Allies and Enemies: How the World Depends on Bacteria Read Online Free PDF

By contrast, the gram-negative cell wall is
    more complex. The thin peptidoglycan in gram-negatives lies in
    between membranes on both the outer and inner surfaces of the cell.
    The thinness of the layer has been proposed as one reason why gram-negative cells cannot hold onto the stain.
    Few hard and fast rules can be attributed to gram-positive and gram-negative populations. Gram-negative bacteria were once
    thought to be more numerous than gram-positives and have a higher
    proportion of pathogens, but these generalizations probably hold little merit. The Gram reaction nevertheless helps gives clues to microbiologists about potential trouble. Food, water, consumer products such as shampoo, and skin with high concentrations of gram-negative bacteria signal possible fecal contamination. That is because E. coli and all other bacteria in its family come from animal intestines. But gram-positive bacteria are not totally benign. Gram-positive bacteria
    recovered from a person’s upper respiratory tract might indicate strep
    throat (from Streptococcus) or tuberculosis. Skin wounds infected
    with gram-positives range in seriousness from Staph infections (from Staphylococcus ) to anthrax. In the environment, the known gram-negative and gram-positive species distribute almost evenly in soils and waters.
    During the time Gram worked out his new procedure, German
    physician Walther Hesse left his job of ten years tending to uranium
    miners in Saxony who were dying of lung cancer (although the disease had not yet been identified). After two years in Munich working
    in public hygiene, he became an assistant to Robert Koch who was
    second only to Louis Pasteur as the world’s eminent authority on microbes. Originally a country doctor in a small German village, Koch
    had already immersed himself in the behavior of anthrax and tuberculosis bacteria in test animals. From these studies he began developing a procedure for proving that a given bacterial species caused a specific disease. In 1876, Koch established a set of criteria that a bacterium must meet in test animals to be identified as the cause of disease. The criteria to become known as Koch’s postulates laid the foundation for diagnosis of infectious disease that continues today.
     
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    allies and enemies
    Medical historians have debated whether the criteria attributed
    to Robert Koch should be called the Henle-Koch postulates. Koch received his early training under German physician Jacob Henle who
    in 1840 published a list of criteria for confirming the cause of infectious disease. The criteria proposed by Koch were similar to Henle’s, but the origin of Koch’s postulates probably came by a gradual evolution of ideas with each new experiment on pathogens. I explain Koch’s postulates here: 1. The same pathogen must be present in every case of a disease.
    2. The pathogen must be isolated from the diseased host and grown in a laboratory to show it is alive.
    3. The pathogen should be checked to confirm its purity and then injected into a healthy host (a laboratory animal).
    4. The injected pathogen must cause the same disease in the new host.
    5. The pathogen must be recovered from the new host and again
    grown in the laboratory.
     
    Some bacteria do not conform to Koch’s postulates. For example
    Mycobacterium tuberculosis , the cause of tuberculosis, also infects the skin and bones in addition to the lungs. Streptococcus pyogenes causes sore throat, scarlet fever, skin diseases, and bone infections.
    Pathogens that cause several different disease conditions can be difficult to fit into the criteria for diagnosing a single disease.
    In developing these criteria, Koch made another contribution to
    the fundamentals of microbiology by introducing a way to obtain pure cultures. For Koch’s postulates to work, a microbiologist
    needed a pure culture of the potential pathogen. Without bacteria in
    pure form, no one would be able to prove bacterium A caused disease A, bacterium B