Bacterial Infection and Immunity Xiao-Kui GUO

Symbioses Commensalism: one partner benefits and the other is neither harmed nor benefited. Mutualism: both partners benefit. Parasitism: one partner benefits at the expense of the other.

Role of the resident flora Members of the resident flora in the intestinal tract synthesize vitamin K and aid in the absorption of nutrients. Members of the resident flora on mucous membranes and skin may prevent colonization by pathogens and possible disease through “bacterial interference”. The normal flora may antagonize other bacteria through the production of substances which inhibit or kill nonindigenous species. The normal flora stimulates the development of certain tissues, i.e., the caecum and certain lymphatic tissues (Peyer's patches) in the GI tract The normal flora stimulate the production of cross- reactive antibodies.

Hospital acquired infection: Infections acquired during hospital stays. Pathgen: A microorganism capable of causing sisease. Nonpathogen: A microorganism that does not cause disease; may be part of the normal flora. Opportunistic pathogen: An agent capable of causing disease only when the host’s resistance is impaired (ie, when the patient is “immunocompromised”). Pathogenicity: The ability of an infectious agent to cause 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 invasion and toxigenicity. LD 50 (age /sex /health /route of entry, etc ) LD50: The number of pathogens required to cause lethal disease in half of the exposed hosts is called an LD 50. ID50: The number of pathogens required to cause disease (or, at least, infection) in half of the exposed hosts is called the ID 50 Adherence (adhesion, attachment): the process by which bacteria stick to the surfaces of host cells. Once 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. Invasion: The process whereby bacteria, animal parasites, fungi, and viruses enter host cells or tissues and spread in the body. Toxigenicity: The ability of a microorganism to produce a toxin that contributes to the development of disease.

Koch’s Postulates Molecular Koch’s Postulates Molecular Guidelines for Establishing Microbial Disease Causation Koch's postulates Isolated Isolated – diseased not healthy people Growth Growth – pure culture Induce disease Induce disease – susceptible animals Re-isolated Re-isolated – susceptible animals

Pathogenesis Pathogenesis is a multi-factorial process which depends on the immune status of the host, the nature of the species or strain (virulence factors) and the number of organisms in the initial exposure.

Source of infection Exogenous infection : patient, carrier, diseased animal or animal carrier. Endogenous condition : most are normal flora, cause infection under abnormal condition. Airborne droplets Airborne droplets Food Food Water Water Sexual contact Sexual contact Transmission Respiratory Gastroenteric Genitourinary tract closely contact insect bitting blood transfusion Parenteral route Mucous membranes Routes of infection

According to infectious sites According to infectious sites Local infection Generalized or systemic infection 1. Toxemia : is the presence of exotoxins in the blood. 2. Endotoxemia : is the presence of endotoxins in the blood. 3. Bacteremia : is an invasion of the bloodstream by bacteria. 4. Septicemia : illness that occurs when poisonous substances (toxins) produced by certain bacteria enter the bloodstream. 5. Pyemia : is caused by pyogenic microorganisms in the blood. Inapparent or subclinical infection Latent infection Apparent infection : cause apparent clinic syndrome Carrier state: carrier According to infectious state

Environmental signals often control the expression of the virulence genes. Common signals include:Temperrature/Iron availability : C diphtheriae /low ion/Osmolality /Growth phase/pH/Specific ions BACTERIAL VIRULENCE FACTORS

1. Adherence Factors 1. Tissue tropism: 2. Species specificity: 3. Genetic specificity within a species: Hydrophobic interactions Electrostatic attractions Atomic and molecular vibrations resulting from fluctuating dipoles of similar frequencies Brownian movement Recruitment and trapping by biofilm polymers interacting with the bacterial glycocalyx (capsule)

Adhesion adhesin EPITHELIUMreceptorBACTERIUM fibronectin lipoteichoic acid F-protein mannose Type 1 galactose – glycolipids – glycoproteins P E. coli fimbriae

2. Invasion of host cells & tissues

3. Toxins 3. Toxins Exotoxins Endotoxins Exotoxins Produce in vitro cause food poisoning: botulin, staphylococcal enterotoxin, etc. Produce in vivo: Systematic toxic effects : e.g. diphtheria, tetanus, and streptococcal erythrogenic toxins. Local toxic effects : e.g. cholera, and toxigenic E. coli enterotoxins.ActiveBindingA Cell surface B Antibodies (anti-toxins) Antibodies (anti-toxins) neutralize – vaccination

E ndotoxins LPS Lipopolysaccharide: core or backbone of CHO side chains of CHO: "O" antigen Lipid A Cell wall lysis required formaldehyde and heat resistant poor antigen as free molecule Endotoxin effects  Fever- pyrogen 1 microgram/ kg  Leukopenia and leukocytosis necrosis  Shwartzman phenomenon and disseminated intravascular coagulation (DIC).  Endotoxemia and shock Lethal 1 milligram/ kg Identification: Limulcyte assay Non-specific inflammation Non-specific inflammation. Cytokine release Cytokine release Complement activation Complement activation B cell mitogens B cell mitogens Polyclonal B cell activators Polyclonal B cell activators Adjuvants Adjuvants

Peptidoglycan of Gram- positive bacteria Peptidoglycan of Gram- positive bacteria May yield many of the same biologic activities as LPS.

4. Enzymes 4. Enzymes Tissue-degrading enzymes IgA1 proteases: split IgA1, an important secretory antibody on mucosal surfaces, and inactivate its antibody activity. 1. H. influenzae 2. S. pneumoniae 3. N. gonorrhoeae 4. N. meningitidis Some pathogens evade phagocytosis or leukocyte microbicidal mechanisms by adsorbing normal host components to their surfaces. A few bacteria produce soluble factors or toxins that inhibit chemotaxis by leukocytes and thus evade phagocytosis. 5. Antiphagocytic factors

Antiphagocytic Substances 1. Polysaccharide capsules of S. pneumoniae, Haemophilus influenzae, Treponema pallidum ; B. anthracis and Klebsiella pneumoniae. 2. M protein and fimbriae of Group A streptococci 3. Surface slime ( polysaccharide) produced as a biofilm by Pseudomonas aeruginosa 4. O polysaccharide associated with LPS of E. coli 5. K antigen (acidic polysaccharides) of E. coli or the analogous Vi antigen of Salmonella typhi 6. Cell-bound or soluble Protein A produced by Staphylococcus aureus. Protein A attaches to the Fc region of IgG and blocks the cytophilic (cell-binding) domain of the Ab. Thus, the ability of IgG to act as an opsonic factor is inhibited, and opsonin-mediated ingestion of the bacteria is blocked.

Protein A inhibits phagocytosis immunoglobulin Protein A Fc receptor BACTERIUM PHAGOCYTE rr r peptidoglycan Complement fibrinogen M protein M protein inhibits phagocytosis

6. Intracellular pathogenicity Some bacteria live and grow within polymorphonuclear cells, macrophages, or monocytes by avoiding entry into phagolysosomes and living within the cytosol of the phagocyte, preventing phagosome- lysosome fusion and living within the phagosome, or being resistant to lysosomal enzymes and surviving within the phagolysosome.

7. Antigenic heterogeneity Antigenic type of bacteria may be a marker for virulence, related to the clonal nature of pathogens, though it may not actually be the virulence factor. Some bacteria may make frequent shifts in the antigenic form of their surface structures in vitro and presumably in vivo, allowing the bacteria to evade the host’s immune system.

Bacterial siderophores compete effectively for Fe 3+ bound to lactoferrin and transferrin. 8. The requirement for iron For the host, the iron metabolism denies pathogenic bacteria an adequate source of iron for growth. For the bacteria, they have developed several methods to obtain sufficient iron for essential metabolism, e.g., the low-affinity iron assimilation system or the high-affinity iron assimilation systems.

Development of the Immune System ery pl mye neu mφmφ lym nk thy CD8 + CD4 + CTL TH2 TH1

Components of the Immune System Humoral Cellular Humoral Cellular Specific Nonspecific complement, interferon, TNF etc. macrophages, neutrophils T cells; other effectors cells antibodies

Balance between Infection and Immunity infection immunity Bolus of infection x virulence immunity Disease =

Response to Infection infection x disease Innate immunity no disease recovery adaptive immunity re-infection no disease x

u Beneficial: u Protection from Invaders u Elimination of Altered Self u Detrimental: u Discomfort (inflammation) u Damage to self (autoimmunity) u Beneficial: u Protection from Invaders u Elimination of Altered Self u Detrimental: u Discomfort (inflammation) u Damage to self (autoimmunity) Significance of the Immune System

Innate Immunity Adaptive Immunity Components of Innate and Adaptive Immunity skin, gut Villi, lung cilia,etc many protein and non-protein secretions phagocytes, NK cell eosinophils, K cells physical barriers soluble factors cells none Immunoglobulins (antibody) T and B lymphocytes

Chemotactic response to inflammatory stimulus Macrophage Attacking E.coli (SEM x8,800)

Adaptive Immunity

Innate Immunity Adaptive Immunity Characteristics of Innate and Adaptive Immunity No Immunologic memory Antigen independent No time lag Not antigen specific Antigen dependent A lag period Antigen specific Development of memory

Immunity of extracellular bacterial infection: antibodies (IgG, IgM, SIgA); phagocytes (neutrophils); complement; humoral immunity mainly. Immunity of intracellular bacterial infection: cell-mediated immunity (delayed-type hypersensitivity, DTH response (DTH) involving TH1and macrophages) mainly.

INADEQUATE IMMUNE RESPONSES TO INFECTIOUS AGENTS Causes immune suppression—an example is infection with HIV, which alters T cell immunity and allows further infection with opportunistic pathogens. Release toxins that function as superantigens, initially stimulating large numbers of T cells to proliferate but, because of the release of cytokines from T cells, ultimately suppressing the immune response and allowing the pathogen to multilply. Evade the immune defenses by altering their antigenic structure—an example is that influenza virus undergoes antigenic variation by two mutational mechanisms called antigenic shift and antigenic drift that creat new antigenic phenotypes which evade the host’s current immunity and allow reinfection with the virus.