholera toxin and staphylococcal enterotoxin  Super antigens  bind directly to the MHC-II molecules on macrophages without being processed and cross link non-specifically with many TCRs on multiple T- cells.  cross linking causes stimulation of up to 1 in 5 T-cells in the body (normal antigens cause stimulation of 1 in 10,000).  results in the secretion of excessive amounts of interleukin-2 (IL-2)  high levels of IL-2 in the blood lead to symptoms such as fever, nausea, vomiting, diarrhea, and malaise.  stimulation of IL-2 secretion can also lead to production of other cytokines such as TNF-alpha, IL-1, IL-8, and PAF, which can lead to SIRS (Systemic Inflammatory Response Syndrome)  e.g., Toxic shock syndrome toxin-1 (TSST-1), produced by some strains of Staphylococcus aureus  Binding of T4-Lymphocytes to Conventional Antigens Conventional antigens are only recognized, after processing and presentation with MHC-II molecules by antigen presenting cells, by specific T4-lymphocytes having a TCR with a shape that corresponds to a peptide of that antigen  Binding of Super antigens Super antigens bind directly to the outside of MHC-II molecules and the TCRs and activate many T4-lymphocytes. A specific TCR is not required for activation. Toxins that act on the extracellular matrix of connective tissue  break down of host macromolecules , such as collagen, hyaluronic acid, proteins including immunoglobulins, etc.  play an important role in disease development by providing nutrients and/or helping in dissemination deeper in body tissues  cause extensive tissue damage  Examples:  Hyaluronidase –aid in the spread of bacteria by degrading extracellular matrix.  Collagenase –aids in dissemination  DNase–reduces viscosity of debris from dead cells  Proteases – tissue damage  A-B toxins  consist of two parts: A (active) - enzymatic component, and B – binding component  determines the host cell specificity of the toxin. Cell surface Active Binding A B  A-B toxins B component binds the exotoxin to a receptor molecule on the host cell After binding the exotoxin is translocated across the host cell membrane (some A-B toxins enter by endocytosis) (some passes directly through the host cell's membrane) A-component of the toxin separates from the B-component A-component is released or enter the cytoplasm of host cell A component then catalyze an enzymatic reaction A-B toxins A-B toxins  A-B toxins - Types  Those with ADP-ribosylating activity e.g. cholera toxin, E. coli heat labile toxin, Pseudomonas aeruginosa and diphtheria toxins.  Those with a lytic activity on 28S rRNA e.g. shiga and shiga-like (vero)toxins.  Those with a partially characterized site of action e.g. botulinum toxin, tetanus toxin and anthrax lethal toxin.  A-B toxins - Examples a) Diphtheria toxin: ADP-ribosylation of host EF-2; host cells are killed due to the blocking of translation of mRNA into polypeptides. b) Cholera toxin: ADP-ribosylation of a cAMP regulatory protein, which causes loss of ion regulation, water loss, diarrhea. c) Shiga toxin: cleaves host rRNA, which blocks translation and kills the host cell. d) Clostridium botulinum: large subunit targets neurons, small subunit cleave snare proteins inhibiting neurotransmitter release from neurons-causes paralysis  ADP-ribosylating A-B toxins Cholera toxin and E. coli labile toxin - ADP-ribosylation of regulator protein Gs - adenylate cyclase activation - cyclic AMP - block intestinal epithelial cells from taking in sodium from the lumen of the intestines - active ion and water secretion - diarrhea  A-B toxins with lytic activity on 28S rRNA Shiga toxin - Shigella and EHEC - lyses rRNA in ribosome - inhibits protein synthesis - death of epithelial cells - poor water absorption - diarrhea  Membrane damaging exotoxins  Cause damage or disruption of plasma membranes, which leads to osmotic lysisand cell death.  Three types of membrane disrupting toxins: a)Enzymes that hydrolyze phospholipids: phospholipase, sphingomyelinase b)Toxins with detergent-like surfactant activity that disrupt membrane by lipid solubilization c)Pore forming toxins (the most common): proteins that insert in the host membrane and form a hydrophilic pore  Membrane damaging exotoxins Examples:  C. perfringens phospholipase or alpha toxin (lecithinase)  Leukocidin - Staphylococcus aureus and Streptococcus pyogenes  Elastase - Pseudomonas aeruginosa  Haemolysins  Virulence Factors that Induce Autoimmune/Hypersensitivity Responses  Producing cross-reacting antibodies or auto reactive cytotoxic T- lymphocytes made in response to bacterial antigens that accidentally cross-react with epitopes on host cells destroying the host cells to which they have bound and/or activate the classical complement pathway that stimulates the inflammatory response resulting in more tissue damage, e.g. rheumatic fever triggered by some strains of Streptococcus pyogenes  Stimulating the production of immune complexes that activate the complement pathway resulting in inflammatory response, which destroys tissues, e.g. acute glomerulonephritis following infection by Streptococcus pyogenes.