Micro-organisms and disease

A brief history of microbiology

Microbiology is the study of very small living organisms. They are too small to be seen with the naked eye so could not to be seen until magnifying lenses were developed in the mid 1600s. In the late 1660s, the Dutch scientist, Anton Van Leeuwenhoek examined pond water using these magnifying lenses and made drawings of the microscopic living "creatures" he observed.

Nearly 200 years later, in the mid 1800s, Louis Pasteur demonstrated that micro-organisms are present in and on all kinds of materials. He was able to use heating methods to destroy micro-organisms and developed methods to prevent micro-organisms from gaining access to heat-treated liquids. Pasteur's pupil Charles Chamberland developed the first pressure autoclave. It looked like a pressure cooker and could reach a temperature of 120o Celsius.

Joseph Lister, an English surgeon, took an interest in Pasteur's work as he was concerned about surgical wound infections. He knew that phenol could kill micro-organisms so used it to disinfect his instruments as well as an operating room spray. The incidence of post-operative infection was significantly reduced in his patients.

By the late 1880s, Robert Koch confirmed that a specific micro-organism causes a specific infectious disease. He also demonstrated that a micro-organism could be passed from one person to another. Studies then began to isolate and identify the micro-organisms responsible for most of the diseases known at that time.

In 1928 Alexander Fleming, a Scottish bacteriologist noticed that a green mould was inhibiting growth on one of his bacterial culture plates. Ten years later, the Australian scientist Howard Florey further researched this mould and from it extracted the first antibiotic, penicillin. The mass production and use of penicillin has saved countless lives since that time.

Activity 1



It is generally accepted that the modern era of sterilisation began in the 1930s with the development of the first pressure and temperature controlled steam steriliser. The first few Central Sterile Supply Departments (CSSD) established after that time processed mainly dressings, ward instruments and linen. Since then the role of the CSSD department has changed dramatically. CSSD staff now process a wide range of delicate and specialised instruments. This was previously the responsibility of staff in the operating theatre.

If you are planning to work in the sterilisation industry you must be well trained and realise how significant your role is in the health care facility. By completing this course you are starting your journey to become a highly skilled sterilisation professional.

Micro-organisms remain a major cause of hospital acquired and community infections.

The challenge for you as a health care professional is to prevent transmission of infection. To achieve this you need to learn more about micro-organisms.


Microbiology is the study of living organisms known as micro-organisms. They are so small that they are only visible with a microscope, therefore the word ‘micro’.

Not all micro-organisms can cause disease in man. However, some do, so it is important that you know a little bit about micro-organisms so that you can properly carry out the steps necessary to control contamination and prevent the spread of disease.

Large numbers of micro-organisms are always present in every human environment. Most live in warm, moist dark areas where an adequate food supply exists. The mouth is such an area, and an enormous number of micro-organisms commonly exist in the normal human mouth.

As I said before, most micro-organisms do not produce human illness. In fact, they co-exist with us and are often valuable allies in many ways. For instance;

Pretty useful to have around – aren’t they? Except viruses, which we’ll talk about soon!


So, what are the ones that are not so useful called?

Any micro-organism that is capable of causing disease is called a ‘pathogen’. Therefore disease producing organisms are said to be pathogenic. It is these micro-organisms that we want to kill or protect ourselves from. Sometimes if we contract a disease caused by bacteria, we are given antibiotics to take, but these antibiotics can also kill the good bacteria which is not so good for us. Also, antibiotics do not kill viruses!

Now let’s look more carefully at all these little creatures!

Categories of micro-organisms

The principal groups of micro-organisms are:

Algae (singular, alga)

Algae contain chlorophyll and carry out photosynthesis similar to plants. The single cells of algae often link together to form structures such as seaweed. Algae mainly live in aquatic environments, both in fresh and salt water. They do not usually cause direct infection in humans. However some algae produce toxins. These algae are eaten by fish and shellfish and then consumed by humans who develop food poisoning. Algae are colourful organisms, often green, brown or red.


Protozoa are single celled microscopic animals. They are transmitted by insects to humans. A good example of this is mosquitos transmitting the protozoan that causes malaria.

Drawing of protozoa



You see some fungi in fields under trees, in shower recesses and in ponds.

They are commonly known as mushrooms, moulds and yeast. In the diagram following, you will see that some of these organisms have thread like strands called filaments. Filaments are tubular branches of fungi that carry the fungi spores.

You may have heard about or seen ringworm? It is a disease caused by a fungus.

A very common dental disease caused by the fungi group yeast is 'candidiasis'. Candida albicans is naturally found on and in the human body, however, at times of poor health, and sometimes if broad spectrum antibiotics are taken, candida multiplies and becomes invasive causing disease. This disease is commonly known as Thrush.

Fungi like to grow in moist, warm and often semi-dark environments. They reproduce by spreading spores. The spores are basically a single cell of the parent plant. These spores are not the same as bacterial spores.

Drawing of fungi



We will start with bacteria because these are probably the types of micro-organism that are most commonly known.

Bacteria are single celled micro-organisms, in other words they are only ONE cell. However, they come in a variety of shapes and sizes.

So, what is this cell made up of? Bacteria consist of a rigid cell wall that protects a thin, rubbery, cell membrane underneath. Both these structures surround the cytoplasm of the cell which is a fluid that holds other structures, each of these having different functions. Some of these structures are:

Bacteria do not have a nucleus like our body cells even though they do have DNA. The DNA is grouped in an area called the nucleoid. They don’t have organelles like our cells, just Ribosomes.

To enable movement some bacteria have long thin appendages projecting from the main body of the cell, these are known as flagella (plural of flagellum).

Bacteria reproduce by cell division that is why when environmental conditions are at an optimum growth is so rapid.

Drawing of bacteria


Now can you think about some of the things bacteria can do?

Capsules and spores

Bacteria can form capsules and spores. A capsule is a protective layer around some bacteria. They can be virulent as they can resist the body's defence mechanism.

Bacteria can change into bacterial spores when the environmental conditions are unfavourable. In this condition bacteria are inactive. When the environmental conditions become more suitable the bacteria can become active and cause disease. Bacterial spores are extremely resistant and can survive most forms of disinfection and inadequate sterilisation.

Drawing of bacterial spores

Bacterial spores

Most bacteria have distinctive cell shapes. The shape and form of bacteria is an important aid in classifying bacteria. The main types are shown in the following table.

Main types of bacteria
Bacteria TypeDescriptionAppearance

Small structures which are sub divided into




Appear to grow in clusters like bunches of grapes.


11 small circles




Appear to grow in chains like strings of beads. 

 5 small circles




Appear to grow in pairs.

2 small circles



 Regular or rectangular

Orange rectangular shape



Comma shaped micro-organisms

Purple comma shape



Spiral shaped rigid celled micro organism

Red spiral



Long thin spiral shaped capable of twisting movements

Thin yellow spiral shape


Micro-organisms, like people, require certain elements to live, grow and reproduce. Not all micro-organisms require the same environmental conditions to live, but once you read the list of their general requirements, you will understand why in the next section it is so important to follow guidelines in the prevention of cross contamination.

Food, water or moisture are required by bacteria for growth and energy. The enzymes in bacteria break down the food and absorb moisture that is stored or used by the cell.

Oxygen: Aerobes are a variety of bacteria that require oxygen in order to grow and survive. Anaerobes are bacteria that grow in the absence of oxygen, and can be destroyed by exposing them to oxygen.

Temperature: Most bacteria grow best at body temperature but they can survive and grow between 1 degree Celsius and 60 degrees Celsius. There's even one species of bacteria called ‘Deinococcus radiodurans’ that can survive radiation blasts a thousand times greater than blasts which would kill a human being.

pH level: This refers to the acidity/alkalinity of a solution. Most bacteria require a neutral or slightly alkaline environment.

Some dental diseases caused by bacteria and the ones of most interest to us are:

Other bacterial diseases include

Activity 2

Click here for activity 2 which tests your knowledge of the shape of bacteria.


While fungi and bacteria can be helpful to our environment, viruses cause disease and death in living organisms and need to be destroyed.

Viruses do not have cellular and metabolic structures like bacteria do, so strictly speaking, should not be classified as micro-organisms. However, it is common practice to treat them as one form of micro-organism. So, for the purpose of your understanding in this topic, we will refer to viruses as micro-organisms. Being metabolically inert, viruses are not susceptible to antibiotics which act on the metabolism of bacteria.

Viruses differ from other micro-organisms as they can only grow and reproduce within the cytoplasm of a host's living cell. They invade the cell and take control of their cellular mechanism to reproduce. They do this by attaching themselves and injecting their genes into the cell.

Bacteriophages are viruses that infect bacteria. They cut a hole in the bacteria’s cell wall to inject their genes. The end result is many new viruses manufactured within the bacteria. When there are too many viruses to fit, the bacterium splits open resulting in its death. All the released viruses move on to infect more bacterial cells.

So you can see viruses are quite different to bacterial micro-organisms.

Some diseases caused by viruses are:

and other diseases such as;


Prions are even smaller particles than viruses and are becoming more of a consideration in sterilisation departments throughout the world. They are simply a tiny bit of infective protein. They cannot be seen with any conventional form of microscope and are usually only identified from the effect on brain cells in brain tissue taken at autopsy.

This infective material has been shown as the cause of scrapie, a slow incubation disease of sheep and bovine spongiform encephalopathy or ‘mad cow disease’ as it is commonly known. Prions have also gained prominence as the cause of a long incubation progressive dementia called Creutzfeldt-Jakob Disease (CJD). In Europe over the past few years, there has been some panic and concern caused by a variant type of this disease which also affects young people. This does not appear to have spread to Australia and may be the result of local issues in Europe.

Prions are extremely resistant to routine disinfection and sterilisation processes. There are special processes to treat instruments used on patients with suspected prion infection.

Micro-organisms and diseases
DiseasesType of micro-organism

Impetigo / boils




Sleeping sickness










Amoebic dysentery


Properties of micro-organisms

The following 6 properties are significant:

  1. Pathogenicity
  2. Size
  3. Motility
  4. Resistance to destruction
  5. Virulence and infectivity
  6. Toxin production.

Let's consider each of these properties.


We can divide micro-organisms into good and bad organisms. Generally the good organisms do not harm us and in many instances they help us. We call these non-pathogenic organisms.

The bad organisms are those which harm us, by causing disease which we call infections. These organisms are called pathogenic organisms.

Sometimes a non-pathogenic organism becomes pathogenic. For example, sub acute bacterial endocarditis occurs where organisms, usually living harmlessly in the mouth, manage to reach the heart which causes endocarditis. This is most likely to occur when the heart is abnormal from birth or from a previous disease such as rheumatic fever.

Some people harbour pathogenic organisms in some part of the body without showing any signs of disease. We refer to these people as carriers. However, the organisms are still pathogenic. If they reach a susceptible person, that person will develop an infection.


Micro-organisms are so small that they can be seen only with the aid of a microscope. When determining their size we use a special unit of measurement called a micron. The Greek letter u is used to represent microns.

To get a better idea of just how minute micro-organisms are let's take the diameter on the average red blood cell as 7 microns. A large member of the protozoa family may be three to four times that diameter, followed in size by fungi, until we get down to viruses which are the smallest of all organisms. Viruses are so small that high-powered electron microscopes are needed to see them.


Some organisms can move by using whip-like outgrowths called flagella, which protrude from their outer covering. The vibration of the flagella propels the organism along. Figure 1 on page 8 shows a micro-organism with a flagellum on Tryanosoma.

Resistance to destruction

Micro-organisms have different susceptibility to unfavourable conditions.

Although very simple, some bacteria have amazing survival abilities. When these bacteria are in unfavourable environmental conditions—high heat or where there is a lack of nutrients, they can change into a resting stage called a spore. They remain alive but they are not growing or multiplying. The water content of the bacterial cell decreases and its outer coating becomes dense. It can stay in this state for very long periods. Once favourable conditions return the bacteria start to grow and multiply again.

Spore forming bacteria are very significant in the field of sterilisation as many of the quality assurance tests (that you will learn about later in the course) use resistant spores. They are also clinically significant as they survive in dust and cause severe infection if they get into wounds through accident or invasive surgery. They can survive disinfection and inadequately performed sterilisation processes.

Viruses do not survive for very long periods outside their living host cells. However, some viruses such as Hepatitis B can survive for some time in dried blood. Most viruses are much easier to kill than bacteria.

Some viruses have an extra layer of material known as lipid as an outer envelope. The envelope helps the virus attach to another cell. Viruses that don't have an envelope are called naked viruses. They are generally more resistant to environmental changes. This is because the envelope can be easily damaged by unfavourable environmental conditions such as extremes in temperature.

Virulence and infectivity

Virulence is the intensity of the disease symptoms produced by a particular organism. The virulence of a disease is determined by the number and power of the invading organisms and the susceptibility of the invaded person.

Infectivity of an organism refers to the proportion of contacts that are infected by it. As an example, the measles and polio viruses are both pathogenic. Out of the 100 susceptible contacts of measles, 70 may develop the disease but out of 100 possible contacts of polio, only one may develop the disease. This shows the infectivity of a measles virus is far greater than that of the polio virus.

Toxin production

Nearly all pathogenic micro-organisms produce poisonous substances called toxins. Often it is these toxins that lead to most of the signs and symptoms of disease.

Let's take staphylococcus bacteria as an example. If food is contaminated with these bacteria, anyone consuming the food will become ill because they are poisoned by the toxins liberated from the bacteria.

Growth and destruction of micro-organisms

To understand how micro-organisms can be destroyed, we must first see how they grow and reproduce.


One fully developed organism divides into two, these grow into two whole independent organisms and then each divides into another two, making eight and so on. This is the main method of reproduction by simple division. This is known as binary fission. Division can occur every 20 minutes.

Millions of new organisms can be produced in 24 hours. Therefore, if one pathogenic organism survives attempts to destroy it, in a short time, under favourable conditions, it can produce millions of new organisms ready to invade and infect through broken skin or surgical wound.

Drawing of micro-organisms reproducing

Reproduction of micro-organisms

Some protozoa have complicated methods of reproduction, as do some of the highly organised species of fungi. However, the study of these processes is beyond the scope of this course. And viruses, as we discussed earlier reproduce in the cytoplasm of a cell.


In general, good hygienic practices are sufficient in most instances to prevent the spread of many infections in the community. Micro-organisms picked up during routine duties are removed leaving only generally harmless micro-organisms that live happily on the skin surface. Sometimes when a person is sick they are kept away from other people to stop the infection spreading to others.

In the health care setting it is necessary to use a range of infection control practices to prevent the spread of micro-organisms from one patient to another.

It is also important to pay special attention to instruments and equipment used on the patient. In general all instruments used in the operating theatre and for invasive procedures in other areas need to be cleaned and sterilised in-between patients. Sterilisation results in the death of all micro-organisms.

Sterilisation can only be achieved by:

Filtration is sometimes used to sterilise air or solutions. This process removes or excludes rather than destroys micro-organisms.

Treatment of infections

Not all infections can be easily treated. It is much better to prevent the spread of infection than treat it. Fortunately there is a wide range of antibiotics that can be used in many situations. Bacterial infections are generally much easier to treat than viral or fungal infections.

Methods of transmission of disease

Micro-organisms and viruses are not able to travel from one person to another or from one object to another on their own. They need a means of transportation, such as:

Organisms can be transmitted from a source by four main modes:

Direct contact

By direct contact we mean direct physical contact between one person and another, such as:

Indirect contact

Transmission of organisms by indirect contact can occur by contact with:

Airborne infection

Airborne infections are caused by pathogenic organisms carried through the air by dust or droplets. Minute droplets containing organisms are projected from the mouth and nose for 1m to 2m through the air whenever a person speaks, coughs or sneezes. Large droplets may directly infect the nose, throat, eyes or a wound of a person close by. Alternatively, these droplets may fall on personal clothing, toys, and furniture (such as equipment table, which in turn contaminates instruments). Small droplets may evaporate, leaving in the air suspended infected particles, known as droplet nuclei, which can be carried a considerable distance.


Vectors are animals or insects that carry pathogenic organisms from one host to another. For instance, flies commonly transfer gastrointestinal organisms from excreta to food.

So, you must bear in mind that every item in your office or testing room, which is not sterile and protected, has living organisms on it. Some of these organisms may be pathogenic. This contamination can be caused by settling of infected dust, contact with clothing, or direct contact with infected body secretions like ear discharges.

Some insects cause infections by depositing organisms on the skin surrounding bites. Scratching the site pushes the organisms into the bites. Insects such as mosquitos actually suck up organisms from an infected person and inject them into another person. Placenta

Some pathogens are able to enter the foetal circulation from the mother via the placental blood vessels, for example, AIDS, cytolomegalovirus (CMV), rubella.

Click here to find out about methods of transmission

Activity 3

Answer true or false


The poisonous substance produced by a micro-organism is a toxin


Answer true or false


Micro-organisms reproduce by simple division called binary fusion.


Answer true or false


All diseases are transmitted by contact with an infected person.


Answer true or false


Virulence refers to the proportion of people infected by an organism.


Answer true or false


Micro-organisms capable of causing disease are called pathogens.


Answer true or false


A flagellum is a structure which enables bacteria to move.


Answer true or false


Viruses have cellular and metabolic structures like bacteria.


Answer true or false


In most instances freezing will destroy micro-organisms.


Role of microbiology in disease

The mucous membranes of the mouth and saliva contain large numbers of potentially disease producing (pathogenic) organisms which, while usually causing no harm to the host, may cause disease if spread to another person. The presence of blood in the saliva which is common in persons with periodontal disease greatly increases the risk if the person is a known or unknown carrier of a disease such as Hepatitis B, C or HIV.

Whether the spread of micro-organisms causes clinical infection depends on the power particular micro-organisms to infect

Activity 4

Click here to test your knowledge of transmission of disease


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