Bacteria
Bacteria are simple, microscopic and unicellular organisms. Bacterial cells are also called prokaryotes because their genetic materials (DNA) are not enclosed in a compartment like the nucleus (compartment of genetic materials for eukaryotes). The absence of a compartment allows bacterial DNA to float freely in the cytosol (Madigan 2006).
Bacterial cells come with different shapes. Bacterial cells that are rod-shape are termed bacillus; those with spherical or ovoid shape are termed coccus; those with corkscrew or curved shape are called spiral; and there are also other bacteria which shaped like star or square. Individual bacteria of a genus or a species may form pairs, chains, clusters, or other groupings (Talaro 2008). These groupings usually characterize a particular genus or species and could help in bacterial identification and classification.
Bacterial cells are similar with plant cells with respect to the presence of cell walls that enclose them. Although they differ in the composition of their cell wall; a plant cell wall is mainly composed of cellulose while a bacterial cell wall is largely composed of peptidoglycan. Peptidoglycan is made up of carbohydrate and protein; it is constructed in such a way that it provides strong structural protection for the bacterial cell. Bacteria reproduce by binary fission, a process wherein a single bacterium divides into two daughter bacterial cells. They derive their energy by absorbing organic chemicals from dead or living organisms. Bacterial species with choloroplast have the ability to produce their own food through the process of photosynthesis. Others utilize inorganic chemicals like sulfur and methane as energy source (Talaro 2008).
There many bacteria that cause disease in humans. For example, the genus Clostridium is the causative agent for botulism, anthrax, and gangrene. Tuberculosis and leprosy are infections caused by Mycobacterium bacteria (Bauman 2007; Madigan 2006). Information on bacterial species that cause diseases could be read in any microbiological books.
Archaea
Archaea are also prokaryotes like the bacteria but they don’t always have the cell wall. What differentiates them from the bacteria is their habitat. They are usually found in extremely arid environments. Archaeal species can be classified into three groups. Species that produce methane as a waste product from respiration are called methanogens. Those that thrive in highly salty environments like the Dead Sea and Great Salt Lake are called halophiles. Lastly, extreme thermophiles are archaeal species found in hot sulfurous water such as hot springs at Yellowstone National Park. Unlike the Bacteria, there are still no reported cases of archaeal infection to humans (Ingraham 2002).
Fungi
Fungi differ from the prokaryotes (bacteria and archaea) through the presence of a distinct nucleus enclosing the cell’s genetic material (DNA), hence they are called eukaryotes. The cell wall of fungi is composed mainly of a substance called chitin (Madigan 2006). Fungi reproduce sexually or asexually and there are unicellular and multicellular fungi on earth:
The multicellular fungi are usually the more noticeable because they could be seen by the naked eye. Examples are the mushrooms we see in the backyard, the edible mushrooms that we put in our soups, poisonous mushrooms in ruminant feces, and among others. Mushrooms may look like plants because of their similarities in growth habit but they differ on plants through the absence of chloroplasts in their cells. Absence of chloroplast means that fungi could not photosynthesize and produce their own food; they however acquire their nourishment by absorbing solutions of organic material from their environment (Madigan 2006).
Unicellular fungi (also called yeasts) are oval shaped microorganisms that are larger than bacteria. Molds are the most typical fungi. They form visible masses called mycelia which are made up of long filaments (hyphae) which branch and intertwine. Like multicellular fungi, unicellular fungi also get their nourishment from organic materials present in the environment (Ingraham 2002).
Some species of fungi produce mycotoxins that are very toxic to humans. For example, the fungus Claviceps purpurea causes the ergot poisoning. An individual infected with the mycotoxin experiences hallucination, gangrene, and blood flow restrictions in his limbs. Humans usually get infected with the fungus after eating cereal grains contaminated with C. purpurea (Bauman 2007).
References:
Madigan, Michael. 2006. Brock Biology of Microorganisms. Upper Saddle River, N.J.: Prentice Hall/Pearson Education.
Bauman, Robert. 2007. Microbiology: with diseases by taxonomy. San Francisco, Calif.: Pearson/Benjamin Cummings.
Talaro, Kathleen. 2008. Foundations in microbiology: basic principles. Boston: McGraw-Hill Higher Education.
Ingraham, John. 2002. Introduction to Microbiology. Pacific Grove, Calif.: Brooks/Cole Pub.
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