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

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    highly basic habitats such as ammonia and soda lakes; xerophiles occupy habitats without water; and radiation-resistant bacteria survive gamma-rays at doses that would kill a human within minutes.
    Deinococcus , for instance, uses an efficient repair system that fixes the damage caused to the DNA molecule by radiation at doses that would kill a human. This system must be quick enough to complete
    the repair before Deinococcus ’ s next cell division.
    All bacteria owe their ruggedness to the rigid cell wall and its main component, peptidoglycan. This large polymer made of repeat—
    ing sugars and peptides (chains of amino acids shorter than proteins
    and lacking the functions of proteins) occurs nowhere else in nature.
     
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    allies and enemies
    Peptidoglycan forms a lattice that gives species their characteristic shape and protects against physical damage. A suspension of bacteria
    can be put in a blender, whipped, and come out unharmed.
    Archaea construct a cell wall out of polymers other than peptidoglycan, but their cell wall plays the same protective role. Furthermore, because archaea have a different cell wall composition than bacteria, they resist all the antibiotics and enzymes that attack bacterial cell walls. This quirk would seem to make archaea especially dangerous pathogens to humans, but on the contrary, no human disease has ever been attributed to an archaean.
    In a microscope, bacteria present an uninspiring collection of
    gray shapes: spheres, rods, ovals, bowling pins, corkscrews, and boomerangs. Microbiologists stain bacteria with dyes to make them
    more pronounced in a light microscope or use advanced types of microscopy such as dark field or phase contrast. Both of these latter
    methods create a stunning view of bacteria illuminated against a dark
    background.
    When bacteria grow, the cell wall prevents any increase in size so
     
    that bacterial growth differs from growth in multicellular organisms.
    Bacteria grow by splitting into two new cells by binary fission. As cell numbers increase, certain species align like a strand of pearls or form clusters resembling grapes. Some bacteria form thin, flat sheets and swarm over moist surfaces. The swarming phenomenon suggests bacteria do not always live as free-floating, or planktonic, beings but can form communities. In fact, bacterial communities represent more than a pile of cells. Communities contain a messaging system in which identical cells or unrelated cells respond to each other and change their behavior. This adaptation is called quorum sensing.
    Quorum sensing begins when cells excrete a steady stream of signal molecules resembling amino acids. The excreted signal travels about 1 ì m so that neighboring cells can detect it with specific proteins on their surface. When the receptors clog with signal molecules, a cell gets the message that other cells have nudged too close; the population has grown too dense. The proteins then turn on a set of
    genes that induce the bacteria to change their behavior. Different types of bacterial communities alter behavior in their own way, yet throughout bacteriology communities offer bacteria a superb survival
    chapter 1 · why the world needs bacteria
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    mechanism. Some communities swarm, others cling to surfaces, and
    yet others can cover a pond’s surface and control the entire pond ecosystem.
    Bacterial communities
    Swarm cells start growing like any other bacterium on laboratory-prepared nutrient medium. (Media are liquids or solids containing gel-like agar that supply bacteria with all the nutrients needed for growth.) They metabolize for a while, split in two, and repeat this until nutrients run low. Rather than halting the colony’s growth, swarm cells signal each other to change the way they reproduce. The swarmer Proteus develops a regular colony when incubated, each cell about three ì m in length. After several hours, cells on the colony’s outer edge elongate to 40 to 80 ì m and sprout