Microorganisms: An Introduction

Microscopic organisms, commonly known as microorganisms or microbes, are found all around us and even inside our bodies. The category ‘microbes’ includes a massive range of organisms including bacteria, fungi, viruses, algae, archaea and protozoa. Some of these, such as bacteria and fungi, are well known, but others such as archaea much less so.

What are microorganisms?

Microorganisms, as their name implies, are so small that they usually require a microscope to see.

The vast majority of microbes on the earth pose no real threat to humans, plants or animals; in fact they actually work alongside humans to make the world go round, aiding decomposition, decay and even helping us to digest our food. However, there are some microorganisms which negatively impact our lives, causing illness, bad odours and damaging products and surfaces. Some of the names we regularly hear in the media are Salmonella, E. coli, MRSA, malaria and bird flu (avian influenza).

The major divisions within microorganisms

Bacteria, perhaps the most well-known microorganism, are a member of the prokaryotes; they have no nucleus within the cell and contain no organelles (specialised cellular ‘organs’). Within bacteria there are two classes; Gram-positive bacteria, which possess a thick peptidoglycan layer and no outer lipid membrane, and Gram-negative bacteria that have a very thin peptidoglycan layer with an outer lipid membrane composed of lipopolysaccharides.

Bacteria are extremely diverse and in terms of number are by far the most successful organism on Earth. Many microflora bacteria can live harmlessly within the human body, often aiding bodily functions such as digestion. In fact there are more bacterial cells within the human body than humans cells, albeit much smaller in size.

Bacteria, of all the ‘living’ microorganisms, cause the most problems in terms of disease in humans, despite only relatively few bacteria being dangerous.

Fungi are eukaryotes which means they have a defined nucleus and organelles. The cells are larger than prokaryotes such as bacteria. Fungal colonies can be visible to the human eye once they have achieved a certain level of growth, for example mould on bread. Fungi can be split into three main groups, 1) moulds which display thread-like (filamentous) growth and multicellular structures, 2) yeasts which are typically non-filamentous and can be single celled and 3) mushrooms which possess a fruiting body for production of spores.

Fungi can be problematic for the immunocompromised and contain significant pathogens which can cause disease in plants. However, we also recognise organisms from this group from widespread use in the food industry, for production of beer and other foodstuffs, and some fungi can be used for the production of antibiotics.

Viruses are considered by many experts to not be living organisms although this is still a controversial topic amongst scientists. They essentially consist of nucleic acid (DNA or RNA,) and a protein coat. A virion (a virus particle) requires a host cell in order to replicate. Within the human anatomy a virus enters a human cell and hijacks it, using the cell’s biochemistry to replicate.

In many cases the immune system detects the presence of the virus and takes action leaving us with the symptoms of a common cold or influenza. Some viruses can cause permanent and irreversible damage to cells, for example HIV.

Algae are a more difficult to define group of organisms, containing both prokaryotes and eukaryotes by some definitions. Unlike other microorganisms, algae are typically photosynthesisers and are generally found in marine environments. Confusingly, cyanobacteria are most commonly referred to as ‘blue-green algae’, however they are in fact prokaryotic bacteria and not algae. They are an ancient group of photosynthetic bacteria, and the name blue-green algae is due to their relation to the chloroplast only in eukaryotic algae. There are other groups such as Archaea and Protozoa which are generally less well known.