How microorganisms
cause disease
objectives
1. Describe routes of transmission of infectious
diseases
2. Discuss the process of adherence,
multiplication, invasion and tissue damage
3. Explain mechanisms bacteria utilize to evade
body immune system
4. Explain endotoxins and exotoxins
1.
PRIMARY INFECTION
initial infection in host
2.
REINFECTION
subsequent infection by the same parasite
3.
SECONDARY INFECTION when a new parasite sets up in a host
whose resistance is lowered by a preexisting infectious disease
4.
CROSS INFECTION
when a person already suffering from a
disease a new infection is set up from another host
5.
NOSOCOMIAL INFECTION
cross infections in hospitals
6.
IATROGENIC INFECTIONS
physician induced infections
7.
INAPPARENT/SUBCLINICAL INFECTION
apparent
8.
Exogenous infection
9.
Endogenous infection
clinical effects are not
SOURCES OF INFECTION
1.
Humans
2.
Animals
3.
Insects. Blood sucking insects like
mosquitoes(vector) may transmit
pathogens to humans
(arthropode borne diseases)
4.
Soil and water: spores , fungal
infections
5.
food
zoonotic infections
METHODS OF TRANSMISSION
OF INFECTION
1.
CONTACT
Direct: EXAMPLES: sexually transmitted diseases
indirect:fomites like pencils, thermometers
2. Airborne INHALATION droplet nuclei and dust examples:
respiratory illness
3.
Faeco-oral INGESTION
examples: intestinal infections
4.
Parenteral INOCULATION
5.
Vector borne INSECTS examples: malaria
6.
CONGENITAL vertical transmission: example: rubella
7.
IATROGENIC
examples: HIV
Respiratory transmission

An infected person can release
microbes(viruses and bacteria) by talking,
sneezing, coughing which are inhaled by
another person

Droplet transmission
 pathogens transferred infection from the
respiratory tract of the infected person
"generally over short distances usually 3-6
feet, necessitating facial protection


respiratory viruses etc.
Airborne transmission
 Very small droplet nuclei (less than 5 micron)
that remain suspended in air for longer time
and inhaled by other person, necessitating
special isolation rooms for the patient
 e.g Tuberculosis , Measles , Chickenpox etc.
Feco-oral transmission
 The fecal–oral route (or
alternatively the oral–fecal route or
orofecal route) is a route of
transmission of a disease, when
pathogens in fecal particles pass
from one host and introduced into
the oral cavity of another host
 Contaminated food and water
 Adequate hand washing after
using toilet to avoid feco oral
transmission
 e.g.
 Bacteria causing Gastroenteritis
Cholera, typhoid,etc
 Viruses like poliovirus,rota virus,
Parenteral transmission
 Parenteral tansmission is the
introduction of microbes via a
route other than the mouth or
rectum, especially via infusion,
injection
 Percutaneous entry through
the skin
 Transfusions, needle pricks,
surgical incision,tatoos, razor cuts
etc.
 e.g hepatitis C & B, HIV etc
Vector borne transmission
 Vector-borne diseases are
infections transmitted by the bite of
infected arthropod species, such as
mosquitoes, ticks, triatomine bugs,
sandflies, and blackflies1
 e.g malaria, leishmania, plague,
dengue fever
Contact transmission
 Contact transmission requires some
form of touch to spread an infection.
 Direct contact transmission involves
immediate contact between two
people( or with an animal)
 lndirect contact transmission involves
fomites an obiect that becomes
contaminated by touch (the fomite)
then spreads the infection by touch.
 Some diseases that are transmissible
by direct contact include:
 Athlete's foot, warts , conjunctivitis
etc.
Sexual transmission
 Unprotected sex with an infected
partner including vaginal ,anal or
oral sex
 e.g gonorrhea, syphilis, HIV
infection, etc.
vertical
 Microbes are transmitted from
mother to offspring
 In utero( transplacental)
 In birth canal during delivery
 Via breast milk
 Horizontal transmission in contrast
is person to person transmission
that is not from mother to offspring
 e.g TORCH infections
Iatrogenic transmission
 Transmission due to medical
procedures, such as injection or
transplantation of infected
material.
Zoonoses
 Diseases for which animals
are the reservoirs are called
zoonoses
 Animals can either
 be the source (reservoir) or
 mode of transmission
 e.g plague(rats),rabies
(vertebrate
animals),brucellosis
(livestock)
Stages of microbial
pathogenesis
Transmission
from external
source into
portal of
entry
Evasion of
primary host
defenses
Adherence
Colonization
Invasion/
inflammation
or Toxin
production
Host
responses
Progression/
resolution
Bacterial Virulence Factors
(ability to cause disease)
 Adherence Factors:
 Multiplication
 Invasion
 Tissue damage by bacteria
ADHERENCE
 Pili, capsules or glycocalyces allow
Bacteria to adhere to the surface of
human cells
 Pili of Ns gonorrhoeae and E.coli :
attach to urinary tract epith
 Glycocalyx of Staph epidermidis and
certain viridans streps : attach to heart
valves
 Adhesins: molecules mediating
attachment to surfaces
 Without adherence the bacteria may
be washed away
Ns gonorrhoeae attaching to urinary
epithelium via pili
ADHERENCE
 Biofilm/slime layer
formation: protective
matrix of
polysaccharides and
proteins
 Protect bacteria from
antibiotics & host
immune defenses e.g.
antibodies and
neutrophils
 Persistence of
Streptococci in dental
plaques
COLONIZATION &
MULTIPLICATION
 After adherence to the surfaces,
bacteria colonize and multiply
INVASION & INFLAMMATION
 Invasion followed by
inflammation
 Invasive bacteria
secrete enzymes that
destroy host tissue
 Collagenase and
hyaluronidase, lipase,
protease etc.
Infectious Diseases
What is an infectious diseases??
Infectious disease is:
①
A group of common diseases
②
Caused by different pathogens
③
Possessing infectivity.
④
To form epidemic.
⑤
Infectious disease is a threat to the health of people.
Factors Influencing Disease Transmission
Agent
Environment
• Infectivity
• Weather
• Pathogenicity
• Housing
• Virulence
• Geography
• Immunogenicity
• Antigenic stability
• Occupational
setting
• Survival
• Air quality
• Food
• Age
Host • Sex
• Genotype
• Behaviour
• Nutritional status
•Health status
Defense Mechanisms in our body
1. Skin:

Prevents entry of infectious organisms, unless injured.

Severe burn patients who die are usually killed by infections. So much skin is
damaged they are very vulnerable to infections.
2. Mucus membrane:

Mucous is usually rich in enzymes that will kill many pathogens
3. Cilia:

These are hair-like structures lining the respiratory tract. They work to sweep
foreign particles out of the respiratory tract.

Damaged by smoking, leaving smokers more vulnerable to infections.
4. Coughing:

Helps remove foreign material from respiratory tract.
5. Personal Hygiene

Helps reduce the number of pathogenic organisms on the skin and other
surfaces of the body.
Factors predisposing to microbial
pathogenicity
Pathogenecity and virulence:
 Pathogenecity denotes the ability of a microbial
species to cause disease
 Virulence is the ability of a strain of a species to
produce disease. The quantitative ability of an agent
to cause disease for example: capsulated bacteria is
more virulent than noncapsulated bacteria
 Determinants of virulence
1. Transmissibility
2. Adhesion
3. Invasiveness
4. Toxigenecity
5. Avoidance of host tissue mechanisms
6. Enzymes like coagulase ,streptokinase, cytolisins
7. Plasmids
8. Bacteriophages
9. communicability
10. Infecting dose
11. Route of infection
4. TOXIGENECITY
EXOTOXINS
 Excreted by living cell; high
concentrations in liquid
medium
 Produced by both grampositive and gram-negative
bacteria
ENDOTOXINS

EXOTOXINS
 Most Exotoxins have A–B subunit structure
 A subunit is active
 possesses pathogenicity
 Usually an Enzyme
 B binds to the receptor
• Exotoxins cause protein
synthesis inhibition,
neurotoxicity or can be
cytolytic
SOME EXAMPLES OF
EXOTOXINS
ORGANISM
TOXIN
C. tetani
Tetanus
toxin
C.
botulinum
Botulinu
m toxin
MECH OF
ACTION
Blocks
release of
inhibitory NT
Glycine &
GABA
Blocks
release of
Acetylcholin
e at NMJ
EFFECT
Spastic
contractio
n of
muscles
Flaccid
paralysis
ENDOTOXINS
 Integral parts of the cell walls
 Endotoxins are lipopolysaccharides
 Toxic moiety is lipid A
 Produce the same generalized
effects of fever and shock
 No toxoids
ENDOTOXINS-BIOLOGICAL
EFFECTS
 Fever due to IL-1 & IL-6
 Hypotension, shock, and impaired
perfusion due to TNF, NO,
Bradykinins
 DIC due to release of tissue factor
 Inflammation & tissue damage due
to activation of alternate pathway
of complement
5. Mechanisms bacteria utilize to evade body
immune system
A. Capsule like streptococcus pneumonia
 it prevents phagocytosis
B. Streptoccocal M protein
C. Resistance to killing by phagocytic
cells(intracellular)
 Some bacteria survive inside neutrophils,
monocytes and macrophages
D. Antigenic variation
 Ns gonorrhoeae outer membrane protein &
pili
 HIV and hepatitis C virus
 leptospira
D. Serum resistance
E. Siderohpore and iron acquisition
Glossary
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Adherence (adhesion, attachment):
Carrier :
Infection:.
Invasion:
Nonpathogen:
Opportunistic pathogen:
Pathogen:
Pathogenicity:.
Toxigenicity:
Virulence: The quantitative ability of an agent to cause disease.
Virulent agents cause disease when introduced into the host in small numbers.
Virulence involves adherence, invasion, and toxigenicity