Bacterial disease

1
Bacterial disease

• Learning objectives
• To be able to describe the factors affecting
  pathogenicity of bacteria
• To be able to describe modes of transmission.
• To be able to describe precautions to avoid
  contamination of food and water.

2
Types of disease

• Disease is a general term meaning disorder of the
  body. There are several different kinds of
  disease
• Infectious
• Genetic
• Dietary deficiency
• Environmental
• Ageing

3
The factors affecting pathogenicity of bacteria

• Pathogenicity is how ill an infection can make
  you
• It is a result of the toxins produced by the
  bacteria.
• Those released by the bacteria as waste products
  are called exotoxins
• Endotoxins are part of the bacterial cell itself
  and are released when the bacterial cell dies and
  breaks up

4
Infectivity

• This is the number of bacteria needed to cause an
  infection
• Most diseases require large numbers, for example
  salmonella, although some, for example typhoid,
  requires relatively few.
• The fewer bacteria required to trigger a disease,
  the more virulent the disease is

5
Invasiveness

• How easily the bacteria can spread within the
  body.
• Bacteria must avoid phagocytosis and overcome the
  bodies defence mechanisms.
• How easily the bacteria can penetrate connective
  and fibrous tissue
• Bacteria who can penetrate tough tissue have to
  secrete enzymes. Very few bacteria can do this
  those that can (eg bubonic plague and anthrax)
  are highly invasive

6
Attachment

• Once the microbes has infected a new host they
  need to attach themselves firmly to the host's
  cells. Why?
• In order to avoid being removed by cilia,
  sneezing, vomiting, diarrhoea, etc.

7

• To do this microbes use molecules called
  adhesins, which are protein or carbohydrate
  molecules on the cell wall of bacteria.
• These adhesin molecules bind to specific protein
  receptor molecules on the cell membranes of host
  cells forming adhesin-receptor complexes, so
  firmly attaching(or adhering) the microbe to the
  host cell.
• Because this involves specific binding, this
  attachment is called specific adherence.

8
Adhesions
Bacterium

• Specific adherence explains why microbes tend to
  be specific for particular species and tissue.
• For example the malaria parasite only grows in
  red blood cells the diphtheria bacterium only
  colonises the membranes of cells in the trachea
  and the polio virus only invades nerve cells.
• It also explains one reason why different people
  have susceptibility to certain infections.
  Individuals without the "correct" receptor cannot
  attach the pathogen and so cannot catch that
  particular disease.

Receptor
Cell membrane
9

• Transmission

10
Transmission by Water.

• Many pathogens can survive (though not reproduce)
  in water and so infect humans drinking that
  water.
• Water-borne infections include cholera, typhoid,
  dysentery, gastroenterisis and food poisoning

11
Transmission by Food.

• Food is an even better way of transmitting
  pathogens, since microbes can grow and reproduce
  in food, which they can't do in water.
• Food can become contaminated by contact with
  humans or insects.
• Food-borne infections include TB, botulism,
  cholera, amoebic dysentery, typhoid and
  gastroenteritis.

12
Transmission by Aerosol Droplet.

• When an infected person coughs, sneezes, talks or
  breathes, they are transmitting their pathogens
  inside droplets of mucus, saliva or water.
• Air-borne infections include many of the viral
  infections such as cold, flu, rubella, and some
  bacterial infections such as TB, diphtheria and
  strep throat.

13
Transmission by Direct Contact.

• The skin is a good barrier to microbial entry,
  but some infections are still possible by contact
  with the skin.
• Sexual intercourse is a special case of direct
  contact, and sexually-transmitted diseases (STDs)
  include the viral infections Hepatitis B and AIDS
  and the bacterial infections gonorrhoea and
  syphilis.

14
Transmission by Vectors

• In the context of disease, a vector is an animal
  that transmits a pathogenic microbe between other
  animals.
• Examples include rabies (transmitted by dogs),
  malaria (transmitted by mosquitoes), sleeping
  sickness (transmitted by tsetse flies) and yellow
  fever (transmitted by mosquitoes).

15
Entry

• Some bacterial and fungal pathogens remain
  attached to the outer surface of human cells or
  colonise extracellular spaces in humans, such as
  the lumen of the gut, alveoli, or tissue fluid.
• These are termed extracellular pathogens.

16

• Intracellular pathogens enter the cell. Entry is
  achieved in different ways
• Some bacteria secrete invasin enzymes, which
  digest the cell membrane of the host cell,
  allowing the bacterial cell to enter it.
• Sometimes the specific adherence described above
  stimulates endocytosis by the host cell, which
  brings the microbe cell inside the host cell. The
  host cell may then try to digest the microbe
  cell, but if the bacterium has a capsule it may
  be protected.

17

• By reproducing inside host cells. Viruses,
  bacteria and protoctists can all colonise living
  cells and reproduce inside them, using up
  cellular resources and preventing the cell from
  carrying out its normal reactions.
• The microbes then usually burst out of the host
  cell, rupturing the cell membrane and killing the
  cell in the process.

18

• Which bacterium is the main cause of diarrhoea?
• How is it spread?
• What do the toxins cause?
• How does ORT help patients suffering from
  diarrhoea?
• What does the exotoxin secreted by the diptheria
  bacterium do?
• The bacterial disease tetanus causes muscle
  spasms. How?
• Exotoxins are detected by the body as foreign
  proteins. How is this effect exploited?
• What effect do endotoxins have on the body?