Medical brain notes 27

  CLASSIFICATION OF MICROORGANISMS   Bacteria  are very simple, single-celled organisms that   are found virtually everywhere. The natural habitats of bacteria include freshwater, saltwater, soil, and other living organisms. Most bacteria are not harmful to us, and within their normal environments, they have the vital role of decomposing dead organic mate-rial and recycling their nutrients. However, a number of bacteria cause human diseases, including strep throat, pneumonia, and meningitis.   Viruses  are not cells; they are even smaller and   simpler in structure than the bacteria. All viruses are parasites because they can reproduce only within the living cells of a host. Therefore, all viruses cause dis-ease. Common human viral diseases are influenza, the common cold, and chickenpox.   Protozoa  are single-celled animals such as amoe-bas. Most protozoa are free living in freshwater or saltwater, where they consume bacteria, fungi, and one...

  NORMAL FLORA   Each of us has a natural population of microorganisms living on and within us. This is our normal flora. These microbes may be further categorized as resi-dents or transients.  Resident flora  are those species that live on or in nearly everyone almost all the time. These residents live in specific sites, and we provide a very favorable environment for them. Some, such as  Staphylococcus epidermidis , live on the skin. Others, such   as  E. coli , live in the colon and small intestine. When in their natural sites, resident flora do not cause harm to healthy tissue, and some are even beneficial to us. However, residents may become pathogenic if they are introduced into abnormal sites. If  E. coli , for exam-ple, gains access to the urinary bladder, it causes an infection called cystitis. In this situation,  E. coli  is con-sidered an  opportunist , which is a normally harmless species that has become a pathogen in sp...

  INFECTIOUS DISEASE   An infectious disease is one that is caused by microor-ganisms or by the products (toxins) of microorganisms. To cause an infection, a microorganism must enter and establish itself in a host and begin reproducing.   Several factors determine whether a person will develop an infection when exposed to a pathogen. These include the virulence of the pathogen and the resistance of the host.  Virulence  is the ability of the pathogen to cause disease. Host  resistance  is the total of the body’s defenses against pathogens. Our defenses include aspects of innate immunity such as intact skin and mucous membranes, and the sweeping of cilia to clear the respiratory tract, as well as adequate nutri-tion, and the adaptive immune responses of our lym-phocytes and macrophages.   To illustrate these concepts, let us compare the measles virus and rhinoviruses (common cold). The measles virus has at least a 90% infectivity rate, mean-ing tha...

  METHODS OF CONTROL OF MICROBES   Microorganisms are everywhere in our environment, and although we need not always be aware of their presence, there are times when we must try to dimin-   ish or even eliminate them. These situations include the use of chemicals for disinfection, especially in hospitals, and the protection of our food and water supplies.     ANTISEPTICS, DISINFECTANTS, AND STERILIZATION   We are all familiar with the practice of applying iodine, hydrogen peroxide, or alcohol to minor cuts in the skin, and we know the purpose of this: to prevent bac-terial infection. The use of such chemicals does indeed destroy many harmful bacteria, although it has no effect on bacterial spores. The chemicals used to pre-vent infection may be called antiseptics or disinfec-tants. An  antiseptic  (anti 5  against; septic  5  infection) is a chemical that destroys bacteria or inhibits their growth on a living being. The chemicals na...

  THE PATHOGENS   In the sections that follow, each group of pathogens will be described with a summary of important char-acteristics. Examples of specific pathogens will be given to help you become familiar with them. Tables  of important human diseases caused by each group of pathogens.   BACTERIA (SEE TABLE 22–3)   Bacteria are very simple unicellular organisms. All are microscopic in size, and a magnification of 1000 times is usually necessary to see them clearly. A bacterial cell consists of watery cytoplasm and a single chromosome (made of DNA) surrounded by a cell membrane. Enclosing all of these structures is a cell wall, which is strong and often rigid, giving the bacterium its char-acteristic shape.   Based on shape, bacteria are classified as one of three groups: coccus, bacillus, or spirillum (Fig. 22–2). A  coccus  (plural:  cocci ) is a sphere; under the micro-scope, cocci appear round. Certain prefixes may be used to describe t...