Allies and Enemies: How the World Depends on Bacteria by Anne Maczulak Read Free Book Online

antibiotic-resistant health risk. (Courtesy of BioVir Laboratories, Inc.) Some of these names are familiar because they also cause illness, and yet a person’s normal bacteria pose no problem on healthy, unbroken skin. The native flora in fact keep in check a variety of
    chapter 1 · why the world needs bacteria
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    transient bacteria collected over the course of a day. Some of these
    transients might be pathogenic, but they do not settle permanently on
    the skin because the natives set up squatters’ rights by dominating space and nutrients, and producing compounds—antibiotics and similar compounds called bacteriocins—that ward off intruders. Such silent battles occur continually and without a person’s knowledge.
    Only when the protective barrier breaks due to a cut, scrape, or burn
    does infection gain an upper hand. Even harmless native flora can turn into opportunists and cause infection because conditions change in the body. Immune systems weakened by chemotherapy, organ
    transplant, or chronic disease increase the risk of these opportunistic infections:
    · Staphylococcus —Wound infection
    · Propionibacterium —Acne
    · Bacillus —Foodborne illness
    · Streptococcus —Sore throat
    · Corynebacterium —Endocarditis
     
    · Pseudomonas —Burn infection
    Anaerobic bacteria do not survive in the presence of oxygen, but
    they make up a large proportion of skin flora. Though the skin receives a constant bathing of air, anaerobes thrive in miniscule places called microhabitats where oxygen is scarce. Chapped and flakey skin and minor cuts create anaerobic microhabitats. Necrotic tissue associated with major wounds also attracts anaerobes, explain—ing why gangrene (caused by the anaerobe Clostridium perfringens) and tetanus ( C. tetani) can develop in improperly tended injuries. Of normal anaerobes inhabiting the skin, Propionibacterium acnes (the cause of skin acne), Corynebacterium, Peptostreptococcus, Bacteroides, and additional Clostridium dominate.
    The mouth’s supply of nutrients, water, and microhabitats creates
    a rich bacterial community. Brushing and flossing remove most but
    not all food from between teeth, the periodontal pockets between the
    tooth and the gum, and plaque biofilm on the tooth surface, which
    holds a mixture of proteins, human cells, and bacterial cells. Anaerobes and aerobes find these places and their relative numbers vary
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    allies and enemies
    from daytime to night as the level of aeration, flushing with drinks,
    and saliva production changes. During the day, more air bathes oral
    surfaces and aerobes flourish. At night or during long periods of fast—
    ing, the aerobes consume oxygen and anaerobes begin to prosper. By
    the nature of their fermentations, anaerobes make malodorous end
    products when they digest food. These bad-smelling, sulfur-containing molecules vaporize into the air and become bad breath.
    Few bacteria live in the esophagus and stomach with the exception of the spiral-shaped Helicobacter pylori, occurring in half of all people with peptic ulcers. The discovery of H. pylori in the stomach in 1975 dispelled the long-held belief that no microorganisms could withstand the digestive enzymes and hydrochloric acid in gastric juice. Most bacteria traverse the half gallon of stomach fluid at pH 2
    by hiding in a protective coat of food particles on the way to the small intestine. H. pylori, however, thrives in the stomach by burrowing into the mucus that coats the stomach and protects the organ from its own acids. Inside the mucus, the bacteria secrete the enzyme urease
    that cleaves urea in saliva into carbonate and ammonia. Both compounds create an alkaline shield around H. pylori cells that neutralize the acids.
    The pH rises in the intestines and bacterial numbers increase a
    millionfold from about 1,000 cells per gram of stomach contents, which to a microbiologist is a small number. Humans, cows, pigs, termites, cockroaches, and almost every other animal rely