Microbes And Disease

Microbes and Disease Human pathogens

What are micro-organisms? Micro-organisms, or microbes for short, are very small living creatures. 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. Most of them cannot be seen without using a microscope.

They are classified into 5 different groups What are micro-organisms? Algae Protozoa Bacteria Fungi Viruses Illustration of microbes

What are micro-organisms? What does the word ‘microbe’ mean to you? To most people microbe only means……. Only a small number cause disease, many more are helpful. 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. Their activity is exploited for the production of medicines, food and enzymes. They are used to breakdown sewage and other wastes. INFECTION But….

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 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

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

Mode of transmission Direct contact – p erson to person Example: cold sore Horizontal – kissing 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: cold sore 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. Common reasons for the virus becoming reactivated are tiredness, illness, stress and sunlight. Herpes simplex virus budding from the surface of a cell.

Mode of transmission Direct contact – person to person Example: syphilis Horizontal – sexual intercourse The bacterium Treponema pallidum causes s yphilis. 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: syphilis The infection is divided into three stages : Primary stage Hard painless sores appear at the site o f infection. Secondary stage A rash may appear followed by ‘ fl u like symptoms. If untreated the infection will progress to the dormant period. Tertiary stage Permanent damage occurs to the various parts of the body particularly the cardiovascular and nervous systems. The bacteria that cause syphilis. Secondary syphilis rash.

Mode of transmission Direct contact – person to person Example: SARS Horizontal – 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 . T he 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 Direct contact – person to person Example: SARS The main symptoms of SARS are a high fever >38.0°C, dry cough and breathing difficulties. Other symptoms may include headaches, loss of appetite and body aches. About 10 -20% of patients have diarrhoea. Most patients develop pneumonia. Routes of infection for SARS virus (spiky balls).

Mode of transmission Direct contact – person to person Example: gastroenteritis Horizontal – touching (faecal-oral route) Norwalk virus causes a type of gastroenteritis. Norwalk viru s 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 Direct contact – person to person Example: gastroenteritis It starts with an attack of vomiting that can go up to 1 metre in distance. Other symptoms include nausea, diarrh o ea, and some stomach cramping. Some people also have a fever, chills, headache and muscle aches . S ymptoms last only about 1 or 2 days. Norwalk virus particles.

Mode of transmission Direct contact – person to person Example: German measles 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 . 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. Eight week old fetus attached to its placenta by the umbilical cord. Vertical across the placenta or via breast milk

Mode of transmission Direct contact – person to person Example: German measles Congenital rubella syndrome is the name given to a group of defects that occur in a child when infected as a fetus. Defects are deafness (most common) eye problems such as cataracts heart disease impaired mental development bone deformities liver damage 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.

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.

Mode of transmission Indirect contact – vehicle borne Example: tuberculosis (TB) a bad cough that is worse in the morning chest pain greenish or bloody sputum weakness or fatigue weight loss night sweats chills fever A person with active TB will have the following symptoms that get more severe over time Mycobacterium tuberculosis bacteria.

Mode of transmission Indirect contact – vehicle borne Example: food poisoning Via food The b acteriu m 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 Indirect contact – vehicle borne Example: food poisoning C. jejuni is responsible for the majority of cases of dia rr h oe a in humans . The number of cases of C ampylobacter infections is rising. Only a small number of bacteria is required to cause infection. The major symptoms are diarrh oe a, stomach cramps, fever and nausea. The symptoms can range from very mild, where there is little sign of illness, through to bloody diarrh oe a and severe stomach cramps.

Mode of transmission Indirect contact – vehicle borne Example: c ryptosporidiosis The parasitic protozoan called Cryptosporidium parvum causes a gut infection called c ryptosporidiosis . Via water The infective stage, the oocyst , (spore) is excreted in the faeces of infected humans or animals. It is spread by drinking contaminated water. C. parvum has a complex life cycle, which it completes in one host , in this case the human.

Mode of transmission Indirect contact – vehicle borne Example: c ryptosporidiosis Symptoms usually appear within 2-10 days after eating the oocysts. They include frequent, watery diarrh o ea, stomach cramps, nausea and vomiting. Symptoms usually last from 1-2 weeks and people who have a healthy immune system will recover without treatment. 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. Intestinal surface (orange) infected with C. parvum (round).

Mode of transmission Indirect contact – vehicle borne Example: athlete’s foot The fungus Trichophyton that causes athlete’s foot can be spread indirectly through towels, changing room floors etc. The fungus thrives in the damp warm environment found between the toes. 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. If the infection is left untreated it can spread to other parts of the body. Fomite - a non-living object that can carry disease-causing organisms. Close-up of athlete’s foot infection.

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: malaria The Plasmodium parasite has a complex life cycle involving the mosquito and the liver and red blood cells of humans. Symptoms of the disease appear when the parasite bursts out of the red blood cells. They include cycles of chills followed by high fever and sweats. An ae mia and jaundice can occur due to the destruction of the red blood cells and enlargement of the liver and spleen . The passage of malaria through the human body.

Mode of transmission Indirect contact – vector borne Example: bacterial dysentery 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 . External – mechanical Common house flies feeding.

Part 3 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 tract (mouth oral cavity) e.g. Vibrio cholerae Urogenital tract e.g. Escherichia coli Break s 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 . T hese help the virus get into the cell. The respiratory tract.

Portals of entry Respiratory tract Example: influenza Inside the cell the virus replicates to produce new virus particles. The host cell is destroyed as the virus particles leave it. Damage to the cells lining the lungs cause s the lining to become inflamed and irritated. Other symptoms include fever about 39  C, aching limbs and a headache. Influenza virus particles.

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 Gastrointestinal tract Example: cholera 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. W ater flows , by osmosis, in the wrong direction, from the cells lining the intestines into the gut. The main symptom of the disease is diarrhoea. This can be mild through to severe watery dia rr hoea often ‘rice water’ in appearance. Large amounts of water can be lost – between 15-20 litres. Vibrio cholerae bacteria.

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. B acteria 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 . Portals of entry Urogenital tract Example: cystitis Artwork of an inflamed bladder (red) caused by cystitis.

Portals of entry Urogenital tract Example: cystitis Symptoms may include any of the following burning/stinging sensation when urinating the urgent need to frequently pass small amounts of urine blood in the urine lower back pain mild fever and chills Cystitis can be treated with a short course of antibiotics. Bacterial infection of the bladder. E. coli bacteria (yellow) on swollen epithelial cells (blue) lining the bladder.

Portals of entry Urogenital tract Example: Chlamydial infection Chlamydial infection is the most common sexually transmitted bacterial infection in the world. 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. C. trachomatis bacteria (background) with the female reproductive tract superimposed.

Portals of entry Urogenital tract Example: Chlamydial infection Symptoms are usually mild and include In women Vaginal discharge , a bdominal pain , b urning sensation when urinating and b leeding between periods . In m en Discharge from and i tching around the penis , b urning sensation when urinating . Chlamydial infection can be treated and cured with antibiotics. In women, if left untreated, the infection can spread from the cervix to the fallopian tubes , damaging the reproductive organs .

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.

Portals of entry Breaking the surface of the skin Example: tetanus The toxin causes muscles to contact uncontrollably , leading to symptoms such as c lenching of the jaw and a rching of the back. Death is usually due to paralysis of the muscles that control breathing. Death rates can be up to 50%. Tetanus can be treated using an antitoxin. The disease is preventable through vaccination. Clostridium tetani bacteria.

Defending ourselves against microbes Microbes are found everywhere in the soil, water and air and on the skin and lining of our digestive tracts. Why aren’t we continually affected by microbes? How do we stop them invading our internal organs and bloodstream? T he body is protected by a complex system of defences that prevent assault from pathogeni c microbes.

Defending ourselves against microbes The bod y’s defences : Prevent microbes getting into the body Destroy microbes once they have got in The body has three lines of defence against invading micro-organisms: Non specific physical and chemical barriers Non specific immune system Specific i mmune system

Non specific physical barriers Physical barriers include : 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. Cilia: The ciliary escalator propels trapped particles out of the respiratory tract. Cilia in the trachea rhythmically beating.

Non specific physical barriers Physical barriers include : Normal flora of the body present on the skin, the lining of the digestive tract and the vagina. The normal flora compete with potential pathogens for sites on our bodies and also nutrients. The y may also produce chemicals, which create unfavourable conditions for pathogens. E. coli bacteria (yellow) in the gut are part of the normal intestinal flora of humans.

Non specific chemical barriers Chemical barriers include : S ebum is produce d by the sebaceous glands . It has antibacterial properties . Acidic pH of gastric secretions is low enough to kill most microbes entering the body. Lysozyme is an enzyme found in saliva and tears. It works by breaking down bacterial cell walls , causing the bacteria to burst and die. The honeycombed sebaceous gland (light brown) produces sebum.

Non specific immune system The n on specific immune system is activated when microbes invade the body. It is called ‘non specific’ as the response is the same for all pathogens. Phagocyte (phago = "eating", cyte = "cell") a type of white blood cell that carries out phagocytosis. Phagocyte s ingest and digest invading microbes. Phagocytosis, a phagocyte (blue) engulfing a yeast cell (yellow).

Specific immune system The s pecific immune system is activated when microbes invade the body. 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 . Two antibodies bound to an antigen.

Specific immune system Antigen Antigens (usually proteins) are structures found on the surface of every cell. The antigens on the surface of microbes are different to the antigens found on the surface of our cells. Th e antigens allow the body to recognize invading microbes as a foreign substance ‘non self’ and stimulate an immune response. Antigens on the surface of a microbe.

Specific immune system Antibody An antibody is a protein that is produced by lymphocytes (type of white blood cell) in response to the presence of a specific antigen. Specific antibodies bind to specific antigens and cause their destruction. Antibody

Specific immune system What is natural immunity? 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. If the person comes under attack from the same microbe the antibodies will recognize and destroy it. The person is protected from the disease through this natural immunity

Vaccination You can become immune to a disease through vaccination. Immunization programmes and the development of new vaccines play an important role in protecting individuals against illness . Vaccination works by safely exposing individuals to a specific pathogenic microbe, artificially increasing their immunity to it. Vaccination

Vaccination Vaccines are made from: L ive micro-organisms that have been ‘treated’ so that they are weakened (attenuated) and are unable to cause disease. D ead micro-organisms. S ome part or product of the micro-organism that can produce an immune response. Vaccine production.

What are antibiotics? Antibiotics are chemical compounds produced by soil fungi and bacteria. They are used to treat bacterial infections. Alexander Fleming accidentally discovered penicillin, the first antibiotic, in 1928. He isolated it from the mould Penicillium notatum and found it prevented the growth of bacteria. Penicillin was not available for commercial use until Florey and Chain purified it in 1940. The 1940 ’ s saw the mass production of penicillin . Antibiotic drugs (discs) prevent the growth of bacteria (white) demonstrated by clear zones around the discs.

How do antibiotics work? Antibacterials exploit the difference between the prokaryotic bacterial cell and the host’s eukaryotic cell. They work by being either : bacteriostatic, preventing cells from multiplying so that the bacterial population remains static, allowing the host’s defence mechanism to fight the infection . bactericidal, by killing the bacteria . Antibiotics acting on bacteria causing them to expand and burst.

Emergence of drug-resistant bacteria Micro-organisms are termed drug-resistant when they are no longer inhibited by an anti biotic to which they were previously sensitive. 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. The emergence and spread of antibacterial-resistant micro-organisms has continued to grow due to the over use and misuse of antibiotics. Methicillin-resistant Staphylococcus a ureus (MRSA) is the most infamous of the resistant bugs.

Why has antibiotic resistance occurred? Factors leading to microbes becoming resistan t to antibiotics include : Pressure on d octors, by patients, to prescrib e antibiotics even when they are not needed. Patients being prescribed antibiotics without the doctor knowing the cause of the infection. Use of antibiotics in animals for growth promotion and prophylaxis, which allows them to enter the human food chain. MRSA

How do microbes leave our body? Pathogens leave the body through portals of exit. The portal of exit is usually the same as the portal of entry. The pathogen leaves the host in : excretions such as faeces and urine secretions such as saliva discharges such as pus and skin scales in blood via puncture wounds Pus from an infected ear.

Conclusion A pathogen is a micro-organism that has the potential to cause disease An infection does not always result in disease 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 Microbes can be transmitted by Direct contact – person to person Indirect contact The body has three lines of defence against invading micro-organisms The se defences : Prevent microbes getting into the body Destroy microbes once they have got in

Microbes And Disease

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Microbes And Disease