-Leading cause of bacterial disease in humans. “Normal flora” colonizes nares.
-Wound infection, bacteremia, otitis, resistant strain: MRSA-methicillin
One of the most prevalent and costly diseases in North America!
Can affect pharynx, mouth, intestines
Opportunistic infections (take advantage of certain situations): do not cause
disease in a healthy host but affect compromised immune system.
Dental caries, plaque formation
A consistent resident of small intestine, together with several other enteric
bacteria (Klebsiella, Enterobacter, Citrobacter).
Intestinal infections, urinary tract infections, and neonatal meningitis.
o Due to poor design of human body – anus very close to urethra and to
where the vagina is.
In the ORAL CAVITY
Cause acid formation, change in pH – development of dental caries.
During reproductive years, colonize vagina, lower the pHwhich inhibits
growth of various pathogens.
Benefit of HOST to Normal Flora
The host provides food and shelter to the normal flora.
1. Synthesize and secrete vitamins, e.g. K and B12 from enteric bacteria.
2. Prevent colonization by pathogens, e.g. lactobacilli in genitourinary tract.
3. Produce substances that inhibit/kill non-indigenous bacterial species - lysozymes.
4. Stimulate development of certain tissues, especially lymphoid related.
5. Stimulate the production of cross reactive antibodies – “natural” antibodies that
are also active against pathogens.
It’s a balance of the relationships:
Symbiosis: nature of the relationship/interaction between living things (e.g.
host and parasite).
Mutualism: a relationship in which some reciprocal benefit accrues to both
parties (advantages for both). E.g. bees on a flower.
Commensalism: a relationship in which one symbiont, the commensal,
benefits while the other, the host, is neither harmed nor helped. E.g.
barnacles on whale. Parasitism: a relationship in which one of the pair benefits from the other,
and the host is usual harmed. E.g. Mosquitoes on host.
What is Pathogenism? (MC QUESTIONS!)
Pathogen: a microorganism capable of causing disease.
Pathogenicity: ability of the microorganism to cause disease in another
o Determined by both host and organism actors.
o Normal flora normally beneficial but may become PATHOGENIC.
Obligate Pathogens: do not associate with their host except in the case of
1. Number of microorganisms.
2. Host: immunocompetence, vaccine status, co-morbidities, genetics, stress, etc
3. Virulence of microorganism (degree of pathogenicity).
Bug Factors: Virulence
Virulence = the DEGREE of pathogenicity. Depends on ability to:
Invade the host – spread to adjacent tissues/cells.
Infect the host – establish a focal point of infection.
Cause damage to the host – directly or via immune response to organism.
A health animal can normally defend itself against various pathogens at
various stages of the infections disease process.
Typically, host defense mechanisms are divided into:
Natural or innate, general protection, inherent to host
Common to all animals
Non-specific: antimicrobial factors in saliva, mucus, physical
barrier of skin, pH change in gat, pH of vagina, commensals.
Must be TURNED ON by exposure to a pathogen.
Usually quite specific.
Immune response. Immunity = the relative state of resistance
of the host to a specific pathogen. Toxigenesis: the ability to produce toxins.
Solube substances that alter the normal metabolism of the host cells with
deleterious effects on the host.
o Endotoxin = LPS: associated with cell wall of Gram Negative Bacteria
(may be released from cell when growing or when cells lysed).
o Exotoxin = proteins: released from bacterial cells and may act at
tissue sites removed from the site of bacterial growth.
Know this slide -
Outbreaks Rare in Well Engineered Systems
Rural versus urban: rural more likely to have a private well.
Developed versus developing: developed has better sanitation.
Increased turbidity (rainfall): most outbreaks associated with high
Spring/summer months are the peak months for waterborne disease –
frozen ground does not facilitate passage of organisms.
E. Coli 0157
Cattle considered primary reservoir
Incubation Period = 1-5 days
Diarrhea and abdominal pain
Hemolytic Uremic Syndrome: triad of lesions – microangiopathic hemolytic
anemia, thromobocytopenia, renal failure.
Waterborne Microbial Pathogens E. coli, Campylobacter spp., Salmonella spp., S. typhi, Shigella, Vibrio cholera,
Helicobacter pylori, Leptospira, Legionelle pneumonophila, Yersinia enterocolitica,
Mycobacterium spp., Bacillus anthracis, Yersinia pestis, Francisella tularensis,
Enterviruses, Norovirus, Hepatitis A/E, Cryptosporidium, Giardia, Toxoplasma,
Cyclospora, Entameba hisolytica, Dracunculus medinensis.
Waterborne Zoonoses Risks (travels from animals to humas)