The Diseases

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): 1012 or more bacteria per gram feces

Bacteria of the GI Tract Stomach pH 3 0 to 103 per g Duodenum, jejunum pH 6-7 0 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

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

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

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

Major Portals of Entry Respiratory Tract Gastrointestinal Tract Skin

Respiratory Tract Aerosols, gases, vapors

Gastrointestinal Tract Food, drink, fecal-oral

Skin Contact, bathing

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

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

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

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.

Bacterial food poisoning Staphylococcus aureus Salmonella Clostridium

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

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).

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

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

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

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

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 ?

Cryptosporidium life-cycle

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

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

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

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

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