Microbes And Disease


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Microbes and Disease is a teaching resource for secondary school science - produced by the society for general microbiology

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Microbes And Disease

  1. 1. Microbes and Disease Human pathogens
  2. 2. Part 1 INTRODUCTION
  3. 3. What are micro-organisms? <ul><li>Micro-organisms, or microbes for short, are very small living creatures. </li></ul>Bacteria (orange) on the tip of a pin. Magnification x1600 when the image is printed 10 cm high. The tip of the pin is approximately 20 µm across and the bacteria are around 5 µm long. <ul><li>Most of them cannot be seen without using a microscope. </li></ul>
  4. 4. <ul><li>They are classified into 5 different groups </li></ul>What are micro-organisms? Algae Protozoa Bacteria Fungi Viruses Illustration of microbes
  5. 5. What are micro-organisms? What does the word ‘microbe’ mean to you? To most people microbe only means……. <ul><li>Only a small number cause disease, many more are helpful. </li></ul><ul><li>Microbes play a key role in maintaining life on earth, fixing gases and breaking down dead plant and animal matter into simpler substances that are used at the beginning of the food chain. </li></ul><ul><li>Their activity is exploited for the production of medicines, food and enzymes. </li></ul><ul><li>They are used to breakdown sewage and other wastes. </li></ul>INFECTION But….
  6. 6. Infection and disease <ul><li>What is a pathogen? </li></ul><ul><li>A pathogen is a micro-organism that has the potential to cause disease. </li></ul><ul><li>What is an infection? </li></ul><ul><li>An infection is the invasion and multiplication of pathogenic microbes in an individual or population. </li></ul><ul><li>What is disease? </li></ul><ul><li>Disease is when the infection causes damage to the individual’s vital functions or systems. </li></ul>An infection does not always result in disease!
  7. 7. How do microbes reach us? <ul><li>The cycle of transmission involves </li></ul><ul><li>Escape from the host or reservoir of infection. </li></ul><ul><li>Transport to the new host. </li></ul><ul><li>Entry to the new host. </li></ul><ul><li>Escape from the new host. </li></ul>Source /reservoir of infection Mode of transmission HUMAN HOST represents the various barriers to infection e.g. the skin represents the target organ e . g . lungs Portal of entry Portal of exit
  9. 9. Mode of transmission Microbes can be transmitted by: Vertical • Placenta • Breast milk Vehicle borne • Air/dust • Food • Water • Fomites Vector borne • Internal • External Direct contact – person to person Horizontal • Kissing • Sexual intercourse • Sneezing less than 1 metre • Touching Indirect contact
  10. 10. Mode of transmission Direct contact – p erson to person Example: cold sore <ul><li>Horizontal – kissing </li></ul><ul><li>Herpes simplex virus causes cold sores. </li></ul><ul><li>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. </li></ul>A couple kissing. Cold sores on lip and mouth.
  11. 11. Mode of transmission Direct contact – person to person Example: cold sore <ul><li>The virus particles invade the cells of the skin around the mouth and enter the nerve tissue where they lie dormant until something triggers their reactivation. </li></ul><ul><li>Common reasons for the virus becoming reactivated are tiredness, illness, stress and sunlight. </li></ul>Herpes simplex virus budding from the surface of a cell.
  12. 12. Mode of transmission Direct contact – person to person Example: syphilis <ul><li>Horizontal – sexual intercourse </li></ul><ul><li>The bacterium Treponema pallidum causes s yphilis. </li></ul><ul><li>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. </li></ul>Interlocking gender symbols representing sexually transmitted diseases.
  13. 13. Mode of transmission Direct contact – person to person Example: syphilis <ul><li>The infection is divided into three stages : </li></ul><ul><li>Primary stage </li></ul><ul><li>Hard painless sores appear at the site o f infection. </li></ul><ul><li>Secondary stage </li></ul><ul><li>A rash may appear followed by ‘ fl u ­ like symptoms. </li></ul><ul><li>If untreated the infection will progress to the dormant period. </li></ul><ul><li>Tertiary stage </li></ul><ul><li>Permanent damage occurs to the various parts of the body particularly the cardiovascular and nervous systems. </li></ul>The bacteria that cause syphilis. Secondary syphilis rash.
  14. 14. Mode of transmission Direct contact – person to person Example: SARS <ul><li>Horizontal – sneezing closer than 1 metre </li></ul><ul><li>SARS - associated coronavirus causes severe acute respiratory syndrome (SARS). </li></ul><ul><li>SARS is transmitted when an infected person coughs or sneezes infectious droplets onto a nearby person . </li></ul><ul><li>T he droplets land on another person ’ s face or hands, and become introduced to the nose or mouth. </li></ul>Jets of droplets erupt from a man’s nose as he sneezes.
  15. 15. Mode of transmission Direct contact – person to person Example: SARS <ul><li>The main symptoms of SARS are a high fever >38.0°C, dry cough and breathing difficulties. </li></ul><ul><li>Other symptoms may include headaches, loss of appetite and body aches. </li></ul><ul><li>About 10 -20% of patients have diarrhoea. </li></ul><ul><li>Most patients develop pneumonia. </li></ul>Routes of infection for SARS virus (spiky balls).
  16. 16. Mode of transmission Direct contact – person to person Example: gastroenteritis <ul><li>Horizontal – touching (faecal-oral route) </li></ul><ul><li>Norwalk virus causes a type of gastroenteritis. </li></ul><ul><li>Norwalk viru s is found in the faeces or vomit of infected people. It is highly contagious. </li></ul><ul><li>Infection occurs by having direct contact with another person who is infected and not maintaining good hygiene e.g. washing hands . </li></ul>Human hand contaminated with colonies of bacteria (blue/pink patches).
  17. 17. Mode of transmission Direct contact – person to person Example: gastroenteritis <ul><li>It starts with an attack of vomiting that can go up to 1 metre in distance. </li></ul><ul><li>Other symptoms include nausea, diarrh o ea, and some stomach cramping. </li></ul><ul><li>Some people also have a fever, chills, headache and muscle aches . </li></ul><ul><li>S ymptoms last only about 1 or 2 days. </li></ul>Norwalk virus particles.
  18. 18. Mode of transmission Direct contact – person to person Example: German measles <ul><li>Rubella virus causes German measles. </li></ul><ul><li>When infection occurs during pregnancy the virus crosses the placenta in the blood leading to infection of the fetus . </li></ul><ul><li>The virus can affect all the organs of the developing fetus . </li></ul><ul><li>T he risk to the baby is highest in the first 3 months – up to 85% of babies are affected if infected during this period. </li></ul>Eight week old fetus attached to its placenta by the umbilical cord. Vertical across the placenta or via breast milk
  19. 19. Mode of transmission Direct contact – person to person Example: German measles <ul><li>Congenital rubella syndrome is the name given to a group of defects that occur in a child when infected as a fetus. </li></ul><ul><li>Defects are </li></ul><ul><ul><ul><ul><li>deafness (most common) </li></ul></ul></ul></ul><ul><ul><ul><ul><li>eye problems such as cataracts </li></ul></ul></ul></ul><ul><ul><ul><ul><li>heart disease </li></ul></ul></ul></ul><ul><ul><ul><ul><li>impaired mental development </li></ul></ul></ul></ul><ul><ul><ul><ul><li>bone deformities </li></ul></ul></ul></ul><ul><ul><ul><ul><li>liver damage </li></ul></ul></ul></ul><ul><li>The number of cases has dropped significantly due to the introduction of rubella vaccine which is offerd to all children as part of the MMR jab. </li></ul>
  20. 20. Mode of transmission Indirect contact – vehicle borne Example: tuberculosis (TB) <ul><li>Air/dust </li></ul><ul><li>The bacterium Mycobacterium tuberculosis causes tuberculosis (TB). </li></ul><ul><li>TB is spread from person to person through the air. </li></ul><ul><li>When a person with active TB coughs or sneezes, droplets loaded with the infectious organism are propelled into the air. </li></ul><ul><li>The moisture evaporates from these particles to leave droplet nuclei that can remain airborne for days and spread long distances. </li></ul><ul><li>The Mycobacterium has a waxy coat, which protects it from drying out allowing it to survive for many months in the air and dust. </li></ul>
  21. 21. Mode of transmission Indirect contact – vehicle borne Example: tuberculosis (TB) <ul><li>a bad cough that is worse in the morning </li></ul><ul><li>chest pain </li></ul><ul><li>greenish or bloody sputum </li></ul><ul><li>weakness or fatigue </li></ul><ul><li>weight loss </li></ul><ul><li>night sweats </li></ul><ul><li>chills </li></ul><ul><li>fever </li></ul>A person with active TB will have the following symptoms that get more severe over time Mycobacterium tuberculosis bacteria.
  22. 22. Mode of transmission Indirect contact – vehicle borne Example: food poisoning <ul><li>Via food </li></ul><ul><li>The b acteriu m Campylobacter jejuni causes a type of food poisoning. </li></ul><ul><li>C. jejuni lives in the gut of many warm-blooded animals , particularly chickens , as part of their normal body flora. </li></ul><ul><li>The infection is transmitted to humans by eating contaminated food especially poultry and milk. </li></ul>Campylobacter jejuni bacterium.
  23. 23. Mode of transmission Indirect contact – vehicle borne Example: food poisoning <ul><li>C. jejuni is responsible for the majority of cases of dia rr h oe a in humans . </li></ul><ul><li>The number of cases of C ampylobacter infections is rising. </li></ul><ul><li>Only a small number of bacteria is required to cause infection. </li></ul><ul><li>The major symptoms are diarrh oe a, stomach cramps, fever and nausea. </li></ul><ul><li>The symptoms can range from very mild, where there is little sign of illness, through to bloody diarrh oe a and severe stomach cramps. </li></ul>
  24. 24. Mode of transmission Indirect contact – vehicle borne Example: c ryptosporidiosis <ul><li>The parasitic protozoan called Cryptosporidium parvum causes a gut infection called c ryptosporidiosis . </li></ul>Via water <ul><li>The infective stage, the oocyst , (spore) is excreted in the faeces of infected humans or animals. </li></ul><ul><li>It is spread by drinking contaminated water. </li></ul><ul><li>C. parvum has a complex life cycle, which it completes in one host , in this case the human. </li></ul>
  25. 25. Mode of transmission Indirect contact – vehicle borne Example: c ryptosporidiosis <ul><li>Symptoms usually appear within 2-10 days after eating the oocysts. </li></ul><ul><li>They include frequent, watery diarrh o ea, stomach cramps, nausea and vomiting. </li></ul><ul><li>Symptoms usually last from 1-2 weeks and people who have a healthy immune system will recover without treatment. </li></ul><ul><li>P eople who are immunocompromised e.g. have HIV , have a much more severe illness that can become life-threatening. They can produce up to 20 litres/day of diarrh oe a. </li></ul>Intestinal surface (orange) infected with C. parvum (round).
  26. 26. Mode of transmission Indirect contact – vehicle borne Example: athlete’s foot <ul><li>The fungus Trichophyton that causes athlete’s foot can be spread indirectly through towels, changing room floors etc. </li></ul><ul><li>The fungus thrives in the damp warm environment found between the toes. </li></ul><ul><li>The skin between the fourth and fifth toe is usually affected first. A flaky itchy red rash develops. The skin becomes cracked and sore and small blisters may appear. </li></ul><ul><li>If the infection is left untreated it can spread to other parts of the body. </li></ul>Fomite - a non-living object that can carry disease-causing organisms. Close-up of athlete’s foot infection.
  27. 27. Mode of transmission Indirect contact – vector borne Example: malaria <ul><li>Internal – biological </li></ul><ul><li>Malaria is a vector-borne disease caused by a single celled protozoan parasite called Plasmodium , which is transmitted by mosquitoes. </li></ul><ul><li>The primary vector for malaria is the mosquito Anopheles gambiae . </li></ul><ul><li>Only female mosquitoes transmit malaria when they feed on the human host’s blood. </li></ul>A. gambiae feeding on human blood.
  28. 28. Mode of transmission Indirect contact – vector borne Example: malaria <ul><li>The Plasmodium parasite has a complex life cycle involving the mosquito and the liver and red blood cells of humans. </li></ul><ul><li>Symptoms of the disease appear when the parasite bursts out of the red blood cells. </li></ul><ul><li>They include cycles of chills followed by high fever and sweats. </li></ul><ul><li>An ae mia and jaundice can occur due to the destruction of the red blood cells and enlargement of the liver and spleen . </li></ul>The passage of malaria through the human body.
  29. 29. Mode of transmission Indirect contact – vector borne Example: bacterial dysentery <ul><li>The bacterium Shigella causes a type bacterial dysentery . </li></ul><ul><li>Flies can spread Shigella when they carry infected faecal matter on their feet to drinking water or food. </li></ul><ul><li>Symptoms can vary from mild diarrhoea through to a more severe disease with watery or bloody diarrhoea, fever, stomach cramps and vomiting . </li></ul>External – mechanical Common house flies feeding.
  30. 30. Part 3 HOW DO MICROBES GET IN?
  31. 31. Portals of entry <ul><li>To cause an infection , microbes must enter our bodies. </li></ul><ul><li>The site at which they enter is known as the portal of entry . </li></ul><ul><li>Microbes can enter the body through the four sites listed below </li></ul><ul><ul><ul><li>Respiratory tract (mouth and nose) e.g. Influenza virus </li></ul></ul></ul><ul><ul><ul><li>Gastrointestinal tract (mouth oral cavity) e.g. Vibrio cholerae </li></ul></ul></ul><ul><ul><ul><li>Urogenital tract e.g. Escherichia coli </li></ul></ul></ul><ul><ul><ul><li>Break s in the skin surface e.g. Clostridium tetani </li></ul></ul></ul>
  32. 32. Portals of entry Respiratory tract Example: influenza <ul><li>Influenza or ‘flu is a highly infectious respiratory tract infection. </li></ul><ul><li>It is caused by a virus. </li></ul><ul><li>Virus is inhaled into the lungs through the mouth and nose. </li></ul><ul><li>The envelope of the virus has around 500 spikes sticking out of it. </li></ul><ul><li>Spikes attach to the cells lining the lungs . T hese help the virus get into the cell. </li></ul>The respiratory tract.
  33. 33. Portals of entry Respiratory tract Example: influenza <ul><li>Inside the cell the virus replicates to produce new virus particles. </li></ul><ul><li>The host cell is destroyed as the virus particles leave it. </li></ul><ul><li>Damage to the cells lining the lungs cause s the lining to become inflamed and irritated. </li></ul><ul><li>Other symptoms include fever about 39  C, aching limbs and a headache. </li></ul>Influenza virus particles.
  34. 34. Portals of entry Gastrointestinal tract Example: cholera <ul><li>Cholera is an acute infection of the intestinal tract. </li></ul><ul><li>It is caused by the bacterium Vibrio cholerae . </li></ul><ul><li>The infection is spread by contaminated water and food , especially seafood, or from one infected person to another via the faecal-oral route. </li></ul><ul><li>The incubation period is 24-72 hours. </li></ul>The gastrointestinal tract.
  35. 35. Portals of entry Gastrointestinal tract Example: cholera <ul><li>The bacteria stick to the cells that line the intestines and release a toxin (poison). The toxin alters the normal process for the absorption of water. </li></ul><ul><li>W ater flows , by osmosis, in the wrong direction, from the cells lining the intestines into the gut. </li></ul><ul><li>The main symptom of the disease is diarrhoea. This can be mild through to severe watery dia rr hoea often ‘rice water’ in appearance. </li></ul><ul><li>Large amounts of water can be lost – between 15-20 litres. </li></ul>Vibrio cholerae bacteria.
  36. 36. <ul><li>Cystitis is the commonest infection of the lower urinary tract. </li></ul><ul><li>Strains of the bacterium Escherichia coli , a normal inhabitant of the human intestine, are responsible for 80% of cases of cystitis. </li></ul><ul><li>B acteria enter the urethra and travel up to the bladder. </li></ul><ul><li>It is more common in women than in men . </li></ul><ul><li>In women the anus and the opening of the urethra are closer to each other . </li></ul><ul><li>The urethra is shorter in women so the bacteria have a shorter distance to travel to the bladder . </li></ul>Portals of entry Urogenital tract Example: cystitis Artwork of an inflamed bladder (red) caused by cystitis.
  37. 37. Portals of entry Urogenital tract Example: cystitis <ul><li>Symptoms may include any of the following </li></ul><ul><ul><ul><li>burning/stinging sensation when urinating </li></ul></ul></ul><ul><ul><ul><li>the urgent need to frequently pass small amounts of urine </li></ul></ul></ul><ul><ul><ul><li>blood in the urine </li></ul></ul></ul><ul><ul><ul><li>lower back pain </li></ul></ul></ul><ul><ul><ul><li>mild fever and chills </li></ul></ul></ul><ul><li>Cystitis can be treated with a short course of antibiotics. </li></ul>Bacterial infection of the bladder. E. coli bacteria (yellow) on swollen epithelial cells (blue) lining the bladder.
  38. 38. Portals of entry Urogenital tract Example: Chlamydial infection <ul><li>Chlamydial infection is the most common sexually transmitted bacterial infection in the world. </li></ul><ul><li>It is caused by Chlamydia trachomatis . </li></ul><ul><li>About 1 in 10 young people have Chlamydia . </li></ul><ul><li>It can’t be caught from kissing, sharing towels or toilet seats. </li></ul><ul><li>Chlamydial infection is often known as the silent disease as approximately 75% of women and 50% of men don’t experience any symptoms. </li></ul>C. trachomatis bacteria (background) with the female reproductive tract superimposed.
  39. 39. Portals of entry Urogenital tract Example: Chlamydial infection <ul><li>Symptoms are usually mild and include </li></ul><ul><li>In women </li></ul><ul><li>Vaginal discharge , a bdominal pain , b urning sensation when urinating and b leeding between periods . </li></ul><ul><li>In m en </li></ul><ul><li>Discharge from and i tching around the penis , b urning sensation when urinating . </li></ul><ul><li>Chlamydial infection can be treated and cured with antibiotics. </li></ul><ul><li>In women, if left untreated, the infection can spread from the cervix to the fallopian tubes , damaging the reproductive organs . </li></ul>
  40. 40. Portals of entry Breaking the surface of the skin Example: tetanus <ul><li>Tetanus is commonly known as lockjaw; it is a neuromuscular disease. </li></ul><ul><li>It is caused by a toxin (poison), which is produced by the bacterium Clostridium tetani. </li></ul><ul><li>C. tetani is found in soil, dust and the guts and faeces of many animals. </li></ul><ul><li>C . tetani produces endospores. </li></ul><ul><li>The endospores usually enter the body through a puncture wound to the skin. </li></ul>Splinter in the finger.
  41. 41. Portals of entry Breaking the surface of the skin Example: tetanus <ul><li>The toxin causes muscles to contact uncontrollably , leading to symptoms such as c lenching of the jaw and a rching of the back. </li></ul><ul><li>Death is usually due to paralysis of the muscles that control breathing. </li></ul><ul><li>Death rates can be up to 50%. </li></ul><ul><li>Tetanus can be treated using an antitoxin. </li></ul><ul><li>The disease is preventable through vaccination. </li></ul>Clostridium tetani bacteria.
  42. 42. Part 4 DEFENCE
  43. 43. Defending ourselves against microbes <ul><li>Microbes are found everywhere in the soil, water and air and on the skin and lining of our digestive tracts. </li></ul><ul><ul><li>Why aren’t we continually affected by microbes? </li></ul></ul><ul><ul><li>How do we stop them invading our internal organs and bloodstream? </li></ul></ul>T he body is protected by a complex system of defences that prevent assault from pathogeni c microbes.
  44. 44. Defending ourselves against microbes <ul><li>The bod y’s defences : </li></ul><ul><ul><ul><li>Prevent microbes getting into the body </li></ul></ul></ul><ul><ul><ul><li>Destroy microbes once they have got in </li></ul></ul></ul><ul><li>The body has three lines of defence against invading micro-organisms: </li></ul><ul><ul><ul><li>Non specific physical and chemical barriers </li></ul></ul></ul><ul><ul><ul><li>Non specific immune system </li></ul></ul></ul><ul><ul><ul><li>Specific i mmune system </li></ul></ul></ul>
  45. 45. Non specific physical barriers <ul><li>Physical barriers include : </li></ul><ul><li>Intact skin: The cells in the outer layer of the skin contain a protein called keratin. The keratin fibres make the cells tough and virtually impermeable to microbes. </li></ul><ul><li>Cilia: The ciliary escalator propels trapped particles out of the respiratory tract. </li></ul>Cilia in the trachea rhythmically beating.
  46. 46. Non specific physical barriers <ul><li>Physical barriers include : </li></ul><ul><li>Normal flora of the body present on the skin, the lining of the digestive tract and the vagina. </li></ul><ul><li>The normal flora compete with potential pathogens for sites on our bodies and also nutrients. </li></ul><ul><li>The y may also produce chemicals, which create unfavourable conditions for pathogens. </li></ul>E. coli bacteria (yellow) in the gut are part of the normal intestinal flora of humans.
  47. 47. Non specific chemical barriers <ul><li>Chemical barriers include : </li></ul><ul><li>S ebum is produce d by the sebaceous glands . It has antibacterial properties . </li></ul><ul><li>Acidic pH of gastric secretions is low enough to kill most microbes entering the body. </li></ul><ul><li>Lysozyme is an enzyme found in saliva and tears. It works by breaking down bacterial cell walls , causing the bacteria to burst and die. </li></ul>The honeycombed sebaceous gland (light brown) produces sebum.
  48. 48. Non specific immune system <ul><li>The n on specific immune system is activated when microbes invade the body. </li></ul><ul><li>It is called ‘non specific’ as the response is the same for all pathogens. </li></ul><ul><li>Phagocyte </li></ul><ul><li>(phago = &quot;eating&quot;, cyte = &quot;cell&quot;) </li></ul><ul><li>a type of white blood cell </li></ul><ul><li>that carries out phagocytosis. </li></ul><ul><li>Phagocyte s ingest and digest invading microbes. </li></ul>Phagocytosis, a phagocyte (blue) engulfing a yeast cell (yellow).
  49. 49. Specific immune system <ul><li>The s pecific immune system is activated when microbes invade the body. </li></ul><ul><li>A specific response occurs when the immune system recognizes an antigen that does not belong in the body and then prepares a specific reaction to it , an antibody . </li></ul>Two antibodies bound to an antigen.
  50. 50. Specific immune system <ul><li>Antigen </li></ul><ul><li>Antigens (usually proteins) are structures found on the surface of every cell. </li></ul><ul><li>The antigens on the surface of microbes are different to the antigens found on the surface of our cells. </li></ul><ul><li>Th e antigens allow the body to recognize invading microbes as a foreign substance ‘non self’ and stimulate an immune response. </li></ul>Antigens on the surface of a microbe.
  51. 51. Specific immune system <ul><li>Antibody </li></ul><ul><li>An antibody is a protein that is produced by lymphocytes (type of white blood cell) in response to the presence of a specific antigen. </li></ul><ul><li>Specific antibodies bind to specific antigens and cause their destruction. </li></ul>Antibody
  52. 52. Specific immune system <ul><li>What is natural immunity? </li></ul><ul><li>Once a person has had a disease they don’t normally catch it again because the antibodies stay in the body and remember the microbe which caused the disease. </li></ul><ul><li>If the person comes under attack from the same microbe the antibodies will recognize and destroy it. </li></ul>The person is protected from the disease through this natural immunity
  53. 53. Vaccination <ul><li>You can become immune to a disease through vaccination. </li></ul><ul><li>  </li></ul><ul><li>Immunization programmes and the development of new vaccines play an important role in protecting individuals against illness . </li></ul><ul><li>Vaccination works by safely exposing individuals to a specific pathogenic microbe, artificially increasing their immunity to it. </li></ul>Vaccination
  54. 54. Vaccination <ul><li>Vaccines are made from: </li></ul><ul><li>L ive micro-organisms that have been ‘treated’ so that they are weakened (attenuated) and are unable to cause disease. </li></ul><ul><li>D ead micro-organisms. </li></ul><ul><li>S ome part or product of the micro-organism that can produce an immune response. </li></ul>Vaccine production.
  55. 55. Part 5 TREATMENT
  56. 56. What are antibiotics? <ul><li>Antibiotics are chemical compounds produced by soil fungi and bacteria. </li></ul><ul><li>They are used to treat bacterial infections. </li></ul><ul><li>Alexander Fleming accidentally discovered penicillin, the first antibiotic, in 1928. </li></ul><ul><li>He isolated it from the mould Penicillium notatum and found it prevented the growth of bacteria. </li></ul><ul><li>Penicillin was not available for commercial use until Florey and Chain purified it in 1940. </li></ul><ul><li>The 1940 ’ s saw the mass production of penicillin . </li></ul>Antibiotic drugs (discs) prevent the growth of bacteria (white) demonstrated by clear zones around the discs.
  57. 57. How do antibiotics work? <ul><li>Antibacterials exploit the difference between the prokaryotic bacterial cell and the host’s eukaryotic cell. </li></ul><ul><li>They work by being either : </li></ul><ul><ul><ul><li>bacteriostatic, preventing cells from multiplying so that the bacterial population remains static, allowing the host’s defence mechanism to fight the infection . </li></ul></ul></ul><ul><ul><ul><li>bactericidal, by killing the bacteria . </li></ul></ul></ul>Antibiotics acting on bacteria causing them to expand and burst.
  58. 58. Emergence of drug-resistant bacteria <ul><li>Micro-organisms are termed drug-resistant when they are no longer inhibited by an anti biotic to which they were previously sensitive. </li></ul><ul><li>In the late 1940’s, only 4 years after mass treatment with penicillin had been introduced, a strain of the bacterium Staphylococcus aureus was shown to be resistant to this drug. </li></ul><ul><li>The emergence and spread of antibacterial-resistant micro-organisms has continued to grow due to the over use and misuse of antibiotics. </li></ul><ul><li>Methicillin-resistant Staphylococcus a ureus (MRSA) is the most infamous of the resistant bugs. </li></ul>
  59. 59. Why has antibiotic resistance occurred? <ul><li>Factors leading to microbes becoming resistan t to antibiotics include : </li></ul><ul><li>Pressure on d octors, by patients, to prescrib e antibiotics even when they are not needed. </li></ul><ul><li>Patients being prescribed antibiotics without the doctor knowing the cause of the infection. </li></ul><ul><li>Use of antibiotics in animals for growth promotion and prophylaxis, which allows them to enter the human food chain. </li></ul>MRSA
  60. 60. Part 6 HOW MICROBES LEAVE
  61. 61. How do microbes leave our body? <ul><li>Pathogens leave the body through portals of exit. </li></ul><ul><li>The portal of exit is usually the same as the portal of entry. </li></ul><ul><li>The pathogen leaves the host in : </li></ul><ul><ul><ul><li>excretions such as faeces and urine </li></ul></ul></ul><ul><ul><ul><li>secretions such as saliva </li></ul></ul></ul><ul><ul><ul><li>discharges such as pus and skin scales </li></ul></ul></ul><ul><ul><ul><li>in blood via puncture wounds </li></ul></ul></ul>Pus from an infected ear.
  62. 62. Conclusion <ul><li>A pathogen is a micro-organism that has the potential to cause disease </li></ul><ul><li>An infection does not always result in disease </li></ul><ul><li>The cycle of transmission involves </li></ul><ul><ul><ul><li>Escape from the host or reservoir of infection </li></ul></ul></ul><ul><ul><ul><li>Transport to the new host </li></ul></ul></ul><ul><ul><ul><li>Entry to the new host </li></ul></ul></ul><ul><ul><ul><li>Escape from the new host </li></ul></ul></ul><ul><li>Microbes can be transmitted by </li></ul><ul><ul><ul><li>Direct contact – person to person </li></ul></ul></ul><ul><ul><ul><li>Indirect contact </li></ul></ul></ul><ul><li>The body has three lines of defence against invading micro-organisms </li></ul><ul><li>The se defences : </li></ul><ul><ul><ul><li>Prevent microbes getting into the body </li></ul></ul></ul><ul><ul><ul><li>Destroy microbes once they have got in </li></ul></ul></ul>
  63. 63. Microbes and Disease