Transcript Infection and disease.med 11

Tvorko M. S.
Infection and disease
What is a pathogen?
• A pathogen is a micro-organism that has the potential to
cause disease.
What is an infection?
• An infection is the invasion and multiplication of
pathogenic microbes in an individual or population.
What is disease?
• Disease is when the infection causes damage to the
individual’s vital functions or systems.
An infection does not always result in
disease!
How do microbes reach us?
The cycle of transmission involves
• Escape from the host or reservoir of infection.
• Transport to the new host.
• Entry to the new host.
• Escape from the new host.
Source/reservoir
of infection
Mode of
transmission
represents the various barriers to
infection e.g. the skin
Portal of
entry
represents the target
organ e.g. lungs
HUMAN
HOST
Portal
of exit
Source of infection:
Sick men
Sick animal
Carrier
Transmission factor: infected food, water,
dirty hands, flies, fomites.
Vectors of pathogenic microorganims:
ticks, louses, fleas mosquitos.
Air-droplet (dust) mechanism of
transmission of infection
Stage of
selection
of invader
Stage of
penetration of
invader
Stage of stay of invader in an
environment
Tiny
phase
Source
infections
Sneeze
Cough
Breathing
Talks
Aerosol
Drying
Out
settling
Dust
phase
Sensitive
organism
Mode of transmission
Direct contact – person to person
Example: SARS
Vertical – sneezing closer
than 1 metre
• SARS - associated coronavirus
causes severe acute respiratory
syndrome (SARS).
• SARS is transmitted when an
infected person
coughs
or sneezes infectious
droplets onto a nearby person.
• The droplets land on another
person’s face or hands,
and become introduced
to the nose or mouth.
Jets of droplets erupt from
a man’s nose as he
sneezes.
Mode of transmission
Indirect contact – vehicle borne
Example: tuberculosis (TB)
Air/dust
•
The bacterium Mycobacterium tuberculosis causes tuberculosis (TB).
•
TB is spread from person to person through the air.
•
When a person with active TB coughs or sneezes, droplets loaded with
the infectious organism are propelled into the air.
•
The moisture evaporates from these particles to leave droplet nuclei that
can remain airborne for days and spread long distances.
•
The Mycobacterium has a waxy coat, which protects it from drying out
allowing it to survive for many months in the air and dust.
Fecal-oral mechanism of transmission of
infection
Stage of
selection
of invader
Stage of stay of invader in an
environment
Factors of transmission
Ways of
selection
Food
products
Water
Source
infections
Feces
urine
sputum
(for some
infections)
Stage of
penetration of
invader
Ways of
transmission
Food
Hands
Objects
domestic
use
Water
Soil
Contactdomestic
Flies
Receptive
organism
Mode of transmission
Direct contact – person to person
Example: gastroenteritis
Vertical – touching (faecal-oral route)
• Norwalk virus causes a type of gastroenteritis.
• Norwalk virus is found in the
faeces or vomit of infected people.
It is highly contagious.
• Infection occurs by having direct
contact with another person who is
infected and not maintaining good
hygiene e.g. washing hands.
Human hand contaminated
with colonies of bacteria
(blue/pink patches).
Mode of transmission
Indirect contact – vehicle borne
Example: food poisoning
Via food
• The bacterium Campylobacter
jejuni causes a type of food
poisoning.
• C. jejuni lives in the gut of many
warm-blooded animals, particularly
chickens, as part of their normal
body flora.
• The infection is transmitted to
humans by eating contaminated
food especially poultry and milk.
Campylobacter jejuni bacterium.
Mode of transmission
Direct contact – person to person
Vertical – kissing
Example: cold sore
• Herpes simplex virus causes cold sores.
• Initial infection occurs through direct skin contact when the
secretions from another person’s cold sore, containing the virus
particles, come into contact with cells of the skin around the mouth.
A
couple
kissing.
Cold sores
on lip and
mouth.
Mode of transmission
Direct contact – person to person
Example: syphilis
Vertical – sexual intercourse
• The bacterium Treponema pallidum
causes syphilis.
• The bacterium enters the body
through very tiny cuts on the skin or
mucous membranes when there is
contact with an infected person or
their bodily fluids.
Interlocking gender symbols
representing sexually transmitted
diseases.
Mode of transmission
Direct contact – person to person
Example: German measles
Horizontal across the placenta or via breast milk
• Rubella virus causes German measles.
• When infection occurs during pregnancy
the virus crosses the placenta in the
blood leading to infection of the fetus.
• The virus can affect all the
organs of the developing fetus.
• The risk to the baby is highest in the
first 3 months – up to 85% of babies are
affected if infected during this period.
Eight week old fetus
attached to its placenta by
the umbilical cord.
Mode of transmission
Indirect contact – vector borne
Example: malaria
Internal – biological
• Malaria is a vector-borne disease caused by a single celled
protozoan parasite called Plasmodium, which is transmitted by
mosquitoes.
• The primary vector for malaria is the mosquito Anopheles gambiae.
• Only female mosquitoes transmit
malaria when they feed on the
human host’s blood.
A. gambiae feeding on
human blood.
Mode of transmission
Indirect contact – vector borne
Example: bacterial dysentery
External – mechanical
• The bacterium Shigella causes a type
bacterial dysentery.
• Flies can spread Shigella when they carry
infected faecal matter on their feet to
drinking water or food.
• Symptoms can vary from mild diarrhoea
through to a more severe disease with
watery or bloody diarrhoea, fever,
stomach cramps and vomiting.
Common house flies feeding.
HOW DO
MICROBES GET
IN?
Portals of entry
• To cause an infection, microbes must enter our bodies.
• The site at which they enter is known as the portal of entry.
• Microbes can enter the body through the four sites listed below
Respiratory
tract (mouth and nose) e.g. Influenza virus
Gastrointestinal
Urogenital
Breaks
tract (mouth oral cavity) e.g. Vibrio cholerae
tract e.g. Escherichia coli
in the skin surface e.g. Clostridium tetani
Portals of entry
Respiratory tract
Example: influenza
• Influenza or ‘flu is a highly infectious
respiratory tract infection.
• It is caused by a virus.
• Virus is inhaled into the lungs through
the mouth and nose.
• The envelope of the virus has around
500 spikes sticking out of it.
• Spikes attach to the cells lining the
lungs. These help the virus get into the
cell.
The respiratory tract.
Portals of entry
Gastrointestinal tract
Example: cholera
• Cholera is an acute infection of the
intestinal tract.
• It is caused by the bacterium Vibrio
cholerae.
• The infection is spread by
contaminated water and food,
especially seafood, or from one
infected person to another via the
faecal-oral route.
• The incubation period is 24-72
hours.
The gastrointestinal tract.
Portals of entry
Urogenital tract
Example: cystitis
• Cystitis is the commonest infection of the lower urinary tract.
• Strains of the bacterium Escherichia coli, a normal inhabitant of the
human intestine, are responsible for 80% of cases of cystitis.
• Bacteria enter the urethra and travel up to
the bladder.
• It is more common in women than in men.
• In women the anus and the opening of the
urethra are closer to each other.
• The urethra is shorter in women so the bacteria
have a shorter distance to travel to the bladder.
Artwork of an
inflamed bladder (red)
caused by cystitis.
Portals of entry
Urogenital tract
Example: Chlamydial infection
• Chlamydial infection is the most common
sexually transmitted bacterial
C. trachomatis bacteria (background) with the
infection in the world.
female reproductive tract superimposed.
• It is caused by Chlamydia
trachomatis.
• About 1 in 10 young people
have Chlamydia.
• It can’t be caught from
kissing, sharing towels or
toilet seats.
• Chlamydial infection is often known as the silent disease as
approximately 75% of women and 50% of men don’t experience any
symptoms.
Portals of entry
Breaking the surface of the skin
Example: tetanus
• Tetanus is commonly known as lockjaw; it is a neuromuscular
disease.
• It is caused by a toxin (poison), which is produced by the
bacterium Clostridium tetani.
• C. tetani is found in soil, dust and the guts and faeces of many
animals.
• C. tetani produces endospores.
• The endospores usually enter the
body through a puncture wound to
the skin.
Splinter in the
finger.
The dynamics of the development of the infectious process consists
of the incubation and prodromal periods, the height of the disease
and period of recovery ( convalescence).
A certain period of time elapses from the moment of penetration of
the pathogenic microbe to the onset of the first sings of the disease,
which has been named the incubation period of the disease.
Pathogenicity
What makes some bacteria dangerous?
Pathogenicity
Ability to cause disease
Virulence –
degree of pathogenicity
Bacterial virulence factors
for adherence, colonization, and invasiveness
How Bacterial Pathogens
Penetrate Host Defenses
Bacterial Adhesion
• Results in colonisation
• One of several pathogenic mechanisms
= a virulence factor
Adhesion to host tissues is mediated by:
• fimbriae (pili)
• cell wall-associated proteins
• other cell wall constituents, e.g.
lipoteichoic acid
•
Many adhesins recognize specific
sugars on the target tissue
Bacterial Adhesion
Tropism
•
•
•
•
•
•
•
•
•
Neisseria meningitidis
Neisseria gonorrhoeae
Vibrio cholerae
Bordetella pertussis
Salmonella typhimurium
Helicobacter pylori
Streptococcus pyogenes
Campylobacter jejuni
Mycoplasma pneumoniae
•
•
•
•
•
•
•
•
•
Nasopharyngeal epithelium
Uretral epithelium
Intestinal epithelium
Respiratory epithelium
Intestinal epithelium
Gastric mucosa
Pharyngeal epithelium
Intestinal epithelium
Respiratory epithelium
Adherence of vibrio
cholera
on the mucose
Fimbriae of human enterotoxigenic
Escherichia coli
• Invasiveness
Spreading factors - enzymes to destroy
intercellular cement
• collagenase,
•
•
•
•
hyaluronidase,
fibrinolysin,
lecithinase,
streptokinase
Penetration into
the Host Cell
Antiphagocytic factors
– Capsules, cell wall proteins
– leukocidin
– coagulase (walls off)
– survival in WBC - intracellular
parasites (mycobacteria, Listeria)
IgA protease - Neisseria
• Components of the Cell Wall
– Proteins in the cell wall can facilitate
adherence or prevent a pathogen from being
phagocytized.
• M protein of S. pyogenes (cell surface and
fimbriae)
– Some microbes can reproduce inside
phagocytes.
• M. tuberculosis
Gram negative
Toll like receptors
(TLR) respond to
bacterial cell wall
molecules
And some virus
molecules
Pathogen
associated
molecular pattern
(PAMP)
LPS
Microbial toxins
According to the nature of production,
microbial toxins are subdivided into
exotoxins and endotoxins.
Endotoxin
Endotoxins
Exotoxins
They are subdivided into three classes.
Class A includes exotoxins secreted into the external
environment: cholerogen (V. cholerae); histotoxin, dermonecrotoxin
and haemolysin (Cor. diphtheriae), alpha- and delta-haemolysin and
beta and epsilon toxin (C/. perfringens) and others.
Class B includes exotoxins which are partly secreted, partly
bound with the microbial cell: alpha toxin (C/. novyi); tetanospasmin
(Cl. tetani); neurotoxin (C/. botulinum).
Class C consists of exotoxins bound with the microbial cell; Sh.
dysenteriae exotoxin; Cl. perfringens enterotoxin
• Cytotoxins:
– Diphtheria toxin (inhibits protein synthesis)
– S. pyogenes erythrogenic toxins (damage
capillaries and produce Scarlet fever*).
• Neurotoxins:
– Botulinum toxin (prevents nerve transmission)
– Tetanus toxin (prevents inhibitory nerve
transmission or a.k.a muscle relaxation pathway).
• Enterotoxins:
– affect the intestines induce fluid and electrolyte loss
from host cells (S. aureus, V. cholerae,
enterotoxigenic E. coli).
Mechanism of molecular action of
cholerae toxin
Development of vibrio cholerae
in the human organism
Mechanism of action of toxin of
Clostridium tetani
Physiology influencing of tetanospasmin
Child with neonatal tetanus
Structural organization of molecule
of diphtherial toxin and mechanism
of his action on a mew
Look of mouth cavity of sick
patient with diphtheria
Mechanisms of action of botulinum
toxin
Place of neuro-muscle contact
In a healthy organism
neuromediator frees oneself
from the nervous ending
in a sinaptic crack
The molecules of botulinum register to the
presinaptical membrane and hinder
to freeing of neuromediator