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What are toxins?What are toxins?

y Toxins are chemicals released by

pathogens, in particular bacteria, that have

harmful effects on the body. The effects

are felt throughout the body as the toxinis transported through the body tissues

from bacteria present in other tissues.

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ExotoxinsExotoxins andand EndotoxinsEndotoxins

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ExotoxinsExotoxins

y Exotoxins are soluble compounds secreted by the bacteria intotheir environments; they interact with cells in the immune systemresulting in fevers and headaches.

y Both gram negative and gram positive bacteria produce exotoxins.Exotoxins may be secreted, or may be released during lysis of thecell. The majority of exotoxins can be destroyed by heating.

Exotoxins can be destroyed by antibodies produced by the immunesystem, but many exotoxins are so toxic that they may be fatalbefore the immune system can respond.

y There are three main types of exotoxins:

o toxins that act upon connective tissue - this allows the furtherspread of bacteria and therefore the infection into underlying

tissueso toxins that act as enzymes, enter cells and effect their metabolism

o membrane damaging toxins - these toxins are designed primarily topuncture the cell membrane killing cells.

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TetanusTetanus

y Tetanus is a medical condition characterised by asustained contraction of skeletal muscle, thesymptoms are caused by tetanospasmin, anexotoxin and neutrotoxin produced by the Gram

positive bacterium Clostridium tetani.y Infection generally occurs through wound

contamination, and often involves a cut or deeppuncture wound. As the infection progresses,

muscle spasms in the jaw develop, hence thecommon name, lockjaw. This is followed bydifficulty in swallowing and muscle stiffness andspasms.

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EndotoxinsEndotoxins

y Endotoxins are produced by Gram negativebacteria, for example, Salmonella. They are notreleased from the bacteria until the cell wall isdamaged or lysis occurs.

y Endotoxins need to be present in relatively largeamounts to have any effect, such as causing afever, inflammation or diaorrhoea. Endotoxins arelipopolysaccharides that form part of the outer

membrane of the Gram negative cell wall.y The only gram positive bacteria that produces

endotoxins is Listeria monocytogenes.

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Salmonella food poisoningSalmonella food poisoning

y Salmonella enteritidis, a pathogen that invades thegut and introduces a toxin there, causes a foodbased infection.

y The symptoms are a sudden onset of headaches,chills, vomiting and diarrhoea.At this stage, thetoxin is mostly still attached to the bacterial walls.

y This is followed by a fever that lasts a few days,and is caused when the dislodged toxin reaches

the blood circulation.y Even at this stage, the bacterium itself remains in

the lumen of the gut, without invading cells.

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F6.8: Discuss the origin andF6.8: Discuss the origin and

epidemiology of one example of aepidemiology of one example of a

pandemicpandemic

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y In any population of organisms there willalways be a small number of people suffering

from an infection caused by a micro-organism

y . An outbreak refers to a small number of people in a small area suffering from a

disease.y An epidemic occurs when a large number of

people in several communities suffer fromthe same infection.

y A pandemic occurs when very largenumbers of people in different countries allsuffer from the same infection.

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y According to the World Heath

Organisation (WHO), a pandemic maystart when the emergence of a disease or

disease strain new to the human

population, the pathogen infects humans,

causing serious illness and the agentspreads among humans.

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y Four strains of the influenza or flu virus

are known: A, B, C and D.Strains C and Dare stable, but new strains of A commonly

emerge.

y Epidemics of influenza occur on a regular

seasonal basis and pandemics also occur,

for example, in 1918 when the Spanish flu

(A2 strain), killed more than twenty

million people worldwide.

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y The influenza virus enters the respiratorysystem in infected droplets and then

attacks the epithelial lining of thebronchioles.

y The symptoms include fever, shivering,headaches, sore throats and a blockednose. Adults often have aches in the back and limbs. A cough often develops due todamage to the trachea and bronchioles.

y

Pathogenic bacteria often invade thedamaged air passages leading to bronchitisand pneumonia.

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y The most famous pandemic of recorded

history is the Black Death which was

introduced to Europe in the 1300·s from

Asia. A quarter of Europe·s population were

killed.

y It has been traditionally assumed that theBlack Death was an example of the bubonic

plague, caused by the bacterium Y ersina pestis

and spread by fleas on the black rat.

y However some recent research suggests

that the Black Death may have been caused

by an Ebola type virus.

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y Many Biologists consider HIV infection to

be a global pandemic. The WHO

estimates that 25 million people have died

since 1981 (when the HIV virus was firstidentified).

y 40 million people are estimated to be HIV

positive. Retroviral drugs prolong lifespans of HIV positive individuals, but

there is currently no cure or vaccine.

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y The World Health Organisation (WHO) isan agency of the United Nations (UN)concerned with international public health.Its major task is to combat infectiousdiseases, for example, Asian bird flu, malariaand AIDS, and to promote the health of theworld·s population.

y It also has programs to develop anddistribute vaccines and low cost HIV testkits.

y The WHO·s vaccination program resulted in

the eradication of small pox in 1979.Itcurrently aims to eradicate polio within the

next few years.

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