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2.4 Bacteria In section 2.1, you learned that bacteria can be classified into two kingdoms: Archaebacteria and Eubacteria. Before we examine the differences, consider the similarities all bacteria possess: • All are unicellular; some stick together in colonies. • Cells are prokaryotic. Bacteria usually lack membrane-bound internal structures, having no organized nucleus, vacuoles, mitochondria, or chloroplasts. • Cells usually have a single chromosome in the form of a DNA loop. • Cells reproduce asexually by binary fission. • Cells thrive only in moist environments and become inactive if the environment dries up. Figure 1 (a) Eukaryotic animal cell (b) Prokaryotic bacterial cell eukaryotic a type of cell that has a true nucleus, surrounded by a nuclear envelope prokaryotic a type of cell that does not have its chromosomes surrounded by a nuclear envelope plasmid a small piece of genetic material (a) There are two types of cells, eukaryotic and prokaryotic. Cells are placed into one of these two categories based on how complex their structure and cell division process is. Plant and animal cells are eukaryotic, meaning “true nucleus”; their structures were outlined in Unit 1 (see Figure 1 for review). Bacterial cells are prokaryotic, meaning “before nucleus.” Genetic material floats freely inside the cell in the form of either a loop or a small piece of genetic material called a plasmid. Bacteria have a rigid outer wall that gives them shape. Underneath this wall is a more fluid membrane called the cell or plasma membrane. There are few recognizable organelles in the cytoplasm, even under the high magnification of an electron microscope. Table 1 compares some properties of eukaryotic and prokaryotic cells. Table 1 DID YOU KNOW ? The Missing Link Some scientists believe that archaebacteria, isolated in the 1970s, are the link between eubacteria and eukaryotes. Over 50% of the genes identified in this kingdom were previously undiscovered in other organisms. 108 Unit 2 (b) Properties of Eukaryotic and Prokaryotic Cells Property Eukaryotes Prokaryotes true nucleus present absent DNA usually many chromosomes usually single chromosome cell division mitosis binary fission ribosomes larger smaller mitochondria present absent chloroplasts can be present absent flagella complex flagella simple flagella size usually > 2 µm diameter usually < 2 µm diameter microorganism examples Euglena, Paramecium, Amoeba Escherichia coli, Bacillus anthracis NEL Section 2.4 Archaebacteria Archaebacteria (archae means “early” or “primitive”) are derived from one of the oldest groups of living organisms, perhaps the first on Earth. These organisms thrive under extreme conditions that other organisms could not tolerate. Many live without oxygen. Three major groups of archaebacteria include the thermophiles, the methanogens, and the halophiles (Figure 2). The thermophiles live in extremely hot environments. Some obtain energy by oxidizing sulfur, and thrive in and around hot sulfur springs. The methanogens grow on carbon dioxide and hydrogen gas to produce methane. They exist in environments such as volcanic deep-sea vents and the intestines of mammals, including humans. The halophiles live in extremely saline environments, such as salt flats and evaporation ponds, and are responsible for the purplish-red colour in these areas. The bright red pigment protects the cells from intense solar radiation, yet allows them to use sunlight for energy. Scientists are putting this kingdom to work to benefit society. Methanogens help digest sewage and oil spills, producing methane, which can be used as an alternative fuel source. Halophiles are used in cancer research. Enzymes produced by archaebacteria are used in food processing, perfume manufacture, and pharmaceuticals. The most famous archaebacterial enzyme is Taq polymerase, a substance isolated from Thermus aquaticus and used in molecular biology. (a) Thermophiles live at temperatures over 45°C. (b) Methanogens are found in environments without oxygen, such as swamps and marshes. Figure 2 Examples of archaebacteria (c) Halophiles colour the landscape around salt lakes. Eubacteria The best representative organism of the kingdom Eubacteria, and likely one of the most studied organisms in the world, is Escherichia coli (Figure 3). As you will discover throughout this unit, this organism is both helpful and harmful to humans. Billions of E. coli live in the human intestine, helping with food digestion and the synthesis of vitamin K and B complex vitamins. Human and animal feces also contain billions of E. coli bacteria. High counts of such bacteria (coliform bacteria) in water indicate fecal contamination and danger to human health. Most bacteria range in size from 0.4 µm in diameter to several micrometres in length. As E. coli is about 1 µm in length, a line of about 250 organisms could just be seen by the naked eye. The structure of E. coli is illustrated in Figure 4, on the next page. NEL Figure 3 E. coli micrograph Microbiology 109 pili (singular: pilus): These hairlike structures help bacteria attach to each other and to surfaces. They can also be involved in movement. genetic material: DNA floats in the cytoplasm; there is no nucleus. It usually consists of one large chromosome ring, and several smaller ones called plasmids. cytoplasm: Not divided into compartments shape: Rodlike capsule: A sticky coating surrounds some disease-causing bacteria, helping protect them from destruction by the host’s immune system. ribosomes: Float freely in the cytoplasm, making protein cell membrane: Regulates movement of materials in and out of the cell flagellum: Flagella rotate like propellers to drive the cell through its aqueous habitat. Figure 4 E. coli—a typical bacterial cell Figure 5 Staphylococcus epidermis is Gram-positive. cell wall: The cell wall is Gram-negative and consists of a thin peptidoglycan layer, an outer membrane, and a periplasm—the area between the cytoplasmic membrane and the outer membrane. The composition of the cell wall varies among bacteria species and is an important means of identifying and classifying bacteria. Eubacteria contain a polymer called peptidoglycan in their cell walls. The Gram staining method (developed by Danish bacteriologist Hans Christian Gram in 1884) differentiates between two major cell-wall types. In this method, the bacterial sample is smeared on a microscope slide, and stained with crystal violet dye (purple). The dye is then fixed to cells with Gram’s iodine, decolourized with ethanol, and counterstained with safranin (red). Gram-positive bacteria such as Staphylococcus (Figure 5) retain the first dye, appearing purple in the light microscope. In Gram-negative bacteria, the decolourizer washes out the violet dye, the counterstain is taken up, and the cells appear pinkish-red. E. coli is a Gram-negative bacterium (Figure 6). Differences in the amount of peptidoglycan in the wall determine whether or not the crystal violet is washed out. Gram-positive bacterial walls contain more peptidoglycan, so these walls retain the purple dye. Figure 6 E. coli is Gram-negative. 110 Unit 2 NEL Section 2.4 monococcus (1) diplococcus (2) bacilli (found singly or in pairs or chains) spirilla streptococcus (chain) staphylococcus (clump) Aside from cell-wall composition, eubacteria can be classified according to shape, configuration, respiration, and type of nutrition. Most organisms display one of three basic shapes—spherical, rod-shaped, or spiral (Figure 7). After division, many bacteria stay together in groups or clusters rather than remain as individual cells. Cocci (singular: coccus), bacilli (singular: bacillus), and sometimes spirilla (singular: spirillum), form pairs, cluster colonies, or chains (filaments) of cells. For example, Streptococcus mutans, the main cause of tooth decay, forms chains. Staphylococcus aureus, a common bacterium found on the skin, forms clumps. When large numbers of cells have grown, they become colonies. Myxobacteria form specialized colonies in one point of their growth called fruiting bodies (Figure 8). Some bacteria are aerobic organisms and must have oxygen to survive. Bacteria that cause tuberculosis are aerobic organisms. Other bacteria are anaerobic and can only grow in the absence of oxygen. Organisms causing gangrene, tetanus, and botulism are anaerobic organisms. Many bacteria can survive and grow with or without oxygen. E. coli, the bacterium in Figure 4, is in this category. Classifying eubacteria by type of nutrition shows the diversity of this kingdom. Some bacteria are autotrophs, making the food they require from inorganic substances. Photosynthetic bacteria convert carbon dioxide and water into carbohydrates by using energy from sunlight. Chemosynthetic bacteria use chemical reactions rather than sunlight as their energy source. Most bacteria are heterotrophs, obtaining their nutrients from other organisms (e.g., by feeding on dead or decaying matter, or as parasites causing disease by feeding on living tissue). Reproduction and Growth of Eubacteria and Archaebacteria Bacteria reproduce asexually by binary fission. Although it bears some resemblance to mitosis, binary fission is much simpler. The single strand of DNA replicates, resulting in identical genetic material being transferred to each new cell. Following replication of the genetic material, the bacterium produces a cross wall, dividing the cell into two identical bacteria, which may separate or remain attached (Figure 9(a), on the next page). NEL Figure 7 Basic shapes of bacteria colony a visible growth of microorganisms containing thousands of cells aerobic requiring oxygen for respiration anaerobic conducting respiration processes in the absence of oxygen Figure 8 Millions of unicellular myxobacteria form a fruiting body. In this configuration, they can last through harsh conditions for long periods of time. CAREER CONNECTION Bacteriology and microbiology technologists work in laboratories culturing bacteria to determine their identity. Many Canadian colleges and institutes of technology offer programs in these fields. Microbiology 111 endospore a dormant cell of bacilli bacteria that contains genetic material encapsulated by a thick, resistant cell wall. This form of cell develops when environmental conditions become unfavourable. (a) Sexual reproduction is not common in bacteria. However, conjugation does occur among some bacteria, such as E. coli and Salmonella. In conjugation (Figure 9(b)), donor and recipient bacteria make cell-to-cell contact by means of a special structure called a sex pilus, where plasmids are transferred, giving the recipient an altered set of characteristics. Following the transfer, the two bacteria separate. Plasmids can also be transferred by other means. During unfavourable environmental conditions, some bacteria survive by forming dormant or resting cells, called endospores (Figure 10). Endospores are resistant to heat and other extreme conditions, and cannot be easily destroyed. When suitable growing conditions return, the endospore sprouts or germinates, and an active bacterium emerges. attachment DNA (b) DNA replicates Cytoplasm is divided in two, and two new cells are formed. In this case, they also separate. Figure 9 (a) Binary Fission. Features normally associated with mitotic cell division such as centrioles, spindle fibres, and visible chromosomes are not involved in this process. (b) Conjugation. A sex pilus joins two bacteria cells, and plasmids are exchanged. endospore Plasma membrane and cell wall grow. plasmids sex pilus Plasma membrane and wall material start growing through the midsection. Section 2.4 Questions Understanding Concepts 1. Explain the statement “archaebacteria thrive on extremes.” 2. List six ways in which archaebacteria contribute to human society. 3. Design a graphic organizer that could be used to classify eubacteria according to shape, respiration, and nutrition. Give examples of each. 4. Where would you expect to find anaerobic bacteria in nature? 5. Why is conjugation considered a form of sexual reproduction? 6. How has endospore formation guaranteed the survival of bacteria? 7. Identify the following bacteria types: (a) (b) (c) 8. In a laboratory experiment, a student grew a colony of bacteria from a fecal sample. She suspected that the organisms were E. coli. Provide a description of the anatomy and physiology of E. coli to help with the identification. Figure 10 An electron micrograph showing an endospore within a bacterium. A thickened cell wall forms around the genetic material and cytoplasm. The remainder of the original cell eventually disintegrates. 112 Unit 2 Making Connections 9. One of the basic techniques in a college microbiology laboratory is Gram staining. Research this practice. Outline the steps in the procedure and list common examples of Gram-positive and -negative bacteria. GO www.science.nelson.com NEL