3. What are bacteria and how are they harmful?


There are three types of micro-organisms: bacteria, fungi and viruses.

Micro means small and an organism is a living thing, so they are extremely small living things.

This lesson is all about bacteria.

Bacteria are small single-celled organisms, and are the simplest of creatures considered to be alive. They are found almost everywhere on Earth and are important to our ecosystem.

The human body is also full of bacteria, and in fact we think humans may have more bacterial cells than human cells!

Bacteria have one main purpose in life: to reproduce.

Bacteria come in different shapes, which are often part of their name.

Round shape (Coccus) eg streptococcus(causes sore throats)

Rod shape (Bacillus)eg Bacillus anthracis or commonly known as the deadly anthrax.

Spiral shape (spirilla) eg treponema pallidum (syphilis).

Some, but not all, bacteria are harmful. Harmful bacteria are called pathogens and cause illnesses like food poisoning. Staphylococcus aureus, E. coli, salmonella and campylobacter are all bacteria that can be found on foods and when eaten, cause symptoms like diarrhoea, vomiting, stomach cramps, fever and nausea. Food poisoning is very common, it is thought that 1 out of 3 people get food poisoning each year.

To prevent bacteria from growing, food should be stored at temperatures below 4°C and cooked to temperatures above 65°C. Bacteria can only reproduce very slowly below 4°C, so the number of bacteria stay very low - often not enough to make us sick. Temperatures above 65°C kill bacteria.

The bacteria on food left on the bench or in a school bag can reproduce very quickly, leading to millions of bacteria in a very short time. All these bacteria excrete wastes which cause us to get sick if we eat the contaminated food. We also say the food has become rotten and notice that it has become smelly or “off” and this is due to these wastes.

Some foods are more likely to cause food poisoning. Chicken and eggs can carry salmonella and campylobacter, processed meats like mince can carry E. coli and creamy cheeses, custards and salads can be contaminated the Staphylococcus aureus.

Other ways to prevent food poisoning include washing your hands before you touch food and after you go to the toilet, thoroughly cleaning cooking equipment like chopping boards and utensils and storing and handling cooked and raw food separately to prevent cross-contamination.

More on: How bacteria live

Bacteria feed by sending out their digestive enzymes into the surrounding living material. They then use this as food to grow and reproduce

All the waste products made by the bacteria moves of the cells into their surroundings.

More on: Growth

Bacteria have a rapid method of growth and reproduction when conditions are ideal for food (this includes water), space, warmth.

When one or more of the conditions become unfavourable some bacteria can form resistant spores.

These spores can survive for years. When the spores land on the correct conditions for growth eg dead or living tissue they quickly germinate and multiply.

More on: Colonies of bacterial cells

Bacteria have a rapid method of growth and reproduction when conditions are ideal for food (this includes water), space, warmth.

Bacterial cells in colonies appear circular in shape and may be glossy or shiny in appearance. A colony the size of a pin head may contain about 10 million bacteria or more!


Microorganism: A very small living thing, including bacteria and fungi or viruses.

Harmful: Describing a microorganism that causes harm. For example a bacteria that causes vomiting and diarrhea in a human that is harmful.

Food poisoning: An illness causes the wastes made by harmful bacteria that are found on foods. Symptoms include vomiting diarrhea, fever and stomach cramps.

Grass Level Reading

Pōhā A Clever Way of Storing Food.mp3
Pōhā A Clever Way of Storing Food-SJ L2 Sept 2014.pdf

Sky Level Reading

Space Food

Sun Level Reading