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  • Bacteria, Protists, & Viruses

    What are Bacteria?

    You can run, but you can't hide when it comes to bacteria. They can be found everywhere on earth, from Antarctica to the inside of your intestines! Some are good, aiding digestion and giving us tasty food like cheeses and yogurt. Many are harmful, causing serious diseases; these are called pathogenic bacteria. But what are bacteria?

    Bacteria (or bacterium if you're speaking of only one) are one-celled or unicellular microorganisms that don't have chlorophyll and don't have a distinct membrane-enclosed cell nucleus, like plant and animal cells do. Instead, the nuclear material--a single strand of DNA--is folded and clumped in the interior of the cell. Microorganisms that don't have a distinct nuclear membrane are called prokaryotic organisms. Bacteria are classified in the kingdom Monera.

    How are individual bacteria classified within the main kingdom? Scientists divide bacteria by shape: sphere, rod, and spiral. Spherical (round) bacteria include streptococcus, the cause of strep throat. Rod-shaped bacillus bacteria include anthrax and tetanus. Spiral bacteria have long bodies with a twist that form a spiral pattern when they are connected together; this group includes cholera.

    Bacteria reproduce most commonly by binary fission where a single parent bacterium divides to form two independent bacteria. This type of reproduction is called asexual because there is no exchange or combination of nuclear material between two organisms. Fission occurs rapidly in as little as 20 minutes. Under perfect conditions a single bacterium could grow into over one billion bacteria in only 10 hours! A large group of bacteria is called a colony, which you can often see without magnification. Different colonies can be identified by their shape, texture, and color.

    Some bacteria can also reproduce asexually by forming thick-walled endospores that are very resistant to conditions of extended heat, cold, or dryness. An endospore is formed within the cell body of a bacterium. Usually a bacterium forms only one endospore and that endospore will produce only a single bacterium. Endospores are difficult to kill except by strong chemicals or high heat. All the species in the Bacillus genus of bacteria produce endospores.

    Gram-positive and Gram-negative Bacteria

    In 1884 Hans Christian Gram discovered that all types of bacteria could be divided into two different groups -- ones that retained a stain (this kind is 'gram-positive') and ones that didn't ('gram-negative').

    Gram's unique method for identifying these two groups became the first step in any bacterial identification process. Even the simple determination that a bacteria specimen is gram-positive or gram-negative can direct a doctor in diagnosis, as different bacteria cause different diseases. For example, the bacteria that causes scarlet fever is gram-positive, while that which causes typhoid or cholera is gram-negative. In addition, this classification process can help a doctor determine proper treatment, as some gram-negative bacteria are able to resist many common antibiotics.

    So, how does it work? The stain will wash from a gram-negative cell because its cell wall contains more lipids (fatty substances) than a gram-positive cell. The washing solvent dissolves the lipid layer in gram-negative bacteria, allowing the color to be drawn from the cell. In contrast, the solvent causes the gram-positive cell wall to dehydrate, closing the pores and trapping the stain inside the cell.

    Kingdom Protista: the Protozoans

    They're not aliens from another planet, in spite of the name! Protists are unicellular eukaryotic organisms: their cell nuclei are enclosed in membranes. They live in water (or watery tissues within the body, in the case of some diseases) and are classified in their own kingdom. You might have heard of some of these protists before: amoeba, euglena, paramecium, dinoflagellates, slime mold, and even most algae. Kingdom Protista seems to be the catch-all category of the cell world!

    Protozoa use different kinds of movement: an amoeba uses amoeboid movement, flowing along with pseudopods, or temporary foot-like extensions. (This is also the way the white blood cells in our bodies move.) A euglena moves with a whip-like tail called a flagellum, and a paramecium uses tiny hairlike-threads called cilia on its body to propel it along.

    Eating habits amongst protozoans vary, too. Some protists, such as euglena or volvox (a type of algae), use chloroplasts to generate energy through photosynthesis similar to the way plants do. Euglenas also serve as decomposers, by feeding off dead organisms. The amoeba, on the other hand, engulfs its prey with its pseudopodia and brings the food into its food vacuole (a sac where the microorganism's food is stored until digested). A paramecium sweeps its food down an oral groove lined with cilia, into a gullet that closes off when full and becomes a food vacuole.

    Many diseases are caused by protozoa, often transmitted through drinking bad water or through an insect bite. Sleeping sickness, malaria, dysentery, and Giardia (an intestinal disease) are all caused by protozoa.

    You can observe protozoa by taking a sample of pond water and viewing it under magnification. A compound microscope is necessary to see any individual protozoa, although you can see the largest colonies of protozoa, such as volvox, with just a 30x stereo microscope. Scoop a cup or so of pond water (or water from a puddle or river) into a jar. You should view the protozoa specimens within 24 hours, as the composition of the sample changes over time. Some pond water specimens, such as daphnia, hydra, and planaria, are visible without magnification, since they are multicellular. However, all protists are too tiny to see without magnification of at least 100x. (If you don't live by a pond, you can use our protozoa hatchery kit to grow your own specimens.

    To look at protozoa with a compound microscope, use a medicine dropper to put a drop of pond water onto a concave slide. Cover the drop with a slide coverslip, and then view the slide on high power.

    What kind of detail do you see? Can you identify different kinds of protists? What physical characteristics (like flagella or pseudopods) can you see?

    Viruses: Dead or Alive?

    We've all had a virus at one time or another, whether it was the flu, chicken pox, or something worse. But what is a virus? It has two main characteristics: its genetic material (DNA or RNA) is encased in a protective protein coat, and it is unable to reproduce itself the way cells can. Because of this inability, viruses are considered to be non-living.

    So how does a virus work? When it infects a body or plant, the virus will inject its genetic material into a cell, taking over the cell's protein production. It uses its genetic 'code' to direct the cell to replicate the virus, until the cell has so many copies of the virus that it ruptures, sending the infection to other cells.

    Thankfully, our bodies have been equipped with 'fighter' cells, like white blood cells, that can devour some viruses and other diseases!

    Fighting Disease: Antibiotics and Vaccines

    Our bodies are specially designed with an immune system to fight disease, but sometimes there's too much infection for us to fight alone. Antibiotics destroy bacteria cells within a person or animal's body, without harming normal cells. They are often able to cure once-fatal diseases, such as the bacterial infection scarlet fever. Amoxicillin, penicillin, and erythromycin are common antibiotics that inhibit bacterial cell functions. Antibiotics are derived primarily from bacteria or fungi (mold), such as Penicillium.

    Antibiotics don't work exclusively against bacteria: some 'broad-spectrum' ones are also effective against protists. Malaria is a disease caused by protozoa that are carried by certain mosquitoes. Antibiotics, such as doxycycline, can be used in both treatment and prevention of malaria.

    In some cases where an antibiotic is used to treat a disease, the disease-causing bacteria or protozoa will develop resistance to the drug, meaning that that particular antibiotic will no longer be effective at destroying the resistant organisms. Antibiotic research is a continuous process, since the need often develops for bigger and better antibiotics to wipe out resistant diseases.

    Antibiotics won't work on viruses. You need vaccines to prevent viral diseases such as hepatitis or polio. A vaccine is a weakened form of a disease, which produces antibodies when injected in a person or animal. These antibodies allow the immune system to recognize and attack a stronger form of the disease.

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