Bacterial Pathogenesis Learning Objectives Define the basic concepts

Bacterial Pathogenesis

Learning Objectives • • • Define the basic concepts of infection • (infectious cycle/transmission chain, infectious disease, communicable disease, transmission routes, adherence, invasion, toxigenicity, infectious dose, carrier) Define the pathogen factors that contributes to host-pathogen interaction • (symbiotic relations, pathogenic/opportunistic/non-pathogenic microorganism, pathogenicity, virulence factors) Explain the importance of virulence factors in tissue destruction • List differences between exotoxin and endotoxin

Learning Objectives • • Describe the pathological consequences of infection and its impact on clinical course • List pathological events that may occur through direct effect • (impairment of cell function following cell lysis, disruption of cell functions, cytopathic effect, cytokine release, disruption of organ functions, mechanical effect, pressure effect) • Tell pathological outcomes of exotoxin effect • (inhibition of protein synthesis, cell lysis, paralysis, loss of fluid, cytokine release, edema, eruption) • Tell that it may cause pathological activation of innate immunity and list the consequences of activation • (cytokine activation, complement activation, edema) • Tell that immune response may have pathological consequences • (allergy, immune complex diseases, granuloma, eruption, autoimmunity) • Tell that it may cause malign transformation in cells Explain the probable clinical courses and factors effective on outcomes of infectious diseases • (recovery, immunity, immune deficiency, persistance, reactivation, autoimmunity, malignancy, mortality)

Mechanisms of Bacterial Pathogenesis Pathogen Mims’s Medical Microbiology, p: 67, Mosby, 2008.

Mechanisms of Bacterial Pathogenesis • • To a bacterium, the human body is a collection of environmental niches (warmth, moisture, and food necessary for growth) Bacteria have acquired genetic traits to: – – • • • enter (invade) the environment, remain in a niche (adhere or colonize), gain access to food sources (degradative enzymes), escape clearance by host immune and nonimmune protective responses (e. g. , capsule). Mechanisms that bacteria use to maintain their niche and the byproducts of bacterial growth (e. g. , acids, gas) cause damage and problems for the human host. Many of these genetic traits are virulence factors, which enhance the ability of bacteria to cause disease. – many bacteria cause disease by directly destroying tissue – some release toxins, which are then disseminated by the blood to cause system-wide pathogenesis. – surface structures of bacteria are powerful stimulators of host responses: • (acute phase: interleukin-1 [IL-1], interleukin-6 [IL-6], tumor necrosis factor [TNF]), which can be protective but are often the significant causes of the disease symptoms (e. g. , sepsis). Production of disease results from the combination of; • damage caused by the bacteria and • the consequences of the innate and immune responses to the infection.

Bacterial Virulence Mechanisms • • Adherence Invasion Evasion of phagocytic and immune clearance Direct destruction of tissues • • Byproducts of growth (gas, acid) Indirect destruction of tissues • Degradative enzymes • Cytotoxic proteins • Toxins • • Exotoxins Endotoxin Superantigen Induction of excess inflammation Capsule Intracellular growth Resistance to antibiotics

Bacterial Disease Production • Disease is caused by damage produced by the bacteria plus the consequences of innate and immune responses to the infection. • The signs and symptoms of a disease are determined by the function and importance of the affected tissue. • The length of the incubation period is the time required for the bacteria and/or the host response to cause sufficient damage to initiate discomfort or interfere with essential functions.

Body surfaces as sites of microbial infection and shedding

Portal of Entry

Adherence mechanisms

The mode of action of dimeric A-B exotoxins The bacterial A-B toxins often consist of a two-chain molecule. The B chain promotes entry of the bacteria into cells, and the A chain has inhibitory activity against some vital function. ACH, acetylcholine; c. AMP, cyclic adenosine monophosphate

Properties of A-B Type Bacterial Toxins

Activities of Endotoxin (Lipopolysaccharide (LPS)). This bacterial endotoxin activates almost every immune mechanism, as well as the clotting pathway, which together make LPS one of the most powerful immune stimuli known. DIC, disseminated intravascular coagulation; IFN-γ, interferon-γ; Ig. E, immunoglobulin E; IL-1, interleukin-1; PMN, polymorphonuclear (neutrophil) leukocytes; TNF, tumor necrosis factor

Endotoxin-Mediated Toxicity • • Fever Leukopenia followed by leukocytosis Activation of complement Thrombocytopenia Disseminated intravascular coagulation Decreased peripheral circulation and perfusion to major organs Shock Death

Endotoxin versus Exotoxin

Superantigen binding to the external regions of the T-cell receptor and the major histocompatibility complex class II (MHC II) molecules

Microbial Defenses against Host Immunologic Clearance • Encapsulation • Antigenic mimicry • Antigenic masking • Antigenic shift • Production of antiimmunoglobulin proteases • Destruction of phagocyte • Inhibition of chemotaxis • Inhibition of phagocytosis • Inhibition of phagolysosome fusion • Resistance to lysosomal enzymes • Intracellular replication

Examples of Encapsulated Microorganisms • Staphylococcus aureus • Streptococcus pneumoniae • Streptococcus pyogenes (group A) • Streptococcus agalactiae (group B) • Bacillus anthracis • Bacillus subtilis • Neisseria gonorrhoeae • Neisseria meningitidis • Haemophilus influenzae • Escherichia coli • Klebsiella pneumoniae • Salmonella spp. • Yersinia pestis • Campylobacter fetus • Pseudomonas aeruginosa • Bacteroides fragilis • Cryptococcus neoformans (yeast)

Examples of Intracellular Pathogens • Mycobacterium spp. • Brucella spp. • Francisella spp. • Rickettsia spp. • Chlamydia spp. • Listeria monocytogenes • Salmonella Typhi • Shigella dysenteriae • Yersinia pestis • Legionella pneumophila

Mechanisms for Escaping Host Defenses • Bacteria are parasites, and evasion of host protective responses is a selective advantage. • Bacteria: • evade recognition and/or killing by phagocytic cells, • inactivate or evade the complement system and antibody, and • grow inside cells to hide from host responses.

Mechanisms for Escaping Host Defenses The capsule is one of the most important virulence factors: • These slime layers function by shielding the bacteria from immune and phagocytic responses. • Capsules are typically made of polysaccharides, which are usually poor immunogens. • The S. pyogenes capsule, is made of hyaluronic acid, which mimics human connective tissue • The capsule is also hard to grasp by a phagocyte. • The capsule also protects a bacterium from destruction within the phagolysosome of a macrophage or leukocyte. • Mutants that lose the ability to make a capsule also lose their virulence; • e. g. Streptococcus pneumoniae and N. meningitidis. • A biofilm of capsular material, can prevent antibody and complement from getting to the bacteria.

Mechanisms for Escaping Host Defenses Bacteria can evade antibody responses by: • intracellular growth; • bacteria that grow intracellularly include mycobacteria, francisellae, brucellae, chlamydiae, and rickettsiae. • control of these infections requires TH 1 T-helper cell immune responses, which activate macrophages to kill or create a wall (granuloma) around the infected cells (e. g. for Mycobacterium tuberculosis). • antigenic variation; • Neisseria gonorrhoeae can vary the structure of surface antigens to evade antibody responses • inactivation of antibody or complement; • Neisseria gonorrhoeae produces a protease that degrades immunoglobulin A (Ig. A). • Streptococcus pyogenes degrades C 5 a component of complement, can limit the chemotaxis of leukocytes to the site of infection.

Mechanisms for Escaping Host Defenses Many bacteria circumvent phagocytic (neutrophil, macrophage) killing in various ways: (a place to grow and hide from immune responses, and a means of dissemination in the body) • produce enzymes capable of lysing phagocytic cells; • streptolysin by S. pyogenes • α-toxin by C. perfringens • inhibit phagocytosis; • capsule and M protein by S. pyogenes • block intracellular killing; • blocking phagolysosome fusion by Mycobacterium • capsule-mediated or enzymatic resistance to the bactericidal lysosomal enzymes or substances, • ability to exit the phagosome into the host cytoplasm before being exposed to lysosomal enzymes • production of catalase by staphylococci can break down the hydrogen peroxide produced by the myeloperoxidase system

Mechanisms for Escaping Host Defenses Bacteria may modify the alternate pathway of complement and antibody: • evade complement action by; • masking themselves • inhibiting activation of the cascade • prevent complement access by; • O antigen of LPS out of the membrane and protects gram-negative bacteria from damage • evade antibody recognition by; • S. aureus makes an immunoglobulin-G-binding protein, protein A, which masks the bacteria and thereby prevents antibody action • S. aureus can also escape host defenses by walling off the site of infection; • S. aureus produce coagulase, an enzyme that promotes the conversion of fibrin to fibrinogen to produce a clotlike barrier; this feature distinguishes S. aureus from S. epidermidis. • M. tuberculosis is able to survive in a host by promoting the development of a granuloma, within which viable bacteria may reside for the life of the infected person; the bacteria may resume growth if there is a decline in the immune status of the person.