Micro-organisms and Disease - Study Notes

Micro-organisms (microbes) are microscopic life forms visible only with a powerful microscope; they are everywhere around us—inside us, on us, in our food, in our homes, in the air we breathe, and in the water we wash in.
Most microbes are useful; some are harmful.
They have been around for $3.8\times 10^{9}$ years and are vital for life on Earth.

Bacteria have three main shapes:
- Spiral (boriella)
- Cocci (little balls)
- Bacilli (rods)
They can move about on their own.
They can join up to form chains called bio-films (e.g., on teeth!).
Most bacteria covering us are harmless.
Some bacteria living in our intestines provide vitamins that our bodies need; this is why probiotic drinks and yoghurts (e.g., Activia) are popular—they are full of good bacteria that help our digestive systems.
Examples mentioned: Strep bacteria; HIV virus; Penicillium.
Not all bacteria are useful: some in the mouth produce acid as they feed on leftover sugars, which can cause tooth decay when good bacteria are not replaced by probiotics or when diet is poor.
Bad bacteria can breed when too many antibiotics are used; doctors may prescribe an antibiotic and a diet to help restore balance.
Good bacteria support digestion and health; probiotic foods aim to maintain this balance.

Viruses come in thousands of different shapes and sizes (i.e., come in $10^3$ s of shapes and sizes).
They are smaller than bacteria and even smaller than cells; they get into our cells and can only reproduce inside a cell, hijacking the host's cells to replicate.
Some people question whether viruses are truly alive since they need host cells to reproduce.
Viruses can be transmitted through the air and via surfaces (e.g., door handles).
Health-impacting viruses can cause illnesses such as:
- The flu
- Cold sores
- Chicken pox

Not just mushrooms: fungi include athletes' foot, penicillin-producing fungi, and yeast.
Yeast is used in bread making and brewing.
Penicillin is produced by fungi and is used in many medicines.
Fungi can cause diseases such as: athlete's foot, ring worm, and psoriasis.
Fungi can be beneficial (e.g., penicillin) as well as pathogenic.

A parasite is an organism that lives on or in a host and gets its food from or at the expense of its host; parasites can cause disease in humans.
Malaria is the most well-known disease spread by insects; it is spread when mosquitoes carrying the malaria parasite bite an uninfected person.
Protozoa are a type of parasite.
Ways to prevent malaria:
- Avoid standing water to reduce mosquito breeding sites; use insecticides to kill mosquitoes.
Approaches to decrease the number of mosquito bites:
- Fit mosquito screens over windows
- Sleep under a mosquito net
- Wear light colours so it is easier to identify if mosquitoes land on you
- Wear long sleeves and long trousers
- Stay indoors at dawn and dusk
- Wear insect repellent

There are different ways diseases can be passed from one host to another (e.g., a mosquito biting a human, hand shaking, sneezing, feet in a pool of water).
Diseases can be spread through contact with saliva, blood, urine, or faeces; they can also be spread by contaminated food and water or by insect vectors (e.g., mosquitoes can spread malaria and other diseases).
The most important factor in stopping the spread of disease is hygiene.

What can you do to avoid spreading diseases?
Is it always possible to prevent a disease from spreading? What might be the barriers to preventing spread?
Hygiene is the key; practices include
- Handwashing: Proper washing of hands can stop many diseases; wash after using the toilet, handling animals, cleaning, playing outside, gardening, taking out the rubbish, etc.; also wash hands before preparing and eating food.
Handwashing reminders include the phrase “WASH YOUR HANDS.”
Avoiding insect bites is also crucial because bugs (mosquitoes, ticks, fleas, and flies) can spread diseases such as malaria, yellow fever, Zika, dengue, chikungunya, and Lyme disease; while some cases are mild, these diseases can be severe with lasting consequences.

The body is constantly under attack from organisms that cause disease (parasites, fungi, etc.). Most defense mechanisms are physical barriers that stop pathogens from entering the body in the first place.
Skin: The skin is the largest organ and acts as a waterproof mechanical barrier; microorganisms on the skin cannot enter if the skin is intact.
Tears, mucus, and saliva: Contain an enzyme that breaks down the cell walls of many bacteria (lysozyme); those not killed immediately are trapped in mucus and swallowed; special cells line and protect the nose, throat, and other passages.
The inner lining of the gut and lungs also produces mucus to trap invading pathogens.
Cilia: Very fine hairs lining the windpipe move mucus and trapped particles away from the lungs; these can be bacteria or other material like dust or smoke.
Stomach acid: Kills bacteria and parasites that have been swallowed.
Urine flow: Flushes out pathogens from the bladder area.