Bacterial diseaseBacterial disease

Learning objectives:Learning objectives:

• To be able to describe the factors To be able to describe the factors affecting pathogenicity of bacteriaaffecting pathogenicity of bacteria

• To be able to describe modes of To be able to describe modes of transmission. transmission.

• To be able to describe precautions to To be able to describe precautions to avoid contamination of food and water.avoid contamination of food and water.

Types of diseaseTypes of diseaseDisease is a general term meaning Disease is a general term meaning

disorder of the body. There are several disorder of the body. There are several different kinds of disease:different kinds of disease:

• InfectiousInfectious

• GeneticGenetic

• Dietary deficiencyDietary deficiency

• EnvironmentalEnvironmental

• AgeingAgeing

The factors affecting The factors affecting pathogenicity of bacteriapathogenicity of bacteria

• Pathogenicity is how ill an infection can Pathogenicity is how ill an infection can make youmake you

• It is a result of the toxins produced by It is a result of the toxins produced by the bacteria.the bacteria.

• Those released by the bacteria as Those released by the bacteria as waste products are called exotoxinswaste products are called exotoxins

• Endotoxins are part of the bacterial cell Endotoxins are part of the bacterial cell itself and are released when the itself and are released when the bacterial cell dies and breaks upbacterial cell dies and breaks up

Infectivity Infectivity

• This is the number of bacteria needed This is the number of bacteria needed to cause an infectionto cause an infection

• Most diseases require large numbers, Most diseases require large numbers, for example salmonella, although for example salmonella, although some, for example typhoid, requires some, for example typhoid, requires relatively few.relatively few.

• The fewer bacteria required to trigger a The fewer bacteria required to trigger a disease, the more virulent the disease disease, the more virulent the disease isis

InvasivenessInvasiveness

• How easily the bacteria can spread within the How easily the bacteria can spread within the body.body.

• Bacteria must avoid phagocytosis and Bacteria must avoid phagocytosis and overcome the bodies defence mechanisms.overcome the bodies defence mechanisms.

• How easily the bacteria can penetrate How easily the bacteria can penetrate connective and fibrous tissueconnective and fibrous tissue

• Bacteria who can penetrate tough tissue Bacteria who can penetrate tough tissue have to secrete enzymes. Very few bacteria have to secrete enzymes. Very few bacteria can do this – those that can (eg bubonic can do this – those that can (eg bubonic plague and anthrax) are highly invasiveplague and anthrax) are highly invasive

Attachment Attachment • Once the microbes has infected a new Once the microbes has infected a new

host they need to attach themselves host they need to attach themselves firmly to the host's cells. Why?firmly to the host's cells. Why?

• In order to avoid being removed by In order to avoid being removed by cilia, sneezing, vomiting, diarrhoea, etc. cilia, sneezing, vomiting, diarrhoea, etc.

• To do this microbes use molecules called To do this microbes use molecules called adhesins, which are protein or carbohydrate adhesins, which are protein or carbohydrate molecules on the cell wall of bacteria.molecules on the cell wall of bacteria.

• These adhesin molecules bind to specific These adhesin molecules bind to specific protein receptor molecules on the cell protein receptor molecules on the cell membranes of host cells forming adhesin-membranes of host cells forming adhesin-receptor complexes, so firmly attaching(or receptor complexes, so firmly attaching(or adhering) the microbe to the host cell. adhering) the microbe to the host cell.

• Because this involves specific binding, this Because this involves specific binding, this attachment is called specific adherence.attachment is called specific adherence.

• Specific adherence explains why microbes tend to be Specific adherence explains why microbes tend to be specific for particular species and tissue. specific for particular species and tissue.

• For example the malaria parasite only grows in red blood For example the malaria parasite only grows in red blood cells; the diphtheria bacterium only colonises the cells; the diphtheria bacterium only colonises the membranes of cells in the trachea; and the polio virus membranes of cells in the trachea; and the polio virus only invades nerve cells. only invades nerve cells.

• It also explains one reason why different people have It also explains one reason why different people have susceptibility to certain infections. Individuals without susceptibility to certain infections. Individuals without the "correct" receptor cannot attach the pathogen and so the "correct" receptor cannot attach the pathogen and so cannot catch that particular disease. cannot catch that particular disease.

BacteriumBacterium AdhesionsAdhesions

Receptor Receptor

Cell membraneCell membrane

TransmissionTransmission

Transmission by Water.

• Many pathogens can survive (though Many pathogens can survive (though not reproduce) in water and so infect not reproduce) in water and so infect humans drinking that water. humans drinking that water.

• Water-borne infections include cholera, Water-borne infections include cholera, typhoid, dysentery, gastroenterisis and typhoid, dysentery, gastroenterisis and food poisoningfood poisoning

Transmission by Food.

• Food is an even better way of Food is an even better way of transmitting pathogens, since microbes transmitting pathogens, since microbes can grow and reproduce in food, which can grow and reproduce in food, which they can't do in water. they can't do in water.

• Food can become contaminated by Food can become contaminated by contact with humans or insects. contact with humans or insects.

• Food-borne infections include TB, Food-borne infections include TB, botulism, cholera, amoebic dysentery, botulism, cholera, amoebic dysentery, typhoid and gastroenteritis.typhoid and gastroenteritis.

Transmission by Aerosol Droplet.

• When an infected person coughs, When an infected person coughs, sneezes, talks or breathes, they are sneezes, talks or breathes, they are transmitting their pathogens inside transmitting their pathogens inside droplets of mucus, saliva or water. droplets of mucus, saliva or water.

• Air-borne infections include many of Air-borne infections include many of the viral infections such as cold, flu, the viral infections such as cold, flu, rubella, and some bacterial infections rubella, and some bacterial infections such as TB, diphtheria and strep throat.such as TB, diphtheria and strep throat.

Transmission by Direct Contact.

• The skin is a good barrier to microbial The skin is a good barrier to microbial entry, but some infections are still entry, but some infections are still possible by contact with the skin. possible by contact with the skin.

• Sexual intercourse is a special case of Sexual intercourse is a special case of direct contact, and sexually-transmitted direct contact, and sexually-transmitted diseases (STDs) include the viral diseases (STDs) include the viral infections Hepatitis B and AIDS and the infections Hepatitis B and AIDS and the bacterial infections gonorrhoea and bacterial infections gonorrhoea and syphilis.syphilis.

Transmission by Vectors

• In the context of disease, a vector is an In the context of disease, a vector is an animal that transmits a pathogenic animal that transmits a pathogenic microbe between other animals. microbe between other animals.

• Examples include rabies (transmitted Examples include rabies (transmitted by dogs), malaria (transmitted by by dogs), malaria (transmitted by mosquitoes), sleeping sickness mosquitoes), sleeping sickness (transmitted by tsetse flies) and yellow (transmitted by tsetse flies) and yellow fever (transmitted by mosquitoes).fever (transmitted by mosquitoes).

EntryEntry

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

• These are termed extracellular pathogens. These are termed extracellular pathogens.

• Intracellular pathogens enter the cell. Entry is Intracellular pathogens enter the cell. Entry is achieved in different ways:achieved in different ways:– Some bacteria secrete invasin enzymes, which Some bacteria secrete invasin enzymes, which

digest the cell membrane of the host cell, allowing digest the cell membrane of the host cell, allowing the bacterial cell to enter it.the bacterial cell to enter it.

– Sometimes the specific adherence described Sometimes the specific adherence described above stimulates endocytosis by the host cell, above stimulates endocytosis by the host cell, which brings the microbe cell inside the host cell. which brings the microbe cell inside the host cell. The host cell may then try to digest the microbe The host cell may then try to digest the microbe cell, but if the bacterium has a capsule it may be cell, but if the bacterium has a capsule it may be protected.protected.

• By reproducing inside host cells. Viruses, By reproducing inside host cells. Viruses, bacteria and protoctists can all colonise bacteria and protoctists can all colonise living cells and reproduce inside them, using living cells and reproduce inside them, using up cellular resources and preventing the cell up cellular resources and preventing the cell from carrying out its normal reactions. from carrying out its normal reactions.

• The microbes then usually burst out of the The microbes then usually burst out of the host cell, rupturing the cell membrane and host cell, rupturing the cell membrane and killing the cell in the process.killing the cell in the process.

• Which bacterium is the main cause of Which bacterium is the main cause of diarrhoeadiarrhoea??

• How is it spread?How is it spread?• What do the toxins cause?What do the toxins cause?• How does ORT help patients suffering from How does ORT help patients suffering from

diarrhoea?diarrhoea?• What does the exotoxin secreted by the What does the exotoxin secreted by the

diptheria bacterium do?diptheria bacterium do?• The bacterial disease tetanus causes muscle The bacterial disease tetanus causes muscle

spasms. How?spasms. How?• Exotoxins are detected by the body as foreign Exotoxins are detected by the body as foreign

proteins. How is this effect exploited?proteins. How is this effect exploited?• What effect do endotoxins have on the body?What effect do endotoxins have on the body?