Host-Microbe Interactions

1. Host-Microbe Interactions Chapter 14

2. Disease • Etiology • Pathogen • Primary vs. opportunistic • Virulence

3. Contamination • Infection vs. Infestation • Pathogenesis • Pathology

4. Symptoms • Signs • Syndrome • Sequelae

5. Anatomical Barriers as Ecosystem • Skin and mucous membranes are physical barriers to infection • May supply foundation for microbial ecosystem

6. Humans are usually sterile in utero • Soon after birth, microbial populations begin to establish • Normal microbiota • Resident flora • Transient flora

7. Isolated colonies in specific body regions • Factors that influence distribution: • Nutrient availability, salinity, oxygen availability, host defenses, mechanical factors • Dominant type of organism may change with age and situation

8. Normal flora play a role in defenses • protection from colonization by pathogens • microbial antagonism • Competitive exclusion • Bacteriocins

9. Symbiotic relationships form between microbes and host Relationships may change depending on state of host and attributes of microbes

10. Mutualism (++) • Intestinal bacteria • Probiotics • Commensalism (+ neutral) • Flora on skin and conjunctiva • Parasitism (+-) • Pathogens

11. Pathogenicity • State of host resistance usually determines extent of infection • primary infection • secondary infection (typically opportunistic) • Sub-clinical (inapparent) infection

12. Distribution of pathogens • Local infection • Systemic (generalized) infection • Septicemia can result from: • Bacteremia • Toxemia • Viremia

13. Many people are carriers of pathogens • Viruses; Neisseria; Salmonella; Streptococcus • Why are they less affected or not at all?

14. Predisposing factors: • gender • genetic background • climate and weather • inadequate nutrition • age • habits and lifestyle • chemotherapy • emotional disturbances

16. Characteristics of infectious disease: • communicable • contagious • Often reflects ID50 • non-communicable disease

17. Etiology of Infectious Diseases • Robert Koch demonstrated that specific microbes caused specific diseases • experimented with grazing animals infected with anthrax

18. Koch’s postulates • Same pathogen must be present in each case of disease • Pathogen is isolated from diseased host and grown in pure culture • Pure culture must cause disease when inoculated into healthy animal • Pathogen must be re-isolated from inoculated animal

20. Exceptions to Koch’s postulates • some bacteria have unique culture requirements • some diseases are caused by multiple pathogens • Polymicrobial diseases (mixed infections) • Ethical considerations

21. Stages of an Infectious Disease

22. Duration of a Disease • acute diseases • chronic diseases • latent disease

23. Establishment of Infection • In order to cause disease pathogen must follow a series of steps • Gain entrance to host • Adherence • Colonization • Avoid Host Defenses • Cause host damage

24. Portals of entry 1. Mucus membranes • Respiratory tract • Gastrointestinal tract • Genitourinary tract • Placenta 2. Skin 3. Parenteral route • Bite, puncture, injection, wound

25. Most microbes have a preferred portal of entry • Streptococci when inhaled may cause pneumonia; when ingested they do not • A few microbes cause illness no matter how they enter • May cause different illness based on portal • Plague has 2 forms; bubonic and pneumonic • Anthrax has three forms

26. Adherence • Critical Step • Binding of adhesins to host receptors is highly specific

27. Virulence factors • Capsules • Incomplete phagocytosis • Fimbrae • Components of cell wall

28. Extra-cellular enzymes (exoenzymes) • Coagulases & Kinases • Form or dissolve blood clots • Hyaluronidase & Collagenase • Dissolve hyaluronic acid and collagen fibers • IgA proteases & Leukocidins • Destroy antibodies or WBCs

29. Damage to the Host • Damage often facilitates dispersal of pathogen • Diarrhea or coughing • Main sources of damage: • Steal nutrients • Binding to and invading host cells • Induce hypersensitivity reactions (allergies) • Production of toxins (Toxigenicity)

30. Exotoxins • G+ bacteria • Produced as part of their metabolism • Secreted externally or following cell lysis • Among most lethal substances • Proteins • Enzymatic nature • Highly soluble • Heat Liable

31. Toxoids • Inactivated exotoxins • Induce antitoxins that provide immunity • Antibodies against a specific toxin • Passive immunity in form of antitoxin can be given as treatment

32. Grouped into functional categories • Neurotoxins • Enterotoxins • Cytotoxins

33. Staphylococcus aureus • enterotoxin; exofoliatin toxin; Toxic Shock Syndrome toxin • Vibrio cholera • cholera enterotoxin • Clostridium species • botulinum neurotoxin; tetanus neurotoxin; gangrene toxin

34. Endotoxins • G- bacteria • Released when cells die and their cell walls lyse • Lipopolysaccharide (LPS) outer membrane • The lipid portion (lipid A) is toxin • Heat stable; not suitable for use as toxoids • Do not cause formation of antitoxins

35. All endotoxins produce the same symptoms • Chills, fever, weakness, aches • May activate blood clotting proteins • May cause septic shock that can be fatal • Salmonella, Proteus, Klebsiella and Neisseria

37. Portals of Exit • Mucus membranes • Respiratory & gastrointestinal are most common • Skin/wounds • Biting insects • Contaminated needles and syringes