General Virus Structure Viruses are very small particles Viruses are classified by what they infect: humans, animals, plants and even bacteria and fungi Below are some examples of viruses: Virus particles (or virions) are unable to reproduce alone and must infect a living host cell in order to make more copies of themselves. Viral Replication: The Lytic cycle Viruses cause serious diseases in animals such as rabies and foot-and-mouth disease Viruses cause many serious human diseases such as hepatitis, polio, influenza and AIDS. Viruses are very small. The virus which causes polio is 20 nanometers in diameter - that means that polio viruses laid out side by side would stretch for one millimeter - about the width of a pinhead This is a three dimensional reconstruction of a polio virus Polio is a small RNA-containing virus. The structure of this virus has been determined by bombarding crystals of virus particles with X-rays. This technique, known as X- ray diffraction shows us the position of every atom in the virus particle. Viral poliomyelitis is now very rare in developed countries due to the success of vaccination programs. Unfortunately polio is still a common illness in developing countries. Polio enters its host via the mouth through contaminated water, food or saliva and replicates itself in the small intestine. In the absence of a strong immune response, the virus enters the blood stream and in about 1% of those infected the polio virus attacks the motor neurons and the central nervous system causing lifelong paralysis or death. Infectious polio virus is excreted in the feces where it can spread to the sewage and water supplies. This is an electron microscope photograph of an adenovirus particle. Adenoviruses are a common cause of sore throats and serious lung infections in humans. This is an electron micrograph of the Herpes Simplex Virus which causes coldsores. Herpes Simplex is an enveloped virus, the DNA and protein part of the viron is surrounded by a membrane which is the thin white structure in this photograph. Viruses cause many diseases in humans, animals and plants. Among the more common viral diseases which you may have suffered from are the common cold (Rhinovirus), chickenpox (Herpes Zoster Virus) and coldsores (Herpes Simplex Virus). Influenza Influenza (or 'flu) is a serious respiratory illness which has killed millions of people. Influenza A viruses can infect man, pigs, chickens, ducks, seals and horses. The influenza virus is transmitted by inhalation of droplets and the virus attaches itself to the cells which line the inside of the throat and lungs. Human Immunodeficiency Virus HIV/AIDS The Human Inmmunodeficiency Virus (HIV) is a retrovirus. Upon entry into the host cell this RNA is copied into DNA by the virus enzyme reverse transcriptase. This cDNA copy of the virus' genetic information can integrate into the host cell chromosomes in the nucleus. This provirus can lay dormant for many cell divisions before being reactivated and producing more infectious retrovirus particles. Viruses cause many serious diseases in humans, what can we do to limit the spread of viral diseases ? Improved standards of sanitation and the supply of clean drinking water can reduce the spread of viruses Other viruses such as the influenza virus and rhinoviruses are spread by sneezing, such viral infections are much more likely to spread in overcrowded conditions. In animals and humans virus infected cells release proteins called interferons which make the cells around them more resistant to attack by viruses What is a vaccine? When a child is immunized, the vaccine causes their immune system to respond and make antibodies against the viruses in the vaccine. These antibodies then destroy the vaccine viruses, but the immune system 'remembers' the virus so that it can fight off infection if the child is ever exposed to the natural viruses Vaccination Many vaccinations have been developed which have nearly eradicated certain diseases Polio, Mumps, Measles, Small pox, chicken pox, perutssis and diptheria are all common vaccines that now prevent the loss of human life Vaccine: Live or Killed A debate continues as to what type of vaccine to use Live may provide better immunity Live may also provide a greater chance of coming down with the disease Bacteria put the tang in yogurt and the sour in sourdough bread; bacteria help to break down dead organic matter; bacteria make up the base of the food web in many environments. Bacteria are of such immense importance because of their extreme flexibility, capacity for rapid growth and reproduction, and great age - the oldest fossils known, nearly 3.5 billion years old, are fossils of bacteria-like organisms. Bacteria lack the membrane-bound nuclei of eukaryotes; their DNA forms a tangle known as a nucleoid, but there is no membrane around the nucleoid, and the DNA is not bound to proteins as it is in eukaryotes. Whereas eukaryote DNA is organized into linear pieces, the chromosomes, bacterial DNA forms loops Classifying bacteria on the basis of their morphology is extremely difficult; bacteria are generally quite small and have simple shapes, though there are some bacteria whose morphology permit classification by shape. In addition to shape, bacteria have traditionally been identified and classified on the basis of their biochemistry and the conditions under which they grow. Bacteria do not contain membrane-bound organelles such as mitochondria or chloroplasts, as eukaryotes do. However, photosynthetic bacteria, such as cyanobacteria, may be filled with tightly packed folds of their outer membrane. The effect of these membranes is to increase the potential surface area on which photosynthesis can take place. Most bacteria may be placed into one of three groups based on their response to gaseous oxygen. 1.Aerobic bacteria thrive in the presence of oxygen and require it for their continued growth and existence. 2.Other bacteria are anaerobic, and cannot tolerate gaseous oxygen, such as those bacteria which live in deep underwater sediments, or those which cause bacterial food poisoning. 3. facultative anaerobes, which prefer growing in the presence of oxygen, but can continue to grow without it. Bacteria may also be classified both by the mode by which they obtain their energy. Classified by the source of their energy, bacteria fall into two categories: 1. Heterotrophs derive energy from breaking down complex organic compounds that they must take in from the environment - - this includes saprobic bacteria found in decaying material, as well as those that rely on fermentation or respiration. 2. Autotrophs, fix carbon dioxide to make their own food source; this may be fueled by light energy a. (photoautotrophic), b. or by oxidation of nitrogen, sulfur, or other elements (chemoautotrophic). While chemoautotrophs are uncommon, photoautotrophs are common and quite diverse. They include the cyanobacteria, green sulfur bacteria, purple sulfur bacteria, and purple nonsulfur bacteria. Bacteria are microscopic single-celled organisms at least 1 micron long. Most bacteria species are harmless to humans; indeed, many are beneficial But some are pathogens, including those that cause Cholera, Diphtheria,Leprosy, Plague,Ppneumonia, Strep throat, Tetanus, tuberculosis, and typhoid fever Bacteria are often maligned as the causes of human and animal disease (like this one, Leptospira, which causes serious disease in livestock). However, certain bacteria, the actinomycetes, produce antibiotics such as streptomycin and nocardicin; Others live symbiotically in the guts of animals (including humans) or elsewhere in their bodies, or on the roots of certain plants, converting nitrogen into a usable form. The cycling of nitrogen is another important activity of bacteria. Plants rely on nitrogen from the soil for their health and growth, and cannot acquire it from the gaseous nitrogen in the atmosphere The cell membrane is surrounded by a cell wall The composition of the cell wall varies among species and is an important character for identifying and classifying bacteria. Bacterium have a fairly thick cell wall made of peptidoglycan (carbohydrate polymers cross-linked by proteins); such bacteria retain a purple color when stained with a dye known as crystal violet, and are known as Gram-positive (after the Danish bacteriologist who developed this staining procedure).. Other bacteria have double cell walls, with a thin inner wall of peptidoglycan and an outer wall of carbohydrates, proteins, and lipids. Such bacteria do not stain purple with crystal violet and are known as Gram-negative Bibliography University of California at Berkley website