Transcript 0-Flora&Bacterial-Pathogenesis

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Batterjee Medical College
Normal flora
Dr. Manal El Said
Head of Microbiology Department
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Normal flora
Normal flora (commensals)
• They are those microorganisms that are permanent
residents of body that everyone has.
• Commensals are organisms that derive benefit from
another host but do not damage that host.
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Normal flora
Normal flora organisms are:
• Bacteria or yeasts (No Viruses, protozoa & helminths)
• Inhabit body surfaces exposed to environment, such as :
Skin- Oropharynx- Intestinal tract -Vagina.
• Differ in number & kind at various anatomic sites.
• Low-virulence organisms.
• In their usual anatomic site, they are nonpathogenic.
• If they leave their usual anatomic site, especially in
immunocompromised individual
disease.
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Normal flora
Colonization
• Some people can be colonized:
- transiently or for long periods
- With certain organism (not members of normal flora).
Colonization resistance
It occurs when members of normal flora occupy receptor
sites on skin & mucosal surfaces
from binding to those receptors.
preventing pathogens
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Normal flora
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Important Members of the Normal Flora
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Bacterial Pathogenesis
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Bacterial Pathogenesis
Pathogen
Pathogen are microbes capable of causing disease in
immunocompetent people.
Opportunistic Pathogen
It refers to microbes that are capable of causing disease only
in immunocompromised people.
Carriers (chronic carriers)
Individuals in whom pathogenic organisms are present in
significant numbers & are source of infection for others.
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Bacterial Pathogenesis
Virulence
It is measure of microbe's ability to cause disease
Asymptomatic or inapparent Infections
These infections occur when our host defenses have
eliminated microorganism before it could multiply to sufficient
numbers to cause symptoms of disease.
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Bacterial Pathogenesis
Infection
Infection has two meanings:
(1) Presence of microbes in body
(2) Symptoms of disease.
Presence of microbes in body does not always result in symptoms of disease
Bacteria cause symptoms of disease by two main mechanisms:
1. Production of toxins (exotoxins & endotoxins)
2. Induction of inflammation.
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Bacterial Pathogenesis
Communicable Infection
• Most bacterial infections are communicable, i.e., capable of
spreading from person to person
Epidemiologic terms used to describe infections
Endemic :Infections that occur at persistent, usually low level
in certain geographic area
Epidemics: Infections occur at much higher rate than usual.
Pandemics: Infection spread rapidly over large areas of globe
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Bacterial Pathogenesis
Stages of Bacterial Pathogenesis
1. Transmission from external source into portal of entry.
2. Evasion of primary host defenses (skin or stomach acid).
3. Adherence to mucous membranes, usually by bacterial pili.
4. Colonization by growth of bacteria at site of adherence.
5. Disease symptoms caused by toxin production or invasion
accompanied by inflammation.
6. Host responses, both nonspecific & specific (immunity)
7. Progression or resolution of disease.
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Determinants of Bacterial Pathogenesis:
Transmission
Mode of Transmission
Comment
I. Human to human
A. Direct contact
Intimate contact, e.g.
•Sexual
•Passage through birth canal
B. No direct contact
Fecal–oral, e.g., excreted in human feces, then
ingested in food or water
C. Transplacental
Bacteria cross the placenta and infect the fetus
D. Blood-borne
• Transfused blood or intravenous drug use can
transmit bacteria and viruses
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Determinants of Bacterial Pathogenesis:
Transmission
Mode of Transmission
Comment
II. Nonhuman to human
A. Soil source
wound in skin
B. Water source
water aerosol are inhaled into lungs
C. Animal source
1. Directly
cat scratch
2. Via insect vector
tick bite
3. Via animal excreta
cattle feces are ingested in undercooked
hamburger
D. Fomite source
towel, are transferred onto the skin
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Determinants of Bacterial Pathogenesis:
Transmission
Vertical Transmission
• It is transmission of Bacteria, viruses & other microbes from
mother to offspring through:
- Placenta
- Birth canal during birth
- Breast milk.
Horizontal Transmission
It is person-to-person transmission (not from mother to
offspring).
Zoonoses
Human diseases for which animals are reservoir
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Determinants of Bacterial Pathogenesis:
Transmission
The main "portals of entry" into body :
• Respiratory tract
• Gastrointestinal tract
• Skin
• Genital tract.
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Determinants of Bacterial Pathogenesis:
Toxin Production
Property
Source
Exotoxin
Endotoxin
Certain species of gram-positive Cell wall of gram-negative
and gram-negative bacteria
bacteria
Secreted from cell Yes
No
Chemistry
Polypeptide
Lipopolysaccharide
Location of genes Plasmid or bacteriophage
Bacterial chromosome
Toxicity
High (fatal dose on order of 1 g) Low (fatal dose on order of
hundreds of micrograms)
Clinical effects
Various effects
Fever, shock
Mode of action
Various modes
Includes TNF and interleukin-1
Antigenicity
Induces high-titer antibodies
Poorly antigenic
called antitoxins
Vaccines
Toxoids used as vaccines
No toxoids formed and no
vaccine
Heat stability
Destroyed rapidly at 60°C
Stable at 100°C for 1 hour
(except staphylococcal
enterotoxin)
Typical diseases Tetanus, botulism, diphtheria
Meningococcemia, sepsis by
gram-negative rods
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Determinants of Bacterial Pathogenesis:
Toxin Production
Exotoxins
• Many exotoxins have A–B subunit structure:
- A (active) subunit possesses toxic activity
- B (binding) subunit is responsible for binding exotoxin to
specific receptors on membrane of human cell.
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Typical Stages of Infectious Disease
1. Incubation period:
time between moment person is
exposed to microbe (or toxin) & appearance of symptoms.
2. Prodrome
period:
time
during
which
nonspecific
symptoms occur.
3. Specific-illness period: time during which characteristic
features of disease occur.
4. Recovery period: time during which symptoms resolve &
health is restored.
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Typical Stages of Infectious Disease
• After recovery period, some people become:
- chronic carriers of organism
- latent infections develop.
• Some people have subclinical infections during which they
remain asymptomatic.