Bacterial Pathogenesis Nov 4, 2013 I-Hsiu Huang Ref:

Normal Flora (Commensal Microbes) Introduction Significance of the Normal Flora Distribution of the Normal Flora Bacterial Pathogenesis Introduction Host Susceptibility Pathogenesis Mechanisms Virulence Factors

Normal flora and pathogenesis Outcomes of exposure to organisms 1.Transient colonization 2.Permanent colonization 3.Disease Colonization vs Infection – Establishment of a site of reproduction of microbes on a person without necessarily resulting in tissue invasion or damage. – Growth and multiplication of a microbe in or on the body of the host with or without causing disease.

Normal flora: The closest neighbor of a human Where: Mucosal surfaces and skin of the entire body When: Right after birth  death How many: bacteris vs human cells “characterization of the human microbiota and analysis of their role in human health and disease.”

The Human Microbiome Project (HMP) 500 to 1000 species of bacteria live in the human gut The weight of all the bacteria in your body is about 1-3% of your body weight Diverse species of bacteria has learned to live in many difference area of our body by using carbohydrates differently Questions: Can we tailor our medicine, diet, and life style according to the bacteria in our body?

Normal flora What can it do for/to us? – Aid the host (i.e. digestion, combat pathogens) – Harm the host (opportunistic bacteria) – Exist as commensals (no effect, or no known effect) What about other microorganisms? – Viruses and parasites are NOT normal microbial flora – At least so far that’s what scientists believe in

Significance of Normal flora Influences our anatomy, physiology, susceptibility to pathogens, and morbidity Germ-free animal studies – Cesarean section  germ-free animals – Free of bacteria, viruses, fungi, others – Germ-free animal lived much longer – 2x longer

Significance of Normal flora Germ-free AnimalsRegular Counterparts LifespanTwiceOne Cause of death Intestinal AtoniaInfection Anatomic & Physiological Changes 1. Alimentary lamina propria under- developed 2. No Ab 3. Intestinal epithelial cell renewal rate down half

Significance of Normal flora How do normal flora help us? – Digestion, Vitamin production – Help developing mucosal immunity – Protect host from colonization by pathogens 10 6 pathogenic microbes GI infection w/ normal flora GI infection w/ reduced flora after Streptomycin treatment 10 pathogenic microbes Vitamin K, B12 Vitamin B groups

Normal Flora Competing with Invading Pathogens

Normal flora: Frenemies (Friends or Enemies?) Opportunistic pathogens Immuno-suppression – AIDS Radiation therapy Chemotherapy – Cancer treatment Damage in mucosa – injuries

Sites of human body that the normal flora microbes colonize Respiratory tract and head – Ear, eye, mouth, oropharynx, nasopharynx – Sterile when healthy: sinuses, middle ear, brain, lower respiratory tract (trachea, brochiole, lung) Gastrointestinal tract – Esophagus, stomach, small and large intestine Genitourinary tract – Urethra, vagina – Sterile when healthy: bladder, cervix, uterus Skin

Distribution of Normal Flora in Human Body Adopted from Samuel Baron “Medical Microbiology”

Factors influencing normal flora Surrounding environment – pH, temperature, redox potential, oxygen, water, nutrient level and availability Diet – Meat vs vegetarian diet – High sugar diet Age Health condition Antibiotic/drug use

Normal Flora (Commensal Microbes) Introduction Significance of the Normal Flora Distribution of the Normal Flora Bacterial Pathogenesis Introduction Host Susceptibility Pathogenesis Mechanisms Virulence Factors

Bacterial Pathogenesis Infection: growth and multiplication of a microbe in or on our body with or without the production of disease The capacity of a bacterium to cause disease reflects its relative “Pathogenicity.” Virulence is the measure of the pathogenicity of a microorganism. Pathogenesis refers both to (1) the mechanism of infection and to (2) the mechanism by which disease develops.

Host susceptibility Susceptibility to bacterial infections – Host Defenses vs Bacterial Virulence Host defenses – Barriers (skin & mucus) – the first line – Innate Immunity (complement, macrophages & cytokines)  the early stage – Adaptive Immunity (Ag-specific B & Tcells)  the later stage Host defenses can be damaged by destructing barriers or defective immune response – e.g. Cystic Fibrosis ( 囊腫纖維症 )  Pseudomonas aerugionsa ( 綠膿桿菌 ) infection

Strict pathogens are more virulent and can cause diseases in a normal person. Opportunistic pathogens are typically members of normal flora and cause diseases when they are introduced into unprotected sites; usually occur in people with underlying conditions.

Transmission of infection Asymptomatic infection – By producing none to mild disease, it help spreading from person to person without detection Carrier – Person or animal without symptoms but can spread the disease Zoonosis – Diseases transmitted between animals and men Hospital & Community-acquired infection

Entry into the human body Nature barrier breakdown Skin, mucus, ciliated epithelium Routes – Ingestion, Inhalation – Trauma, Needlestick – Catheters, Bug bite – Sexual transmission

How do we measure pathogenicity? Transmissibility Adherence to host cells Invasion of host cells and tissues Evasion of immune system Toxigenicity Bacterium may cause disease by – Destroying tissues – Producing toxins – Over-stimulate immune response

Pathological mechanism of bacteria infections Bacteria-mediated Host-mediated Virulence factors – Bacterial factors that cause disease – e.g. toxins

How do bacteria become virulent?

Bacterial virulence mechanism Adherence Invasion Byproducts of growth (gas, acid) Toxins – Degradative enzymes – Cytotoxic proteins – Endotoxin – Superantigen Induction of excess inflammation Evasion of phagocytic and immune clearance Capsule Resistance to antibiotics Intracellular growth Ref: Chapter 14. Box “Medical Microbiology”

Bacterial virulence factors Adhesins Pili (fimbriae) Nonfimbrial adhesins Invasion of host cells Tissue damage Growth byproducts Tissue-degrading enzymes Toxins Exotoxins (cytolytic enzymes and A-B toxins); enterotoxins; superantigens; Endotoxin and other cell wall components Antiphagocytic factors Intracellular survival Antigenic heterogeneity Antigenic variation Phase variation Iron acquisition Siderophores Receptors for iron-containing molecules Resistance to antibiotics

Adhesion Adherence to epithelial or endothelial cells Without adhesion, there’s no colonization Adhesins – Structures allowing bacteria to adhere – Pili (fimbriae). e.g. E. coli P fimbriae  erythrocytes and uroepithelial cells – Lipotechoic acid (Gram-positive) – Other surface proteins (Collagen-binding protein)

Adhesion Biofilm – Ability of large number of bacteria to form a “community” structure – Bound by sticky polysaccharide – Allow bacteria-bacteria and bacteria-host attachment – Dengue plaque, catheters, – Immune evasion, stress resistant

Examples of biofilm formation

Invasion without prior damage Shigella, Salmonella, Yersinia Adherence Injection of proteins Endocytosis Bacteria replication Dormancy Cell death

Tissue destruction Bacterial growth byproduct – Through fermentation  acid, gas, etc Degradative enzymes Ex: anaerobic pathogens Clostridium perfringens – Collagenase, protease, hyaluronidase, toxins, acid, gas

Endotoxin (LPS) mediated activity Lipid A of LPS is responsible for endotoxin activity Pathogenesis of sepsis (septicemia)

Endotoxin-mediated toxicity Fever, leukopenia followed by leukocytosis, Activation of complement, thrombocytopenia, Disseminated intravascular coagulation, Decreased peripheral circulation and perfusion to major organs (multiple organ system failure), Shock and death. Peptidoglycan, teichoic and lipoteichoic acids of gram-positive bacteria stimulate pyrogenic acute phase responses and produce endotoxin-like toxicity.

Exotoxins Made by both gram-positive and gram-negative bacteria Often encoded on plasmid  gene transfer Cytolytic toxins – α-toxin of C. perfringens  membrane-disruption (sphingomyelins, membrane phospholipids) AB-toxins – Binding domain, Action domain. Tissue-specific. – Ribosomes, transporters, intracellular signaling Superantigens

Superantigen-mediated toxicity Bind to TCR and activate T- cells w/o Ag Autoimmune-like response Excess amount of interleukins (cytokine) Staphylococcus aureus – Toxic shock syndrome toxin-1 Streptococcus pyogenes – Streptococcal pyrogenic toxin (SpeA/C)

A-B toxins A chain has the inhibitory activity against some vital function B chain binds to a receptor and promotes entry of the A chain Modes of action – Inhibit protein synthesis – Inhibit neurotransmitter release – Hyperactivation

Immunopathogenesis Symptoms produced not by the bacteria themselves But, through excessive immune responses – Innate, immune, inflammatory Host protective mechanism overload – When limited, it help to clear the pathogen – When out of control, can be life threatening Cytokine storm – superantigen, toxins TB granuloma formation  tissue destruction

Bacteria fights back Encapsulation (Inhibition of phagocytosis and serum bactericidal effect) Antigenic mimicry, masking, phase variation Intracellular multiplication Escape phagosome Inhibition of phagolysosome fusion Resistance to lysosomal enzymes Inhibition of chemotaxis Destruction of phagocytes Production of anti-immunoglobulin proteases

Mechanisms for escaping phagocytic clearance and intracellular survival

Summary Normal flora – Skin and mucosal surfaces – Helps host in many ways Digestion, vitamin production, mucosal immunity development, combat pathogen colonization – Could become pathogen Opportunistic infection Immune-suppressed individuals, barrier-broken

Summary Host defenses – Barriers – Front line defense – Innateimmune response (macrophages, cytokines) – Adaptive immune responses (Ag-specific B & T cells) Balance between host defense vs bacterial virulence (the see-saw model)

Summary Disease symptoms can be caused by: 1.Bacterial mediated - toxins, metabolic byproducts 2.Host mediated - Hyper-stimulation of immune-responses leading to further damages or even death

You be the pathogen! A fun game to play 瘟疫公司 Plague Inc