Chapter 4 Bacterial Infection and Pathogesis

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.

IDENTIFYING BACTERIA THAT CAUSE DISEASE Koch’s Postulates Molecular Koch’s Postulates Molecular Guidelines for Establishing Microbial Disease Causation

1. Normal Microbial Flora the population of microorganisms that inhabit the skin and mucous membranes of healthy normal persons.

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 stimulate 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. __ To produce disease under certain circumstances : If forcefully removed from the restrictions of certain environment and introduced into foreign locations, they may become pathogenic.

Numbers of bacteria that colonize different parts of the body Numbers of bacteria that colonize different parts of the body. Numbers represent the number of organisms per gram of homogenized tissue or fluid or per square centimeter of skin surface

Normal microbial Flora

The normal flora competes with invading pathogens.

Clinical conditions that may be caused by members of the normal flora

Infection No Infection Carrier Inapparent Infection Latent Infection Acute infection/chronic infection） Local infection/generalized infection or systemic infection Toxemia; Endotoxemia; Bacteremia; Septicemia; Pyemia

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.

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. 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 invasion and toxigenicity. LD 50 (age /sex /health /route of entry, etc )

Source of infection Exogenous infection : patient, carrier, diseased animal or animal carrier. Endogenous condition : most are normal flora, cause infection under abnormal condition.

TRANSMISSION OF INFECTION Airborne droplets Food Water Sexual contact

THE INFECTIOUS PROCESS Mucous membranes Respiratory tract Gastorintestinal tract Genitourinary tract Skin Parenteral route

THE CLONAL NATURE OF BACTERIAL PATHOGENS

REGULATION OF BACTERIAL VIRULENCE FACTORS 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

Adherence Factors

Specific Adherence of Bacteria to Cell and Tissue Surfaces 1. Tissue tropism: 2. Species specificity: 3. Genetic specificity within a species:

Nonspecific adherence 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 BACTERIUM adhesin receptor EPITHELIUM

S. pyogenes lipoteichoic acid F-protein fibronectin

E. coli with fimbriae

E. coli fimbriae Type 1 mannose P galactose glycolipids glycoproteins

Invasion of host cells & tissues

Toxins Endotoxins Exotoxins Endotoxins 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.

A-B toxins Cell surface Active Binding A B

Diphtheria toxin and Pseudomonas exotoxin A ADP-ribosylate elongation factor (EF2) inhibit protein synthesis

Cholera toxin and E. coli labile toxin ADP-ribosylate adenylate cyclase cyclic AMP active ion and water secretion diarrhea

Shiga toxin - shigellosis Shiga-like toxin - enterohemorraghic E. coli Lyses rRNA in ribosome Death of epithelial cells Poor water absorption Diarrhea

Tetanus toxin inhibits glycine release inactivates inhibitory neurons muscles over-active rigid paralysis

Botulinum toxin inhibits acetylcholine release inhibits nerve impulses muscles inactive flacid paralysis

Exotoxins - extracellular matrix of connective tissue Clostridium perfringens - collagenase Streptococcus - hyaluronidase

Membrane damaging toxins Proteases Phospholipases Detergent-like action

C. perfringens phospholipase Destroys blood vessels Stops influx inflammatory cells Creates anaerobic environment Allows growth of this strict anaerobe.

Exotoxins Antibodies (anti-toxins) neutralize vaccination

Endotoxins 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

Endotoxins Endotoxin effects Lethal 1 milligram/ kg 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

Endotoxins Non-specific inflammation. Cytokine release Complement activation B cell mitogens Polyclonal B cell activators Adjuvants

Endotoxemia

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

Enzymes Tissue-degrading enzymes IgA1 proteases: split IgA1, an important secretory antibody on mucosal surfaces, and inactivate its antibody activity. H. influenzae S. pneumoniae N. gonorrhoeae N. meningitidis

Antiphagocytic factors 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.

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 PHAGOCYTE Fc receptor immunoglobulin Protein A BACTERIUM

M protein inhibits phagocytosis Complement fibrinogen M protein r peptidoglycan

Intracellular parasite lysozome phagosome No fusion Fusion Enter cytoplasm

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.

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.

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.

bacterial siderophores compete effectively for Fe3+ bound to lactoferrin and transferrin.