Infection and Response B3

Pathogen definition

• A microorganism that causes communicable (infectious) disease in plants or animals.

Four types of pathogens

• Bacteria, Viruses, Protists, and Fungi.

Bacteria and how they cause illness

• Single-celled organisms rapidly reproduce themselves inside the body and produce toxins that damage cells and tissues, making you feel ill.

Viruses and how they cause illness

A micro-organism that are not cells or not living and can’t reproduce themselves.

• Once inside host (another organism) cells, utilise cell machinery to reproduce rapidly until the cell bursts and is destroyed, and taken over another organism cell which damages our body/makes us ill.

Fungi and how they cause illness

Can be uni-cellular or multi-cellular ones that have long thread-like structures called hyphae. These spread through soil to penetrate surface of plant or human skin to cause disease.

Also hyphae produce spores that spread the infection easily and produce new fungi.

Protists and how they cause illness

Usually single-celled organisms, disease-causing protists are called parasites that live on or inside another organism harming it.

They are transported by vectors (other organisms like mosquitos) between different host organisms, but the vector doesn’t get the disease itself.

Pathogen spread: Direct contact

• Spreading through skin-to-skin touching, contaminated surfaces, or sexual contact (e.g. HIV, athlete's foot).

Pathogen spread: Water and Food

• Spreading by drinking or bathing in dirty, sewage-contaminated water (e.g. cholera) or eating contaminated food (e.g Salmonella)

Pathogen spread: Air

• Spreading through airborne droplets exhaled when coughing, sneezing, or talking (e.g. flu, common cold, COVID-19).

Type of viral disease: Measles

• Spread by inhalation of droplets when an infected person sneezes or coughs.

Symptoms include a red skin rash and a fever.

Can be serious/ fatal, so young children are vaccinated against it.

Type of viral disease: HIV (human immunodeficiency virus)

• Spread through sexual contact or exchange of bodily fluids like blood (e.g when sharing needles).

Initially causes a flu-like illness with fever, tiredness and aches.

Later virus attacks immune cells, leading to AIDS where the immune system fails being so damaged.

AIDs used to be final stage, now if antiretroviral drugs early on can prevent virus replicating and have normal life.

Type of Plant viral disease: Tobacco Mosaic Virus (TMV)

• Causes a distinctive 'mosaic' discoloration pattern on leaves.

Destroys chloroplasts, reducing photosynthesis and limiting plant growth e.g for tobacco or tomato plants.

Type of bacterial disease: Salmonella

• Food poisoning spread by consuming contaminated bacteria in unhygienic food.

Symptoms include fever, abdominal cramps, vomiting, and diarrhoea caused by bacterial toxins.

Type of bacterial disease: Gonorrhoea (STD)

• Spread by sexual contact.

Symptoms include a thick yellow or green discharge from the vagina or penis and pain when urinating.

Treated with antibiotics and prevented with condoms.

Type of Fungal Plant disease: Rose Black Spot

• Purple or black spots develop on leaves, which turn yellow after more damage and drop off. Reduces photosynthesis, causing less plant growth.

Spread by environment water or wind.

Treatment is to chop off infected leaves/ destroy them or spray plant with fungicide to kill fungus.

Type of protist disease: Malaria

• Caused by protist parasites (needs a host like human or other animal to survive). Spread by mosquitoes acting as vectors that feed on blood of an infected animal to pick up protist and bites next organism to inject it into their bloodstream.

Symptoms of recurrent episodes of severe fever/ headaches and can be fatal.

To prevent malaria stop vectors spreading the protists by reducing number of mosquitoes, destroying their breeding sites, killing with insecticides or stop them feeding on us with mosquito nets and repellents.

Stop pathogen spreading: Kill Vectors

• An organism that spreads the infection by transferring pathogens from one organism to another without catching the disease themselves (e.g. mosquitoes with malaria protists). For insects kill them with chemical insecticides.

Stop pathogen spreading: Hygiene and quarantine

Washing hands and cleaning cooking items

If people catch disease can isolate/ quarantine them when serious disease

Skin physical barrier defense

• Forms a physical outer barrier to lock out pathogens, and secretes oils/ antimicrobial substances to kill microorganisms.

Nose physical barrier defense

• Contains hairs and sticky mucus that physically trap dust particles and pathogens before they can enter.

Trachea/ Oesophagus & Bronchi physical barrier defense

• Lined with mucus to trap pathogens, and to prevent mucus build up they covered in hair-like cilia that move the mucus upwards to the throat to be swallowed.

Stomach chemical barrier defense

• Produces hydrochloric acid that kills pathogens (as so acidic with pH2) swallowed down in food or mucus before they can pass further into the digestive system.

Phagocytosis Immune system defence

• White blood cells (phagocytes) tracks down, binds to the pathogen (with flexible membrane) then engulf (surrounding) them as the enzymes in phagocytes breaks down to destroy them.

Antibodies production Immune system defence

• White blood cells (b-lymphocytes/ b-cells) produce specific, uniquely shaped protein molecules called antibodies that lock onto complementary antigens on the surface of a pathogen to act as signals to white blood cells to target it for destruction.

Antigens- the surface on the pathogen our immune system that detects as being foreign.

Antitoxin production Immune system defence

• White blood cells produce antitoxins that chemically neutralise by binding and counteract the specific poisonous toxins released by invading bacteria.

Stop pathogen spreading: Vaccination

• Injecting small quantities of dead or inactive forms of a pathogen (with same antigens) into the body to stimulate white blood cells to produce specific antibodies, so cannot become ill as our body becomes immune for future. They work against bacteria and viruses.

Immune- when the body recognises the same type but live pathogen so the secondary response with production of antibodies is much quicker and stronger destroying pathogen, preventing illness.

Pros- control of common diseases, prevent outbreaks that kill lots of people with herd immunity.

Cons- don’t always work (don’t grant full immunity), bad reactions with swelling, fevers and seizures.

Herd Immunity definition

• When a large percentage of a population is vaccinated, it prevents the pathogen from spreading easily, protecting the unvaccinated individuals.

Drug that treat disease: Antibiotics

• Medicines (like penicillin) that kill bacteria inside the body (or stop more growing) without harming body cells. They cannot kill viruses because viruses live/hide inside human cells, so cannot destroy without destroying own cells.

Problem with antibiotics: Antibiotic Resistance

• Bacteria randomly mutate (change characteristics) so bacteria is less affected by antibiotic and become immune to it.The resistant bacteria survive, where there’s no competition (as all normal bacteria is killed) and reproduce rapidly (have gene for antibiotic resistance) creating a strain that antibiotics can no longer kill.

An example of antibiotic resistent strain is MRSA

Ways to prevent antibiotic resistance

• Doctors must avoid over-prescribing antibiotics for mild or viral infections, patients must always finish their entire prescribed course and restrict agricultural use of antibiotics on livestock.

Drug that relives symptons: Painkillers

• Drugs (like aspirin or paracetamol) used to treat the symptoms of disease and reduce pain, but they do not kill the pathogen causing the illness.

Digitalis origin

• A drug used to treat heart problems, traditionally discovered and extracted from foxglove plants.

Aspirin origin

• A common painkiller used to lower fevers, traditionally discovered and extracted from willow bark

Penicillin origin

• Discovered by Alexander Fleming from the Penicillium mould (fungus), which produced a substance called penicillin that killed the surrounding bacteria in an unwanted petri dish

Drug Trial Phase: Preclinical Testing Step 1

• Testing potential new drugs in laboratories on isolated cells, tissues, then live animals to check for toxicity (how harmful drug is causing side effects) efficacy (how well the drug works), and basic dosage (how much concentration should you give- if you give more it’s more effective but have more side effects).

Cons- Testing on isolated cells cannot show whole-body side effects, and testing on animals does not guarantee the drug will act the same way in humans.

Drug Trial Phase: Clinical Testing Step 2

• Testing the drug on a small group of healthy human volunteers using very low doses to monitor safety and check for negative side effects.

Drug Trial Phase: Clinical Testing Step 3

• Testing the drug on larger group of human patients suffering from the specific illness to check for efficacy and find optimum dosage (most effective with fewest side effects).

Make testing fair to get valid results: Placebo

• A fake drug or treatment (like a sugar pill) that looks identical to the real medicine but contains no active ingredient, used as a control group in medical trials.

Blind Trial vs Double-Blind Trial

• Blind: Patients do not know if they are receiving the real drug or the placebo.

Double-Blind: Neither the patients nor the doctors know who gets the real drug until the trial ends, eliminating bias.

Peer Review definition

• Other independent scientists analyse and check the drug trial methods and results to ensure they are valid before publication to prevent false claims.

Monoclonal Antibodies definition (Triple Only)

• Monoclonal antibodies are identical copies of a single type of antibody produced by a B-lymphocyte (white blood cell), that all bind to the same specific antigen.

How Monoclonal Antibodies are made (Triple Only)

• A mouse is injected with specific antigens (complementary to the specific antibody), so its b-lymphocytes start producing those antibodies. Those b-lymphocytes are taken from the mouse, which on their own they don’t divide quick enough.

In a laboratory the b-lymphocytes are fused with tumor cells, which do divide quickly to create hybridoma cells (that both makes the anti-bodies and quickly divide)

This hydridoma cell is cloned to make many identical hydbridoma cells which all produce many of the same anti-bodies (monoclonal) so are collected and purified.

Uses of Monoclonal Antibodies (Triple Only)

• Monoclonal antibodies bind to one specific thing e.g attaching drugs or radioactive material on the monoclonal antibodies as they are specific to the antigens on cancer cell, it can locate and destroy the cancer without causing harm to healthy body cells. Cons- unexpected side effects e.g fever.

Used in pregnancy tests where if a woman is pregnant her urine contains HCG hormone, so when she urinates on the strip it has monoclonal antibodies which are specific and bind to the HCG hormones, also turning a blue colour to indicate pregnancy.

Plant Diseases and Common signs (Triple Only)

• Plants catch diseases from micro-organisms, larger organisms e.g insects and deficiency diseases (not enough essential minerals like magnesium or nitrate ions)

Stunted growth, spots on leaves, areas of decay (rot), growths, malformed stems or leaves, discoloration, and visible presence of pests.

How to Identify Plant Diseases (Triple Only)

Compare known symptoms to gardening manuals or websites.

Take infected plant tissue to a laboratory where scientists identify the specific pathogen under microscope or DNA testing.

Using monoclonal antibody test kits to detect specific pathogens.

Plant Ion Deficiency: Nitrate (Triple Only)

• Nitrates are needed for making amino acids then joined together to make proteins enabling plant growth. Deficiency causes severe stunted growth (not enough growth)

Plant Ion Deficiency: Magnesium (Triple Only)

• Magnesium is needed to make chlorophyll for photosynthesis. Deficiency causes chlorosis (lack of chlorophyll so yellow leaves).

Plant Defenses: Physical barriers (Triple Only)

• Cellulose cell walls, a tough waxy cuticle on leaves, and layers of dead cells around stems (bark) that physically block pathogens from entering.

Plant Defenses: Chemical barriers (Triple Only)

• Plants secrete chemicals e.g antimicrobial substances to kill bacteria/ fungi, or poisons to deter insects from eating the plant.

Plant Defences: Mechanical functions (Triple Only)

• These work against the animal that might eat/ damage the plant rather than against pathogens. Thorns and hairs to deter animals, leaves that droop or curl when insects land on them, and mimicry to trick animals into avoiding them.