1. The Diseases

2. Not all microbes are pathogens
“Normal” flora (endogenous, indigenous), resident or transient: commensal, symbiotic. Harmless, may be beneficial.
Outcompete pathogens
Provide necessary nutrients
Skin: yeasts, mites, bacteria
Corynebacteria, propionibacteria
Staphylococcus spps,
S. epidermis, S. aureus
Intestine (enteric flora): or more bacteria per gram feces

3. Bacteria of the GI Tract
Stomach
pH 3
0 to 103 per g
Duodenum, jejunum
pH to 104 per g
Enterobacteria
Bacteroides
Escherichia coli
Bifidobacteria
Steptococci
Clostridia
Small intestine
pH 7-7.5
105 to 108 per g
Large intestine
pH 7-8
1010 to1012 per g

4. Opportunistic Pathogens
Pathogenic only under special circumstances:
Susceptible host: Immune system weakened/compromised/suppressed
Indigenous flora decreased (eg antibiotic treatment)
“Indigenous” flora in the “wrong” location
Frank Pathogens
Pathogenic at all times

5. Indicator organisms
Traditional approach to determining the "sanitary" quality of water (food) with respect to fecal contamination – fecal coliforms
Can also serve to monitor effectiveness of disinfection measures
Bacteria commonly present in intestines of warm blooded animals; surrogates for pathogens, especially bacterial pathogens
Easily quantitated, not infectious to humans
May not be reliable indicators of viruses and parasites

6. Pathogenicity – ability to cause disease
Virulence – extent or severity of disease
Both depend on ability to infect the host and to cause damage
Infection:
Colonization (usually at site of entry)
Attachment and/or invasion (Adhesins or pili specific to receptors on target cell)
Multiplication of pathogen
Persistence of pathogen

7. Major Portals of Entry
Respiratory Tract
Gastrointestinal Tract
Skin

8. Respiratory Tract
Aerosols, gases, vapors

9. Gastrointestinal Tract
Food, drink, fecal-oral

10. Skin
Contact, bathing

11. Spread within initial host
“Local” : pathogen confined to single area, usually point of entry
“Systemic” or “generalized”: pathogen invades or is carried to other tissues, organs
Latent infection
Chronic infection
Liver
Duodenum

12. Target organs Location, environment
Consider route of exposure
Specific attachment (by piliated pathogens)
Hepatitis: HAV, HBV, HCV etc

13. Some Enteric Diseases Cholera Vibrio cholerae
Typhoid fever Salmonella typhi
Amebic dysentery Entamoeba histolytica
Hepatitis Hepatitis A virus
Norwalk disease Noroviruses
Polio Polioviruses

14. Viruses in the Environmment
Must be able to survive outside host cell
Non-enveloped viruses are more persistent than enveloped viruses
Lipid envelope more easily damaged, protein coat confers stability
Enteric viruses are almost all non-enveloped
Hepatitis A, poliovirus, noroviruses, rotaviruses
Transmitted by direct and indirect contact, fecally contaminated water, food, fomites and air.

15. Bacterial food poisoning
Staphylococcus aureus
Salmonella
Clostridium

16. Airborne pathogens Respiratory viruses Bacteria Molds
Flu, parainfluenza, respiratory syncytial virus, common cold (rhinovirus)
Bacteria
Legionnaire’s Disease (Legionella pneumophila )
Molds
Allergy, pneumonitis, mycosis, mycotoxicosis

17. Respiratory viruses, mostly enveloped
adenoviruses, coronaviruses, rhinoviruses, influenza viruses,
Transmitted by direct and indirect contact, air (aerosols) and fomites (some also by water and food).

18. Course of infectious disease
Exposure
Entry
Spread
Multiplication
Damage
Immune response
Outcome
Infection
Disease

19. Spread to new host Person-to-person spread
Communicable – no intermediate host
Contagious – easily transmitted (aerosol droplets, saliva)
Spread by contact – direct or via inanimate object (vehicle, fomite)
Environmental spread: intermediate host, reservoir and/or vector may be involved

20. Vector-borne cycle of infection
Disease agent is a microorganism
Reproduces in a reservoir or host
Is transmitted by a vector

21. Vector-borne cycle of infection
Example: West Nile
Target organisms
Disease agent
Reservoirs
Flavivirus
Vector ?

22. Animal Reservoirs Cryptosporidium parvum Single host, eg Beef, calves
Oocyst excysts, releases 4 sporozoites
Sporozoites invade intestinal epithlial cells
Sporozoites replicate asexually, differentiate into microgametes and macrogametes
Sexual replication
More oocysts
Oocyst ?

23. Cryptosporidium life-cycle

24. Comparison – viruses and bacteria
Ability to infect
Ability to spread
Ability to invade
Ability to cause damage
Ability to survive outside host

25. Spread to new host Person-to-person spread
Communicable – no intermediate host
Contagious – easily transmitted (aerosol droplets, saliva)
Spread by contact – direct or via inanimate object (vehicle, fomite)
Environmental spread: intermediate host, reservoir and/or vector may be involved

26. Comparison – viruses and bacteria
Ability to infect
Ability to spread
Ability to invade
Ability to cause damage
Ability to survive outside host

27. Some viruses are carcinogens
Epstein-Barr virus
Herpesviruses
Nasopharyngeal carcinoma, Burkitt’s lymphoma

28. Prions “Infectious proteins” Creutzfeld-Jacob disease
Bovine spongiform encephalopathy (BSE)
Template for protein misfolding
Transmission ?