A microbe or microorganism is any organism that is too small to be seen with the unaided eye. Single celled organisms like bacteria, fungi, protozoa, algae, and viruses are all examples of microbes
7
. Although their stature is small their numbers are immense . There is an average of 1 billion bacteria in a liter of ocean water
8
. Consequently, microbes make up most of Earth's biomass. Not much thought is given to these tiny creatures because of their small size. Scientists weren't even aware of their existence until Antoine van Leeuwenhoek developed a crude magnifying instrument which allowed him to see microbes in water and scrapings from his mouth
9
. Today, microscopes have developed greatly and although most microbes can be seen under one hundred times magnification the details are still obscure. Scientists employ the scanning electron microscope (SEM) and the transmission electron microscope (TEM), which both shoot high energy beams of electrons onto the specimen to get a better look
9
.
Microbes are living organism if all prerequisites are met
7
. The biological definition of life includes five requirements. The first requirement is that the organism has a metabolism. Through its open system it can interact with its environment and take in chemicals and reorganize and rearrange them for biological function and eliminate the waste. The second is that the organism can grow or reproduce. The third is differentiation, through which the organism can produce new cell structures. The fourth is communication. The organism must have the ability to communicate and locate other like organisms. The communication is not restricted to audible signals, it can be chemical signals. The last requirement is evolution. The organism must be able to change characteristics and pass these down to their offspring. By this definition, the prokaryotes, Bacteria and Archaea are living organisms
7
.
All life can be divided into three domains: The Bacteria, the Archaea and the Eukarya. Current scientific theory suggests that these three groups of life forms share a common ancestor, for now called the universal ancestor. As they diverged into their separate groups the Bacteria and Archaea, the prokaryotes, maintained their single-celled morphology, whereas the eukaryotes, aside from the protozoa, evolved into the multicelluar organisms we know today as plants, fungi, and animals
7
.
Eubacteria and Archaea are very similar. Within both domains, single-cell structures include a cell wall, cell membrane, cytoplasm, chromosomes, ribosomes, plasmids, granules, capsules, and flagella. Both domains can thrive under aerobic and anaerobic conditions. Both can be found in all habitats on Earth, however the Archaea tend to occupy the extreme environments like hydrothermal vents
7
.
Unlike the kingdoms Animalia and Plantae, both kingdoms of prokaryotes, the Eubacteria and the Archaea are classified by their metabolism. Microbes transform the molecules in their environment into chemicals that make up the cell, increasing its mass. This process is energy intensive and the cells must be able to obtain energy to forward the process. Some of these organisms obtain their energy from light like plants and the rest obtain energy by oxidizing chemical compounds. Microbes break down chemical compounds and release energy in a process called catabolism
1 0
.
Figure 1.
All the names given the energy obtaining processes end in the suffix troph, from Greek, meaning feed. The organism's energy source is the starting point for its classification. An organism can obtain its energy from either a light source (phototroph) or a chemical source (chemotroph). From there the organisms are then divided into self-feeders (autotrophs) or heterotrophs (other-feeders)
1 0
. (See Figure 1.)
