All bacteria are unicellular prokaryotes, meaning they do not have a defined cellular nucleus. Their genetic information is in their nucleoid, - single, circular tightly- packed DNA molecule. According to their shape, all bacteria are divided into three groups:
- spirilla (with a spiral body shape);
- cocci (with a spherical body shape);
- bacillus ( with a rod (stick) shaped body).
Some types of bacteria live on their own and others form colonies. Some bacteria are quite mobile and others 'stay put' for their whole life. Bacteria move using their cytoplasmic tail - flagella, or by secreting slimy substances that allow them to slide along surfaces.
The cell walls of most bacteria contain a polysaccharide called peptidoglycan. Differences in their cell wall structure is a major feature used in classifying these organisms. The staining abilities of bacteria are also based on their cell wall structure. According to the way they stain, bacteria can be classified as either Gram - positive or Gram - negative.
CYANOBACTERIA
Cyanobacteria are aquatic, photosynthetic organisms. They can be unicellular or colonial. Cyanobacteria are sometimes called blue-green algae. Like other plants, they make their energy through photosynthesis, but besides that, they do not have anything in common with plants. Millions of years ago, the atmospheric oxygen that we depend on was generated by cyanobacteria. Most cyanobacteria live in water, damp soil and rocks
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PROTOZOA
Protozoa is a sub kingdom of unicellular, mostly aerobic, eukaryotic organisms. Sometimes they are also called protists. They are neither plants nor animals. They make up the largest group of organisms in the world in terms of numbers and biomass. Some protozoans, like Euglena, have chloroplasts like plants and make their own food, which makes them autotrophs. Others, like amoeba, are heterotrophs. Protozoans can be free-living or parasitic, unicellular or colonial. Some parasitic protozoans can cause diseases in humans. Protozoans move around using their flagella or pseudopodia - cytoplasmic temporary 'feet'.
Because heterotrophic protozans consume bacteria, they play a very important role in controlling biomass
Viruses
Although viruses are not considered living organisms, they are sometimes classified as microorganisms. Viruses are much smaller than common microbes. They are made of a DNA molecule covered with a protein shell called a capsid. Retroviruses are made of an RNA molecule covered with a capsid. Capsids can take many shapes. Viruses cannot reproduce outside the host cell, but they cannot be called parasites either. Scientists still argue today about whether viruses are true living forms because they are not cells and they cannot metabolise on their own.
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