Bacterial Pathogenesis

1. Bacterial Pathogenesis Infectious disease - cycle of biological interactions. Most interventions in preventing infectious disease do not directly involve the physician. By understanding the complete process of bacterial pathogenesis, you will be better equipped to aid in intervention and prevention at the numerous steps other than by administering antibiotics or vaccines.

2. A. Definitions and concepts 1. Disease - damage caused by presence of microorganisms or their products 2. Infection - presence of bacteria in or on the body (note that some use this term synonymously with disease) 3. Colonization- presence of microorganisms at a site (some infer no damage) 4. Carrier state - inapparent infection (colonization) with a pathogen 5. Pathogen - any organism that has the potential to cause disease 6. Overt (primary) vs. Opportunistic pathogens a. overt pathogens have a high probability of causing disease in an otherwise healthy host b. opportunistic pathogens have a low probability and usually require a debilitated or compromised host

3. B. Normal flora 1. Definition - frequently found on or within the body of healthy persons 2. They can cause disease under the right conditions (endogenous infection/disease) 3. Some tissues, organs are dense with normal flora, others are normally sterile a. colonized sites i. alimentary/intestinal tract ii. upper respiratory tract iii. distal genitourinary tract iv. skin b. normally sterile sites i. blood ii. CSF iii. interstitial fluid and spaces iv. lymph

4. 4. Normal flora and disease a. breach of tissues/barriers b. endocarditis with oral streptococci c. peritonitis after bowel trauma 5. Protective effects of normal flora a. priming immune system - specific (natural antibodies) b. exclusion of pathogens from colonized surfaces (crowded bar) c. production of nutrients - vitamin K 6. Considerations of normal flora in medicine a. Manipulations that enable normal flora to cause disease (surgery, chemotherapy, antibiotic use) b. presence of bacteria doesn't always mean disease is ongoing

5. C. Functions/stages of pathogens/disease 1. encounter 2. entry 3. spread (+/-) 4. multiplication 5. evasion of host defenses 6. damage 7. move to new host (+/-)

6. 1. Encounter a. Exogenous vs. endogenous infection and normal flora b. sources: food, water, air, body fluids (sex, etc.), insects, animals, fomites (things) c. Is there a reservoir in other animals or in the environment? d. Can we eliminate the source or reservoir?

7. 2. Entry - where in/on the body do we initially contact the bacteria a. surface of skin b. mucosal membrane open to outside i. ingestion ii. inhalation iii. sex c. direct inoculation (trauma, bite, injection, surgery) d. virulence function - adherence i. mucosal surface with moving fluid, bacteria must stick or be washed away ii. specific ligand-receptor interactions - not glue iii. virulence factors ►structures: pili/fimbriae, fibrillae ► surface proteins ► extracellular matrix polysaccharide and lipoteichoic acid of oral streptococci

9. 3. Spread - movement from surface through tissues and body a. not all bacteria spread to cause disease b. bacteria cannot penetrate intact skin c. mucosal surface is usually first barrier d. virulence functions i. cellular invasion - invading into host cells - intracellular ►professional phagocytes (macrophages and PMN) by phagocytosis ► non-professional phagocytes (epithelial cells, endothelial cells, hepatocytes, etc.) - bacterial mediated endocytosis resulting in a phagosome/endosome (can involve type III secreted proteins - injected into host cell affecting actin polymerization) ► some bacteria escape the phagosome into the cytoplasm

12. ii. tissue invasion - through tissues, either through cell or between cells ► degrade extracellular matrix ► disrupt tight junctions e. use of host cells to move through blood or lymph (intracellular pathogens)

13. 4. Multiplication a. don’t encounter enough bacteria to cause damage and disease without their increase in number in your body b. variety of nutritional environments in the body i. intestinal lumen, blood, urine ii. intracellular bacteria- cytoplasm vs. phagolysosome c. inoculum size and disease ► threshold for bacterial inoculum to replicate to high enough numbers to cause disease d. incubation period ► how long it takes the inoculum to result in clinical damage e. acquisition of Fe from transferrin, lactoferrin, or other Fe-binding proteins (e.g. siderophores as virulence factors)

14. 5. Evasion of host defenses ► consider where the bacteria are and which defensesthey will encounter a. Complement - beyond mucosal surface i. functions ►opsonization - C3b ►lysis of gram-negatives - C5-9 (Membrane Attack Complex [MAC]) ►inflammation - C3a, C5a ii. virulence mechanisms for evasion ►do not bind and/or activate complement - polysaccharide capsules ► cause inhibition of activation and amplification cascade - bind factor H (M protein of streptococci) ►keep activation away from membrane - smooth LPS ►degrade complement - secrete proteases

15. b. Phagocytes - beyond mucosal surface i. EXTRACELLULAR vs. INTRACELLULAR pathogens ii. extracellular antiphagocytic functions ►inhibit recruitment - inhibit complement, cytokines ►kill the phagocytes - toxins ►prevent phagocytosis - prevent opsonization, prevent binding (carbohydrate capsules) iii. intracellular pathogen functions ►inhibit phagosome-lysosome fusion ►escape phago(lyso)some into cytoplasm ►inhibit oxidative burst ►resist antimicrobial functions

19. c. Antibodies i. antigenic mimicry - surface components look like host ►polysialic acid capsule of Neisseria meningitidis ii. antigenic cloaking - bindhost proteins to bacterial surface ►protein A of Staphylococcus aureus iii. antigenic variation - change antigenic composition ►pili of Neisseria gonorrhoea iv. antigenic variety -numerous serological types among strains in the world (each strain is antigenically stable) ►M protein of Streptococcus pyogenes v. degrade antibodies ►IgAse of Haemophilus influenzae d. Cell-mediated immunity ►alter host response from cell-mediated (Th1) to antibody (Th2) response - Mycobacterium leprae

20. 6. Damage a. cytotoxicity (kill host cells: necrosis vs. apoptosis [programmed cell death]) i. from outside - toxins ii. from inside - intracellular growth iii. apoptosis - type III secreted (injected) proteins b. pharmacology/physiology (alter host cell function) - toxins c. host immune/inflammatory response (host causes damage to itself) i. nonspecific: inflammation, abscess, cytokines ii. specific immune response: antibodies (Rheumatic fever - Streptococcus pyogenes), cell-mediated (Reiter’s syndrome - bacterial heat shock proteins)

21. d. Toxins i. Endotoxin vs. Exotoxin ii.Endotoxin (LPS) from gram-negatives only ►damage is caused by host through macrophages producing cytokines (TNF-α, IL-1, IL-6) iii. Exotoxins (proteins) ►several have A-B motif - A=active portion, B=binding portion ►lytic (lyse host cells - creating pores or lipase activity) ►cytotoxic(kill cells by altering functions, e.g., protein synthesis) ►pharmacological (alter host cell function, e.g., cAMP levels) ►extracellular enzymes (degrade extracellular matrix, e.g., DNAse, hyaluronidase) ►superantigens (stimulate host T cell responses in antigen-independent manner (binding to Vβ of T cell receptor) resulting in cytokine cascade - similar to endotoxin) ►type III secreted proteins (injected into host cell) - induce apoptosis, affect actin polymerization

22. 7. Spread to new host a. completes the cycle to lead to new encounter - step 1 b. not all pathogens spread to new human host (e.g., Legionella pneumophila) c. not all pathogens are contagious - humans may be dead end hosts d. most bacterial pathogens are contagious - some examples of how: i. fecal-oral ii. sexually transmitted iii. droplet spread (respiration) iv. skin-skin v. fomite vi. vertical - in utero, at birth