23 Jan 2019

Microbiology and the spread of microorganisms

It wasn’t long ago that the cause of illness was a complete mystery. Striking anyone randomly at any time, disease inflicted terrible pain, fever, and often led to death. It’s only in the last few hundred years that we have come to grasp the scientific mechanisms at play; with the invention of the microscope in 1674, a single drop of rainwater was revealed to be a whole new world teeming with miniature life!

Microbiology is the study of small or microscopic life.

This microscopic life – mainly consisting of protozoa, bacteria and viruses – answered the question of what causes illness. In 1877, French biologist Louis Pasteur discovered the connection between a rod-like type of tiny being, or microorganism, and the cattle disease splenic fever, also known as anthrax. When this tiny creature entered the cow’s body, the cow would contract anthrax. Pasteur also discovered that heating milk before its consumption killed many of these microorganisms, thus making the milk safer to consume. And thus, the term pasteurisation was born.

Good (and bad!) things come in small packages...

Microorganisms can be good or bad, depending on their composition and where they are found. We need them to digest our food, make bread rise, and brew beer, as well as break down waste. Indeed they are just about everywhere and on everything - from our skin and bodily fluids to the air we breathe and the water we drink.

Microorganisms can be grouped into one of the following: worms, fungi, protozoa, bacteria, viruses and prions. When these microorganisms cause disease we call them pathogens. While worms and fungi can often be seen with the naked eye, protozoa, bacteria, viruses and prions can only be detected with microscopes (or extremely powerful electron microscopes).

Bacterial Wonder

Let’s take the case of bacteria. Some bacteria are essential to the survival of life, like the nearly 500 types of bacteria that live in our intestinal tract and help us digest our food. And some bacteria we would like to do without, for example the bacteria that cause Streptococcus (strep throat), Tuberculosis, Whooping Cough, and Gonorrhoea. There are thousands and thousands of bacteria, usually ranging in size from 0.3-10 μm (micrometres) and are all comprised of one single cell. In order to survive, bacteria must eat, breathe, produce waste and be able to reproduce. They are either rod-shaped, round, or spiral (as seen in the image below).

Bacteria grouped according to shape

Going Viral

Viruses are even smaller than bacteria, indeed about one-thousandth smaller! They are not comprised of cells like bacteria, but rather protein-encased genetic material like DNA or RNA. They cannot survive on their own and therefore are actually parasites, beings which must grow inside the cells of the host organism. Whereas bacteria are actual live beings and reproduce, viruses are not actually alive (think of them as messages transferred between cells) and must replicate themselves in order to proliferate. Famous examples of viruses include everything from the common flu to polio, hepatitisrabiesHIVSARS and Ebola.

Microorganism Warfare

Microorganisms are not too picky about where they set up home. As long as they have sufficient food, an agreeable temperature and moisture, they will start to multiply - and quickly! They will begin to divide themselves in the first 20 minutes, and within just six hours can increase their numbers by  65,000 individuals!

Infection of the body by microorganisms, showing how they multiply inside the body

Considering the vast variety and sheer number of microorganisms in our environment combined with their ability to rapidly multiply, it’s amazing that we don’t get sick more often. The body’s first line of defence against infection bymicroorganisms is our skin – a physical barrier preventing the entry of bacteria into the body. If the skin fails in its protective function, our immune system takes over and brings out the big guns such as macrophages, a type of white blood cell that “eats” foreign cells.  But if the immune system fails (which is more likely when the body is weak), and the microorganism takes hold inside of our bodies, then we say the person has an infection, which may or may not be helped by drugs (antibiotics). Infection is often accompanied by fever and inflammation.

The Battlefronts

We say that we are contaminated when disease-causing microorganisms enter our body and when instruments are used in an operation on a diseased person. In summary, anywhere that microorganisms live - on a body or a thing - is said to be contaminated. Even though, technically, instruments are not contaminated unless directly exposed tomicroorganisms, nevertheless all instruments used in surgery are treated as if they are contaminated, so that proper measures are taken to kill all microorganisms before the equipment is used again.

Microorganisms are spread in the following ways:

  •          Direct bodily contact by touch
  •         Air that we breathe or particles and droplets in the air that land on our skin
  •         Contaminated food
  •         Contaminated animals and insects
  •         Contact with contaminated materials and equipment.

What are the body’s chances of success in fighting against microorganisms? It depends on which battlefront the fight takes place. There are three general areas of microorganism entry in the body: low risk, medium risk, and high risk:

  • Low risk: Skin. The skin is the body’s first line of defence and is able to prevent most microorganisms from entering the body.


  • Medium riskMucous Membranes. These are the special coatings found on the surfaces of many organs that are in direct contact with the outside world. The slimy fluid produced by the mucous membranes is called mucous and is capable of killing many microorganisms. We have mucous in our tears, saliva, stomach acid, and sexual organs, just to name just a few areas.


  • High risk: Open Wounds, Sterile Organs, Tissues and Fluids. Usually microorganisms are found on the skin, mucous membranes, in the genito-urinary system, and in the digestive tract. In all of the other organs (known as the sterile organs, tissues, and fluids), there are no microorganisms. Examples of sterile fluids are blood and cerebrospinal fluid. When skin and mucous membranes are damaged as in the case of open wounds, the usual barriers are compromised; with nothing standing between the microorganisms and these sterile organs, tissues and fluids, there is a strong likelihood of infection caused by microorganisms.


The low, medium, and high-risk areas of the body


Mighty Microorganisms Demystified

To summarise, we learned that microbiology is the study of microscopic life, which includes microorganisms like bacteria and viruses. When those microorganisms cause disease, we call them microorganisms. Under the right conditions, microorganisms can multiply very quickly, and if they are not killed by the skin or immune system, they can cause an infection. Microorganisms are spread through contact with contaminated food, animals, materials and equipment, as well as direct bodily contact and the air. Where the microorganisms enter our body also strongly determines our chances of infection. The low risk area is our skin, followed by the medium risk area of our mucous membranes, and finally the highest risk area where microorganisms enter our bodies via open wounds and sterile organs.

This is one of several articles written to help you better understand and optimise your infection control procedures. Each article in the series has been written using content contained in the Autoclave Sterilisation Basics series from the Sterilisation and Infection Control blog by Tuttnauer.

Our next post in this series "Preventing the Spread of Infection in Dental Practices", is COMING SOON. This article will develop your understanding as to how to kill microorganisms. We will explore the important concepts of cleaning, disinfection, and of course, sterilisation.

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