Any micro-organism is just what its name suggests – a very small organism. Most micro-organisms are unicellular (the whole organism consists of just one cell), although some do contain more than one cell.
There are fie main groups of micro-organisms, although each group can be subdivided. These groups are:
• protozoa • some fungi
• some algae • viruses
Protozoa, fungi and algae Protozoa are unicellular organisms that lack a cell wall. Most of them are motile (able to move), and include organisms such as
Amoeba, Plasmodium (the organism that causes malaria), and
Although the yeasts are the only unicellular fungi, other fungi are also classed as micro-organisms. Many fungi produce a mycelium of microscopic strands called hyphae. Thy release enzymes from these strands that digest whatever the fungus is growing on. Th products of digestion are then absorbed into the fungus to help with its growth and reproduction. Remember, fungi do not have true roots, stems and leaves. Some fungi live on or in living organisms, as parasites. Others live on dead material as saprobionts, organisms that digest their food externally and absorb the products.
Algae are an important group of organisms. Many are large (the seaweeds are all algae), but some algae are unicellular. Th unicellular algae are part of the plankton, the collections of small microscopic plant and animal organisms that flat or drift in large numbers in fresh or salt water, providing food for fish and other larger organisms. These unicellular algae in the oceans produce far more oxygen during photosynthesis than all the forests in the world together.
Some unicellular algae are motile – they can move. Chlamydomonas, which has two flagella at one end to propel it through the water. Viruses are sometimes referred to as micro-organisms, although some biologists say that, strictly, they are not organisms at all. Viruses cannot independently carry out any of the processes common to all living organisms. Thy can only reproduce inside other cells. So they are all parasites. Some parasitic bacteria, some parasitic plants and others parasitise animals. Th basic virus is not even a cell – it has no nucleus and no cytoplasm – but it does have genetic material surrounded by a protein coat.
What are bacterial cells like?
Bacteria have prokaryotic cells. In prokaryotic cells there is no true nucleus separated from the rest of the cell by a membrane. Instead, the DNA of the bacterium forms a continuous loop that is intermingled with the cytoplasm. Figure below shows the structure of a generalized bacterial cell. Not all bacteria have all the structures shown in the diagram. For example, not all bacteria have a capsule and many do not have a flagellum. All bacteria do have a cell wall (but it is not made from cellulose like plant cell walls and instead is made from a substance called peptidoglycan, which makes it rigid), a cell membrane, cytoplasm, ribosomes and DNA.
Although bacterial cells vary a great deal in size, they are usually much smaller than eukaryotic cells. Bacterial cells are usually between 1 and 10 µm long, whereas eukaryotic cells are between 10 and 100 µm long. (1 µm is 0.001 mm, one-thousandth of one millimetre.)
Th nucleus, mitochondria and chloroplasts found in eukaryotic cells are all surrounded by a double membrane. Thy are sometimes called ‘membrane-bound organelles’. So, biologists sometimes say that prokaryotic cells do not contain membrane-bound organelles. Because of this, photosynthesis and respiration are carried out differently in bacterial cells. Photosynthesis takes place in the plasma membrane or membranes in the cytoplasm. Many of the reactions of respiration take place in the cytoplasm, with some also occurring on the plasma membrane.
Do Bacteria even come in one boring shape?
Hold your horse there buddy, No – there are several shapes, sizes and arrangements. Bacterial cells come in three main shapes:
• cocci (singular, coccus) – spherical bacteria
• bacilli (singular, bacillus) – rod-shaped bacteria
• spirochaetes – spiral or corkscrew-shaped bacteria
Whatever their shape, bacterial cells are sometimes found singly; sometimes two cells are stuck together; and sometimes the cells exist in chains.
So how are we do we tell them apart? Bacteria Classification
Bacteria can be classified in other ways, besides their shape. One of these ways is whether or not they are colored by Gram’s stain. This test gives two categories:
• Gram-positive – these bacteria are stained purple by Gram’s stain
• Gram-negative – these bacteria are stained pink by Gram’s stain Because Gram’s stain produces different results with different types of bacteria, it is called a differential stain.
Th difference is due to the structure of the cell wall of the different bacteria. Gram-negative bacteria have much less peptidoglycan in their cell walls. This is the part of the wall that absorbs the stain. Thy also have a membrane outside the peptidoglycan cell wall, which Gram-positive bacteria do not have. This outer membrane secretes endotoxins (a type of toxin that is a structural component of these bacteria) and is also quite resistant to many antibiotics. This makes diseases caused by Gram-negative bacteria more difficult to treat. Gram-negative bacteria, on the whole, cause more serious diseases, although there are exceptions – the bacterium that causes tuberculosis is a Gram-positive bacterium.
Gram staining is used much less than it was in diagnosing disease, as more advanced and more reliable biochemical techniques have become available.