1
Q
Define host.
A
An animal capable of supporting growth of a microbe.
2
Q
Define pathogen.
A
Any virus, bacterium, or other agent that causes disease.
3
Q
Define pathogenicity.
A
Condition or quality of being pathogenic, or the ability to cause disease.
4
Q
Define virulence.
A
The degree or intensity of pathogenicity of an organism as indicated by case fatality rates and/or ability to invade host tissues and cause disease.
5
Q
Define colonization.
A
Establishment of a site of microbial reproduction on an “external” host surface (transiently or permanently) without observable clinical symptoms or immune reaction.
6
Q
Define infection.
A
Presence and replication of a microbe in or on a host.
7
Q
Define infectious disease.
A
Where the interaction between microbe and host leads to pathologic process characterized by damage to the host.
- may result from microbial factors (proliferation, production of cytotoxic enzymes or toxins)
- may result from host’s immune response
8
Q
Define symbiosis.
A
All associations in which one species lives in or on the body of another.
9
Q
Define strict pathogens.
A
Pathogens always associated with human disease.
Examples: Mycobacterium tuberculosis (TB), Neisseria gonorrhoeae (gonorrhea)
10
Q
Define opportunistic pathogen.
A
A microbe which is not pathogenic under normal conditions, but may cause infection and disease if introduced into a normally sterile body site (or into a host with a compromised immune system(
- may be members of patient’s normal microbial flora
Ex. Staphylococcus aureus, Escherichia coli, Candida albicans
11
Q
Define commensalism.
A
Symbiotic relationship in which one benefits and the other neither benefits nor is harmed.
12
Q
Define mutualism.
A
Symbiotic relationship in which both members benefit from their interaction.
13
Q
Define parasitism.
A
Symbiotic relationship in which one organism derives benefit while harming its host.
14
Q
Contrast the various types of symbiosis (mutualism, commensalism, parasitism).
A
Commensalism, mutualism and parasitism form a continuum as associations can merge and change given the circumstances of the relationship.
15
Q
What is normal flora?
A
- = total microbial population routinely found associated with a healthy human
- found in parts of body that are exposed to or communicate with the external environment (skin, nose and mouth, intestinal and genitourinary tracts)
- primarily bacteria (although minor numbers of viruses, fungi and protozoans may be present)
- ~10^14 bacteria cells (~1.2 kg) associated with the ~10^13 cells that make up the human body
16
Q
Describe the development of the normal flora.
A
- human fetus lives in protected, sterile environment
- during and after birth, colonization of skin, oropharynx, gastrointestinal tract and other mucosal surfaces occurs (from mother’s genital tract flora, flora of medical personnel, environmental flora)
- diet influences normal flora of GI tract:
Ex. breastmilk is high in lactose –> GI flora of breastfed infants contains lactic acid streptococci and lactobacilli; bottle-fed infants show greater variety of organisms - normal flora may be: resident (months to life) or transit (hours to days)
17
Q
What determines the composition of normal flora?
A
Composition of normal flora is in a continual state of flux determined by:
- local physiological conditions:
- types and amounts of nutrients, pH, oxygen concentration, etc.
- presence of local antibacterial substances such as lysozyme in tears
- types of other flora present (competition for nutrients, inhibition by metabolites etc.) - systemic host factors (age, diet, hormonal state, health, personal hygiene)
17
Q
What are the beneficial effects of normal flora?
A
- participate in metabolism of food products –> degradation of dietary oxalates (helps prevent formation of calcium oxalate kidney stones)
- gut bacteria provide essential growth factors (vitamin K and B vitamins)
- stimulates normal development of immune system –> germ-free animals have poorly developed immune systems –> more susceptible to normally non-pathogenic organisms
- protects against infection with virulent microbes
18
Q
Describe the mechanisms of how normal flora protect against virulent microbes.
A
- exclusion of potential pathogens –> competition for attachment sites, nutrients, etc.
- production of toxic metabolites
- Bifidobacterium in gut of breast fed infants –> lowers pH due to acetic acid production –> protects against GI pathogens (can’t grow at low pH)
- skin bacteria produce fatty acids and vaginal lactobacilli maintain acid environment –> discourage other species from invading
- production of antibacterial factors
- ex. Bacteriocins
19
Q
What are the potential negative effects of normal flora?
A
- source of pathogens: normal flora often has the potential to cause disease if presented with the right opportunity
- skin flora enters blood/deeper tissues after cuts, bites
- peritonitis caused by intestinal flora after ruptured colon
- oral Streptococcus enter bloodstream (extraction of teeth) –> infect heart valves
- Pseudomonas infections in burn victims
- Perianal organisms ascend urethra –> urinary tract infection
- overgrowth of certain organisms when normal flora changes (ex. after antibiotics) or when immune system becomes compromised
- metabolic activity of intestinal flora may be detrimental
- biotransformation of natural compounds into carcinogens
- release of toxic waste products from bacterial metabolism
- relationship of normal flora to obesity and type II diabetes
20
Q
How can normal flora be unintentionally altered?
A
- longterm antibiotic use –> indigenous flora in GI tract is eliminated –> Clostridium difficile proliferates –> diarrhea, colitis
- hospitalization –> replacement of normally avirulent microbes in the oropharynx with gram-negative rods (ex. Klebsiella, Pseudomonas) that can invade lungs –> pneumonia
21
Q
How can normal flora be intentionally altered?
A
- bone marrow transplant patients (immunocompromised): give antibiotics to remove normal flora and minimize risk of opportunistic infections
- “probiotics”: consumption of live bacteria so as to alter the balance of intestinal flora and improve health
- Lactobacillus and Bifidobacterium are commonly used (gram positive, lactic acid producing species)
- considered “safe” (already a part of normal GI flora)
- already routinely consumed with many different food products (sauerkraut, yogurt, etc.)
- Enteric coating may ensure better delivery to colon
22
Q
What are “prebiotics”?
A
Non-digestible food ingredients that beneficially affect the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon, thus improving host health
- selectively fermented by desired microbes
- “soluble fiber”
- “dietary fiber”
- Ex. Inulin (a plant polysaccharide), raw oats, unrefined wheat
23
Q
What are the benefits of probiotics?
A
May help reduce:
- bloating and diarrhea from lactose intolerance
- diarrhea that is a side effect of antibiotics
- diarrhea in infants with rotavirus enteritis
- infections in the digestive tract
24
Describe the stages of an infectious disease.
Pathogenic microbes must attach to, or penetrate, the host's body surfaces (must avoid/circumvent surface defences) and eventually shed from the host (either in large numbers in secretions/excretions or are available in blood for uptake via blood-sucking arthropods or needles).
Stages of an infectious disease:
- attachment (+/- entry into body)
- spread within the body (local or general)
- multiplication
- evasion of host defences
- exit (shedding from the body)
- transmission (source and/or reservoir)
- cause damage to host (not obligatory but often occurs)
25
List the mode of entry into the human body.
- ingestion
- inhalation
- trauma
- needlestick
- arthropod bite
- sexual transmission
26
How does receptor molecules play a role in mode of entry?
Receptor molecules:
- there are often specific receptor molecules on host cells
- these have some specific function in the life of the cell - are merely exploited by the pathogen
- after binding to susceptible cell, microbe can multiply at the surface (ex. Bordetella pertussis) or enter cell and infect it (ex. viruses, Chlamydia)
- presence, absence and concentration of receptors are critical determinants of cell (and host) susceptibility
Ex. CD4 molecule for HIV
Ex. Vibrio cholerae bind to receptors on surface of intestinal epithelial cells
- Vibrio cholera can attach to receptors on surface of intestinal epithelial cells
* in order to avoid elimination due to peristalsis and excretion, this microbe attaches itself to host cell to colonize and multiply
27
What are intracellular pathogens?
Intracellular pathogens take metabolic requirements directly from pool of nutrients inside host cell
- somewhat protected against many of host's defines mechanisms while intracellular (ex. antibodies)
- take advantage of problem of selectively targeting drugs or antibiotics against pathogen inside host cell (--> tissue damage)
- obiligate intracellular pathogen: rely on host synthetic machinery so must live within host cells (ex. Viruses, Chlamydia, Richettsia)
28
What are extracellular pathogens?
- take metabolic requirements from tissue fluids or by feeding directly on host cells (ex. Entamoeba histolytic - causes amebic dysentery)
- can grow and reproduce freely
- may move extensively within body tissues (may move away from areas where immune response is activated)
- if large, size can protect against phaocytosis
- if have protective external layers, can withstand attack
- BUT, extracellular location may leave them vulnerable to many of host's defines mechanisms (ex. antibody, complement, phagocytic cells)
29
Describe the exit and transmission of human pathogens.
- successful pathogenic microbes must leave the body and then be transmitted to fresh hosts
- organism with poor transmission from person to person will have little impact on the host population as a whole (ex. Ebola virus; Legionella pneumophila)
- transmission depends on three factors:
1. Number of microbes shed
2. Stability of microbe in the environment
3. Number of microbes required to infect fresh host
* "infectious dose"
* Efficiency of infection
- most of shed microbes die - only occasional one survives to perpetuate the species
- microbes which cause the shed of large numbers of progeny have greater chance of reaching fresh host
30
How does the stability of microbes in the environment play a role in transmission?
- microbes will have greater chance of reaching fresh host if they:
* resist drying
* resist thermal inactivation
- microbes that have developed resistant forms (ex. clostridial spores can survive years in soil) are particularly successful
- microbes that are sensitive to drying depend on close contact, vectors, or contamination of food and water for spread
31
What are the number of microbes required to infect fresh host?
- infectious dose (efficiency of infection) varies greatly between microbes
* 10 Shigella dysenteriae (from human) --> infection
* 10^6 (from animal) needed for infection
- route of infection has impact on infectious dose
* 10 gonococci --> infection in urethra
* 10 000 gonococci required to infect oropharynx or rectum
32
What factors affect transmission
1. microbial genetic factors
- some strains are more readily transmitted than others
- mechanism may not be clear
2. Activity of infected host may be affected by microbe in such a way as to increase efficiency of shedding and transmission
- coughing, sneezing (are reflex activities:)
* benefit current host by clearing foreign material from respiratory tract
* benefit microbe by enhancing transmission
* therefore, microbes that are able to increase fluid secretion and irritate respiratory epithelium will induce more coughing/sneezing --> effective transmission (and will be SELECTED for)
- diarrhea
* benefit host by eliminating infection more rapidly
* benfits microbe by effectively contaminating environment --> spread to new host
3. increase in nasal secretions --> coughing, sneezing --> promotes effective spread
- sneezing: up to 20 000 droplets produced per sneeze (many may contain pathogen)
33
What are the three basic types of transmission?
1. Human to human (most effective)
- respiratory route
- fecal-oral route
- venereal route
2. Vertebrate to human - animal used for food or as a pet
- Zoonoses are animal disease that can be transmitted to humans via animal vector
3. Biting arthropod and other invertebrates to human
- insects, ticks and mites are the most important of these (ex. Fleas and the plague; lice and typhus; mosquitoes and malaria)
34
Define vertical transmission.
Direct from parents to offspring via sperm, ovum, placenta, milk, blood
- can continue as long as do not affect viability of host
- Ex. Certain retroviruses; commensal bacteria
35
Define horizontal transmission.
Individual infecting both related and unrelated individuals
- contact
- respiratory
- fecal-oral
36
Describe the passive carriage by insects.
- relatively less common
- via mouthparts, on body, within intestines
- transfer onto food or host is result of insect feeding, regurgitating, or defecating
- Ex. Chlamydia trachomatis can cause trachoma (eye infection)
- specialized mouthparts may act as "contaminated hypodermic needle" transferring infection between individuals
37
Describe the biologic transmission by insects.
- more common
- bloodsucking vector acts as necessary host for multiplication and development of pathogen
- pathogen is re-introduced into human host after a period of time, at the next blood meal
- transmission by:
* direct injection, usually in vector's saliva (malaria, yellow fever)
* contamination from feces or regurgitated blood deposited at time of feeding (typhus, plague)
38
Describe the passive transmission via invertebrates used for food.
- shellfish (molluscs, crustacea) associated with food poisoning and acute gastroenteritis
* animal accumulates microbes in its body, taking them in from contaminated waste and then transfers them passively (especially in filter-feeders)
Ex. Vibrio cholerae
- some parasites (ex. worms, flukes) use the shellfish as an intermediate host - large numbers of infective stages escape from the invertebrate to infect human through skin
39
Describe the transmission of vertebrates to human.
- Zoonoses are animal disease that can be transmitted to humans via animal vector
- transmission via:
* contact
* inhalation
* bites
* scratches
* contamination of food or water
* ingestion as food
- epidemiology of zoonoses depends on frequency and nature of contact:
1. geographical localization
2. occupation
3. domestic animals (pets)
40
Define reservoir.
site or natural environmental location in which the pathogen is normally found living and from which infection of the host can occur.
41
Define carrier.
infected individual who is a potential source of infection
42
Define active carrier.
has overt clinical case of the disease
43
Define convalescent carrier.
has recovered from the disease but continues to harbour large numbers of the pathogen
44
Define healthy carrier.
harbours the pathogen but is not ill
45
Define incubatory carrier.
incubating pathogen in large numbers but not yet ill
46
Define casual, acute or transient carriers.
harbor pathogen for only a brief period (hours, days, weeks)
47
Define chronic carriers.
harbor pathogen for long periods (months, years, life)
48
Define fomites.
inanimate object that is not harmful itself, but may harbour pathogenic organisms (ex. bedding, eating utensils, un-sterilized surgical instruments)
49
Define vectors.
organisms that spread disease from one host to another
- Zoonoses: animal diseases that can be transmitted to humans (ex. anthrax, tuberculosis)
- arthropod vectors: mosquitoes, ticks, fleas, mites
50
Define sporadic disease.
occurring occasionally at irregular intervals (ex. typhoid fever)
51
Define endemic disease.
steady, low-level frequency at a moderately regular interval (ex. common cold)
52
Define hyper endemic disease.
increase in occurrence frequency beyond endemic level but not yet to epidemic level (ex. common cold in winter months)
53
Define epidemic disease.
sudden increase in occurrence above expected level (first case in epidemic = index case) (ex. influenza in some years)
54
Define pandemic disease.
increase in disease occurrence within a large population over a very wide region (usually the world) (ex. AIDS; influenza in 1960s)
55
What are the 4 types of vertical transmission?
1. prenatal
2. perinatal
3. postnatal
4. germline
56
What is the route and examples of prenatal (vertical) transmission?
Route: placenta
Examples: rubella, cytomegalovirus, syphilis, toxoplasmosis
57
What is the route and examples of perinatal (vertical) transmission?
Route: infected birth canal
Examples: gonococcal/chlamydial, conjunctivitis, hepatitis B
58
What is the route and examples of postnatal (vertical) transmission?
Route: Milk or direct contact
Examples: cytomegalovirus, hepatitis B, HIV, HTLV1
60
What is the route and examples of germ line (vertical) transmission?
Route: viral DNA sequences in human genome
Examples: many retroviruses
FMED - IDMM
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