Methanogens are anaerobic archaea that synthesize organic compounds in a process called methanogenesis that produces methane. Methanogenic archaea thrive in swamps, hot springs, fresh water, and marshes. They are also found in the rumen, an expanded upper compartment of the stomach of cows that contains regurgitated and partially digested food. Methanogenic archaea use hydrogen and carbon to produce methane (CH
4
), and the methane can react with oxygen in the atmosphere producing CO
2,
thereby reducing the amount of atmospheric oxygen.
Bacteria and Diseases
Much of our knowledge about bacteria is the result of the study of diseases that they cause in humans. Pathology is the scientific study of diseases. Bacteria can cause diseases by producing poisons called toxins. These toxins are of two types. The first type is known as exotoxin. Exotoxins are made of protein and released by living Gram-positive bacteria, which secrete these toxins into the surrounding environment. The second type of toxin is called endotoxin, which are made of lipids and carbohydrates, and are associated with the outer membrane of Gram-negative bacteria such as E.coli.
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These toxins are not released until the bacteria are dead and once they are released they cause body aches, fever, weakness, and damage to the vessels of the circulatory system. Another method of causing diseases used by bacteria is destroying body tissues. Some bacteria adhere to cells and secrete digestive enzymes, which further attack the tissues. An example is Streptococci which produce a blood-clot- dissolving enzyme that enables these infectious bacteria to spread into other tissues.
Antibiotics
Antibiotics are chemicals produced my microorganisms [and plants] that selectively kill or inhibit the growth of other microbes. Antibiotics interfere with various cellular functions. For example, penicillin interferes with cell wall synthesis, while tetracycline interferes with bacterial protein synthesis. Many antibiotics are derived from chemicals that bacteria and fungi produce, while others are synthesized in the laboratory such as the sulfa drugs. It is important to say that the most effective agents were isolated from the mold Penicillium. Another significant producer of antibiotics are the streptomyces. They provide most of the world's antibiotics. Over 50 distinct antibiotics have been produced by streptomyces. It is also important to mention that antibiotic-producing organisms are resistant to their own antibiotics, but remain sensitive to antibiotics produced by other species of streptomyces.
Streptomyces is a large group of filamentous bacteria that resemble fungi. They are part of the Actinomycetes family. These filamentous bacteria form spores called conidia. The spore-bearing structures are called sporophores. They are usually pigmented and mature colonies are colored, which makes them easier to detect on agar plates. Streptomyces are capable of metabolizing different compounds such as sugar, alcohol, amino acids and some aromatic compounds. They also produce extra –cellular enzymes to help them digest starch and cellulose. Although they can be found in aquatic habitats, these bacteria usually inhabit the soil, and are responsible for the earthy smell of the soil due to the secretion of geosmins. Streptomyces are obligate aerobes. They prefer alkaline and neutral soil, and can be found in well-drained soil such as sandy loams or soils covering limestone. Streptomyces can also degrade organic matter and, therefore, found in compost piles.
When a population of bacteria is exposed to an antibiotic, the bacteria that are most susceptible to antibiotics die first. The therapeutic effectiveness depends on the sensitivity of the pathogen to the drug. Other microbes may vary in their responses. Antibiotic resistance may develop in microbes within the population. Antibiotics do not create resistant cells, but selectively favor the survival and reproduction of the drug-resistant strains. Antibiotic resistance occurs due to mutations in the pathogen's chromosome. In this case, most of the sensitive bacterial cells are killed or inhibited, but a few resistant cells are uninhibited and continue to grow. The prolonged exposure to the antibiotic prevents the sensitive cells from repopulating the area allowing the resistant microbes to become predominant. The second reason for antibiotic resistance is the direct transfer of R-factor plasmids, a small, closed-loop molecule of DNA, from the antibiotic-resistant to the sensitive recipients through the process of conjugation. R factors carry genes for multiple resistances, fortifying the bacterial recipient with protection from a number of drugs.
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Bacteria use many mechanisms to resist antibiotics. These mechanisms include the ability to either inactivate or destroy the antibiotic by producing extra cellular enzymes, or by decreasing the antibiotic uptake by modifying or reducing the permeability of the cell membrane to drugs or by cross resistance, whereby a single cellular modification may provide resistance to all tetracycline, for example.
Antibiotics are also added to animal feed of pigs; cattle, chickens and other farm animals to encourage meat production, but this practice has promoted the survival of antibiotic- resistant microorganisms. Most of this is the R factor type that unfortunately might transfer antibiotic resistance to human pathogens when these animals shed, or when they are slaughtered. For example, the antibiotic- resistant Salmonella can be transferred from livestock to humans. Many countries such as the Netherlands passed a law banning the use of antibiotics that are used to treat humans as a growth –boosting supplements in animal feed.
Useful Bacteria
Most humans think of bacteria as harmful organisms causing diseases and threatening our lives. In fact, most bacteria are harmless and many are beneficial and vital to our living world. Some of the following are benefits of bacteria.
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1. Nitrogen Fixation
Animals depend directly or indirectly on plants for energy, and plants depend on bacteria to get nitrogen. Nitrogen is necessary for plants to build amino acids and proteins. But plants cannot get the nitrogen directly from the atmosphere; they depend on bacteria to convert the nitrogen into a form that can be used by plants. This process is called nitrogen fixation. Bacteria in the soil can change nitrogen into ammonia ((NH
3
), some of these bacteria are free living, while others such as Rhizobium bacteria can form a symbiotic relationship in which soy beans provide nutrition for the bacteria, while bacteria convert nitrogen into ammonia. Nitrogen is also cycled by processes of bacterial nitrification. The most important groups of microorganisms involved in this process are the chemolithoautotrophic ammonia- and nitrite-oxidizing bacteria in the family Nitrobacteraceae.
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Nitrosomonas includes a number of genera that oxidize ammonia to nitrite, and then Nitrobacter oxidizes the nitrite to nitrate.
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2. Production of food and beverages.
Bacteria are used in the production of a wide variety of food and beverages. For example, Lactobacillus and Streptococcus thermophilus are used in making yogurt through fermentation. Fermentation is a process by which energy is released from food molecules in the absence of oxygen. Bacteria produce lactic acid as a waste product in the process of fermentation, and this is used for the production of a variety of food and beverages such as yogurt, kimchi, sauerkraut, buttermilk, pickles and cheese. Also, lactic acid bacteria produce antimicrobial substances, sweeteners, aromatic compounds, vitamins, and useful enzymes.
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3. Genetic engineering and medicine.
Bacteria can be transformed by using recombinant DNA. The foreign DNA can be joined to a small DNA molecule known as a plasmid. Plasmids are small, circular, extra chromosomal DNA molecules that replicate independently. A plasmid is very useful in DNA transfer, because it has a DNA sequence that helps promote plasmid replication. Therefore, if the plasmid containing the foreign DNA manages to get inside a bacterial cell, this sequence ensures that it will be replicated.
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Classroom Activities
The following activities have been designed to guide and help the students understand the concepts that have been introduced, and to encourage students to use critical thinking, draw conclusions and make a connection.
