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CHAPTER 28: MICROBIOLOGY LECTURE OUTLINE 28.1 The Microbial World Leeuwenhoek was among the first to view microscopic life forms in a drop of water. Louis Pasteur devised the experiments that disproved the theory of spontaneous generation. Microbiology is the study of microbes, a term that includes the viruses, bacteria, archaea, protists, and some fungi. Many microbes have beneficial, and sometimes essential, roles to play in human health and the biosphere. 28.2 Bacteria and Archaea Both bacteria and archaea are prokaryotes. Prokaryotes do not have nuclei or membrane-bound organelles. Biology of Bacteria Bacteria have three basic shapes: rod, spherical, and spiral. All bacterial have a plasma membrane and a cell wall. Bacteria are differentiated by a Gram stain. Most bacteria have a single circular chromosome located in the nucleoid region of the cytoplasm. Bacterial Reproduction Bacteria reproduce asexually by binary fission. Bacterial Metabolism Bacteria demonstrate a remarkable range of metabolic abilities. Some are heterotrophs, some are chemoautotrophs, and some photosynthesize. Bacterial Diseases in Humans Most bacteria do not cause disease, but a significant number do. Pathogenic microbes often carry genes that code for specific virulence factors that determine the type and extent of illness they are capable of causing. Streptococcus Infections More different types of human disease are caused by bacteria from the genus Streptococcus than by any other type of bacterium. Streptococcus bacteria cause pneumonia, meningitis, middle ear infections, tooth decay, strep throat, impetigo, rheumatic fever, and necrotizing fasciitis. Tuberculosis Tuberculosis is one of the leading worldwide causes of death due to infectious disease. It is caused by Mycobacterium tuberculosis, a very slow growing organism. Food Poisoning Various bacteria cause food poisoning, including Salmonella, Staphylococcus, and Clostridium botulinum. Chlamydia Infections Chlamydia bacteria are responsible for trachoma and one of the most common sexually transmitted diseases in the United States. Drug Control of Bacterial Diseases Antibiotics kill or inhibit bacteria by interfering with their unique metabolic pathways. Growing resistance of bacteria to antibiotics has become a significant problem. Biology of Archaea Archaea and bacteria are not close relatives. Most of the archaea studied to date live in extreme environments. Archaeal Structure The plasma membrane of archaea differs markedly from those of bacteria and eukaryotes. Archaea reproduce asexually by binary fission. Archaeal Metabolism Some archaea are heterotrophs, while others are autotrophs. 28.3 Protists Protists are eukaryotes. For the most part, protists are unicellular and microscopic. Protists are structurally diverse. Biology and Diversity of Algae Algae can be unicellular or form colonies or filaments. Some types are multicellular seaweeds. Algae contain chloroplasts and perform photosynthesis. In aqueous environments they are termed phytoplankton. Green Algae Green algae are believed to be closely related to the first plants. Green algae are symbiotic with other organisms, particularly fungi and lichens. Diatoms Diatoms are the most numerous unicellular algae in the oceans. Diatoms have a wide variety of elaborate shells made of silica. Dinoflagellates Dinoflagellates are best known for the red tide they cause when they greatly increase in number. One species in red tides produces a very potent toxin. Red Algae Red algae are mainly multicellular and produce a number of useful gelling agents. Brown Algae The brown algae are the conspicuous multicellular seaweeds. Euglenoids Euglenoids are freshwater unicellular organisms that typify the problem of classifying protists. Many have chloroplasts, but some do not. Biology and Diversity of Protozoans A protozoan is a usually motile, eukaryotic, unicellular protist. This text groups the protozoans as all heterotrophic by ingestion. In aquatic environments, they are part of the zooplankton. Ciliates Ciliates are the largest group of protozoans. All of them have cilia. Paramecium is the most widely known ciliate. Amoeboids Amoeboids move by pseudopods, processes that form when cytoplasm streams forward in a particular direction. Parasitic amoeboids in the genus Entamoeba cause amoebic dysentery. Zooflagellates Zooflagellates are heterotrophic protozoans that propel themselves using one or more flagella. Most are symbiotic, and many are parasitic, causing African sleeping sickness, Chagas disease, and epidemics of diarrhea. Sporozoans Sporozoans produce spores and are either intercellular or extracellular parasites. Malaria Malaria is the most widespread and dangerous sporozoan disease. The malaria parasite, one of several Plasmodium species, has a complex life cycle that involves transmission by a mosquito vector. Other Sporozoan Diseases Other sporozoan diseases include Toxoplasma infection and Crytosporidium infection. Molds as Protists Water molds and slime molds are classified as protists. Water Molds Most water molds are saprotrophic. They usually live in water. Slime Molds Slime molds feed on dead plant material in forests and woodlands. Cellular Slime Molds Cellular slime molds exist as individual amoeboid cells. 28.4 Fungi Fungi are a structurally diverse group of eukaryotes that are strict heterotrophs. Most fungi are saprotrophs. Biology of Fungi The body of a fungus is composed of a mass of individual filaments called hyphae; collectively, the mass of filaments is called a mycelium. The cell walls of fungal cells contain chitin, not cellulose. The energy reserve of fungi is not starch, but glycogen. Fungi produce windblown spores during both asexual and sexual reproduction. Diversity of Fungi Fungi are differentiated on the basis of their mode of sexual reproduction. Zygospore Fungi Stalks bear sporangia, capsules that produce zygospores. The common bread mold belongs to this group. Sac Fungi The sac fungi are named for their characteristic cuplike sexual reproductive structure called an ascocarp. Important sac fungi include morels and truffles, and the original source of penicillin. Yeasts The term yeasts is generally applied to unicellular fungi, and many of these organisms are sac fungi. Brewer’s yeast is a representative of budding yeasts. Club Fungi Club fungi are named for their characteristic sexual reproductive structure called a basidium. Mushrooms and shelf fungi are basidiocarps. Environmental Aspects of Fungi Fungi often have symbiotic relationships with other organisms. Fungi and Photosynthesizers Lichens are associations between fungi and cyanobacteria or green algae. Lichens are efficient at acquiring nutrients and moisture, and therefore can survive in poor soils as well as on rocks. Mycorrhizal fungi form mutualistic relationships with the roots of most plants, helping them grow more successfully in dry or poor soils. Fungal Diseases of Plants Smuts and rusts are club fungi that parasitize cereal crops. Fungal Diseases of Humans Mycoses, fungal diseases of humans, vary in levels of seriousness. Moldlike fungi cause infections of the skin called tineas. Histoplasma capsulatum can cause a mild “fungal flu” or serious disease. Candida albicans causes the widest variety of fungal infections. Control of Fungi The strong similarities between fungal cells and human cells make it difficult to design antimicrobials against fungi. 25.5 Viruses, Viroids, and Prions Viruses are not composed of cells, and are not alive. Viroids are strands of RNA that can reproduce inside a cell, and prions are protein molecules that cause other proteins to become prions. Biology of Viruses There is considerable variety in the structure and reproduction of viruses. Viral Structure Most viruses are much smaller than bacteria. They come in a variety of shapes. A virus always has at least two parts—a capsid composed of protein subunits and an inner core of either DNA or RNA. In some viruses, the capsid is surrounded by a membrane called an envelope. Viral Reproduction Viruses are specific to a particular host. The life cycle of a typical enveloped animal RNA virus has six steps: attachment, entry, replication, biosynthesis, assembly, and budding. Latency Some animal viruses can become latent (hidden) inside the host cell. Retroviruses have a genome of RNA, but are able to convert their genome into DNA because they contain an enzyme called reverse transcriptase. Viral Diseases in Humans Viruses cause many important human diseases. The best protection against most viral diseases is immunization utilizing a vaccine. The Common Cold and Influenza Colds are most commonly caused by rhinoviruses, and the symptoms usually include a runny nose, mild fever, and fatigue. The flu is caused by the influenza virus and is characterized by more severe symptoms. The influenza virus can change rapidly via antigenic drift and antigenic shift. Measles Measles is one of the most contagious human diseases. It is a major killer worldwide. A vaccine exists for measles. Herpesviruses Herpesviruses cause chronic infections that remain latent for much of the time. These include cold sores and fever blisters, chickenpox, and infectious mononucleosis. Antiviral Drugs Because viruses use the machinery of host cells for viral replication, it is difficult to develop drugs that affect viral replication without harming host cells. Antibiotics are not effective against viral infections. Viroids and Prions Viroids and prions are also acellular pathogens. Viroid replication causes diseases in plants, the only known hosts. Prions are proteinaceous infectious particles that cause degenerative diseases of the nervous system in humans and other animals. These disease include scrapie, Creutzfeldt-Jakob disease, and mad cow disease.