Objective 3 Viruses & Bacteria 1. Compare the structure and functions of viruses to cells and describe the role of viruses in causing diseases and conditions such as acquired immune deficiency syndrome, common cold, smallpox, influenza and warts. 2. Identify and describe the role of bacteria in maintaining health such as in digestion and causing diseases such as in streptococcus infections and diphtheria The genetic material found in a virus can be either DNA or RNA, but not both. The DNA or RNA can be either double-stranded or single-stranded. The genetic material of the virus contains the instructions necessary for the production of the capsid and spike proteins and also directs the replication of the viral genetic material. Some viruses are able to insert a copy of their genetic material into the DNA of the host cell, thereby ensuring that each time the host cell replicates the viral genetic material is also copied. The insertion of the viral genetic material into the host genome can result in uncontrolled growth of the cell, leading to tumor formation. Unlike viruses, bacteria display all of the characteristics associated with living organisms. Bacteria play an important role in the recycling of nutrients, photosynthesis, nitrogen fixation, and bioremediation. Most bacteria are beneficial to humans; however, some bacteria are pathogenic (i.e., able to cause illness). Bacteria possess a number of structures that allow them to attach to target surfaces or tissues. Many bacteria are surrounded by a capsule, which is composed of polysaccharides and glycoproteins. The capsule is important in bacteria that cause disease because it prevents phagocytosis by the host's white blood cells. The sticky nature of the capsule also allows the bacterium to bind to target surfaces. For example, the slimy feel of your teeth in the morning is caused by the capsules of bacteria that have attached to your teeth. Another factor that makes the capsule important is that other microorganisms can stick to the capsule, resulting in the formation of a biofilm (a community of microorganisms). Pili are short, hair-like appendages that protrude through the capsule and serve in the attachment of bacteria to surfaces. The ability of the pili to attach to host cell receptors determines the type of tissues that a bacterium can colonize. A specialized pilus known as the sex pilus is involved in the transfer of DNA from one bacterium to another via a process known as conjugation. This ability to transfer DNA is an important factor in the spread of antibiotic resistance genes through a microbial community. Another difference between bacteria and viruses is motility. Viruses do not possess structures that enable them to move purposefully through the environment. Bacteria, on the other hand, are able to move through the environment using a structure known as the
flagellum. Surrounding the bacterial plasma membrane is a cell wall composed of peptidoglycan. Peptidoglycan is composed of two glucose derivatives (N-acetylmuramic acid and N-acetylglucosamine) that are joined together into long chains. These long carbohydrate chains are then joined together by short amino acid chains, and the entire molecule is stabilized by additional peptide chains. The cell wall has a number of functions, including maintaining the shape of the cell, preventing the rupture of the cell in response to osmotic stress, and in some cases, contributing to the virulence of a bacterium. Antibiotics such as penicillin are effective because they disrupt the synthesis of peptidoglycan, thereby making the bacterium more susceptible to osmotic stress. Some bacteria, referred to as gram-negative bacteria, possess an additional outer membrane that surrounds the cell wall. A few groups of bacteria are able to produce structures referred to as endospores in times of nutritional stress. These endospores are difficult to kill and will remain dormant until introduced into an environment that is nutrient rich. Some examples of bacteria that produce these structures are Clostridium tetani (the causative agent of tetanus) and Bacillus anthracis, the causative agent of anthrax. Most of us at one time or another have had colds or the flu, and we are especially vulnerable during the cold and flu season. The symptoms -- fever, congestion, coughing, sore throat -- spread through offices, schools and homes, no matter where in the world we live. Colds and flu (influenza) are caused by viruses. Viruses are responsible for many other serious, often deadly, diseases including acquired immunodeficiency syndrome (AIDS), Ebola hemorrhagic fever, infectious hepatitis and herpes. How can viruses cause so much trouble? What makes us so vulnerable to them, and what makes them spread? A virus particle, or virion, consists of the following: Nucleic acid - Set of genetic instructions, either DNA or RNA, either single-stranded or double-stranded Coat of protein - Surrounds the DNA or RNA to protect it Lipid membrane - Surrounds the protein coat (found only in some viruses, including influenza; these types of viruses are called enveloped viruses as opposed to naked viruses)
Regardless of the type of host cell, all viruses follow the same basic steps in what is known as the lytic cycle (see figure): 1. A virus particle attaches to a host cell. All viruses have some type of protein on the outside coat or envelope that "feels" or "recognizes" the proper host cell(s). 2. The particle releases its genetic instructions into the host cell. 3. The injected genetic material recruits the host cell's enzymes. 4. The enzymes make parts for more new virus particles. 5. The new particles assemble the parts into new viruses. 6. The new particles break free from the host cell Your immune system responds to the infection, and in the process of fighting, it produces chemicals called pyrogens that cause your body temperature to increase. This fever actually helps you to fight the infection by slowing down the rate of viral reproduction, because most of your body's chemical reactions have an optimal temperature of 98.6 degrees Fahrenheit (37 degrees Celsius). If your temperature rises slightly above this, the reactions slow down. This immune response continues until the viruses are eliminated from your body. However, if you sneeze, you can spread thousands of new viruses into the environment to await another host
Bacteria & Disease The first thing the bacterium has to do is enter your system. We come into contact with millions of bacteria every day. They are in the air we breathe, in and on the food we eat and on the surfaces of most things we touch. Apart from our normal flora, bacteria that come into contact with us have to pass our various defense mechanisms, our dry skin and our acid stomach. Physical actions such as the movement of matter through our alimentary canal, brushing our teeth and washing all help to make life difficult for bacteria. Those bacteria which do colonize our system generally do so by breaking through the mucus barrier that lines most of our alimentary canal (mouth to anus), or entering through damaged tissue, ie wounds and bites, etc. Once a bacterium has entered the system it is free to grow and spread Infection simply means the bacterial or other agent entering the system. It does not equate with disease or damage. You can be infected by an organism that never makes you ill. An infectious agent is simply an organism that is capable of getting past your defenses and then living/growing inside or you. Bacteria rarely, if ever, cause disease merely by being present The pathogenicity of invasive bacteria, or their ability to cause disease is generally the result of toxins. Substances produced by the bacterial cell, sometimes simply as a by-produce of its normal metabolism, which interact negatively with our body, by interfering with the normal functioning. This is often done by simply damaging the specific cells, blocking the transmission of some sort of internal signals or other or by over stimulating some sorts of cells so they malfunction. The ecological reasons for the production of these toxins is not always understood. Also, because of the vagaries of bacterial genetic reproduction, otherwise harmless species can acquire a gene which causes them to secrete a toxin, thus making them a pathogenic strain of a normally harmless species. A good example of this is Escherichia coli which lives harmlessly in most people's intestines but which occasionally makes us sick. Toxins that are leaked or secreted out of the bacteria cell and into its host (you and me) is called an Exotoxin. Cholera toxin works by disrupting the ionic balance of cells' membranes which results in the cells of the small intestine secreting large amounts of water into the intestine. This keeps happening and has two effects; all the water in the small intestine causes diarrhea and thus the body dehydrates. Eventually the small intestine loses water faster than the large intestine can reabsorb it and death follows from dehydration. Clostridium botulinum produces toxins which are among the most poisonous or toxic substances known. One milligram of pure Botulinum toxin is enough to kill 1 million guinea pigs. Beneficial Bacteria: Help make essential soil mineral elements, available to the plant Nitrogen Fixation Decompose organic matter and improve physical properties of the soil Vast numbers of bacteria live in our bodies. One example is found in the intestine. This bacteria and humans have formed a symbiosis with each other. The bacteria help us with digestion and to produce vitamins. In exchange, they soak up a little
extra food for themselves Most dairy products are made by or with the help of bacteria. Some dairy foods are cheese, buttermilk, yogurt, and sour cream. Some other kinds of foods that involve bacteria in their production are pickles and high fructose corn syrup. Bacteria help in the production of fuel in two major ways. Some bacteria decompose compost, garbage and sewage and help make methane. Methane is a valuable natural gas. It is used widely as a fuel. Also, over time, the earth's pressure has changed dead and decomposed animals and plants into coal, which is also a widespread fuel. Bacteria also make, or help to make, drugs, hormones, or antibodies. Bacteria can even help to break down oil to make clean-up after an oil spill easier.