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Spirochetes have a gram-negative cell wall but #102 | | |
are too small to be seen with the light microscope and so must be visualized with a special darkfield micro scope. Spirochetes are all very slender and tightly coiled. From the inside out, they have a cytoplasm surrounded by an inner cytoplasmic membrane. Like all gram-negative bacteria they then have a thin peptido glycan layer (cell wall) surrounded by the LPS contain ing outer lipoprotein membrane. However, spirochetes are surrounded by an additional phospholipid-rich outer membrane with few exposed proteins; this is thought to protect the spirochetes from immune recog nition ("stealth" organisms). Axial flagella come out of the ends of the spirochete cell wall, but rather than protrude out of the outer membrane (like other bacteria shown in Figure 2-1), the flagella run sideways along the spirochete under the outer membrane sheath. These specialized flagella are called periplasmic flagella. Rotation of these periplasmic flagella spins the spirochete around and generates thrust, propelling them forward. #103 | | |
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Mycoplasma do not have a cell wall. They only #105 | | |
have a simple cell membrane, so they are neither gram positive nor gram-negative. #106 | | |
Fig. 1-9. Summary of morphological differences among the bacteria. #107 | | |
CYTOPLASMIC STRUCTURES Bacterial DNA usually consists of a single circle of #108 | | |
double-stranded DNA Smaller adjacent circles of double-stranded DNA are called plasmids; they often contain antibiotic resistance genes. Ribosomes are composed of protein and RNA and are involved in the translation process, during the synthesis of proteins. Bacteria, which are procaryotes, have smaller ribo somes (70S) than animals (80S), which are eucaryotes. Bacterial ribosomes consist of 2 subunits, a large subunit (50S) and a small subunit (30S). These numbers relate to the rate of sedimentation. Antibiotics, such as erythromycin and tetracycline, have been developed #109 | | |
that attack like magic bullets. They inhibit protein syn thesis preferentially at the bacterial ribosomal subunits while leaving the animal ribosomes alone. Erythromycin works at the 50S subunit, while tetracycline blocks protein synthesis at the 30S subunit. #110 | | |
METABOLIC CHARACTERISTICS Bacteria can be divided into groups based on their #111 | | |
metabolic properties. Two important properties include: #112 | | |
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how the organism deals with oxygen, and 2) what the #114 | | |
organism uses as a carbon and energy source. Other #115 | | |
properties include the different metabolic end-products that bacteria produce such as acid and gas. #116 | | |
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How bacteria deal with oxygen is a major factor in their #118 | | |
classification. Molecular oxygen is very reactive, and #119 | | |
when it snatches up electrons, it can form hydrogen per oxide m202), superoxide radicals (02-), and a hydroxyl #120 | | |
radical (OH•). All of these are toxic unless broken down. In fact, our very own macrophages produce these oxygen radicals to pour over bacteria. There are 3 enzymes that some bacteria possess to break down these oxygen products: #121 | | |
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Catalase breaks down hydrogen peroxide in the #123 | | |
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Peroxidase also breaks down hydrogen peroxide. #127 | | |
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Superoxide dismutase breaks down the super #129 | | |
oxide radical in the following reaction: #130 | | |
Bacteria are classified on a continuum. At one end are those that love oxygen, have all the preceding protective enzymes, and cannot live without oxygen. On the oppo site end are bacteria which have no enzymes and pretty much kick the bucket in the presence of oxygen: #131 | | |
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Obligate aerobes: These critters are just like #133 | | |
us in that they use glycolysis, the Krebs TCA cycle, and the electron transport chain with oxygen as the #134 | | |
final electron acceptor. These guys have all the above enzymes. #135 | | |
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Facultative anaerobes: Don't let this name #137 | | |
fool you! These bacteria are aerobic. They use oxygen as an electron acceptor in their electron transfer chain and have catalase and superoxide dismutase. The only difference is that they can grow in the absence of oxygen by using fermentation for energy. Thus they have the faculty to be anaerobic but prefer aerobic conditions. This is similar to the switch to anaerobic glycolysis that human muscle cells undergo during sprinting. #138 | | |
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Microaerophilic bacteria (also called aerotol #140 | | |
erant anaerobes): These bacteria use fermentation and have no electron transport system. They can toler ate low amounts of oxygen because they have superoxide dismutase (but they have no catalase). #141 | | |
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Obligate anaerobes: These guys hate oxygen #143 | | |
and have no enzymes to defend against it. When you are working on the hospital ward, you will often draw blood for culture. You will put the blood into 2 bottles for growth. One of these is an anaerobic growth media with no oxygen in it! #144 | | |
Fig. 1-10. The oxygen spectrum of the major bacterial groups. #145 | | |
Carbon and Energy Source #146 | | |
Some organisms use light as an energy source (pho #147 | | |
totrophs), and some use chemical compounds as an energy source (chemotrophs). Of the organisms that use chemical sources, those that use inorganic sources, such as ammonium and sulfide, are called autotrophs. Others use organic carbon sources and are called heterotrophs. All the medically important bacteria are chemoheterotrophs because they use chemical and organic compounds, such as glucose, for energy. #148 | | |
Fermentation (glycolysis) is used by many bacteria for oxygen metabolism. In fermentation, glucose is broken down to pyruvic acid, yielding ATP directly. There are different pathways for the breakdown of glucose to pyru vate, but the most common is the Embden-Meyerhof pathway. This is the pathway of glycolysis that we have all studied in biochemistry. Following fermentation the pyruvate must be broken down, and the different end products formed in this process can be used to classify bac teria. Lactic acid, ethanol, propionic acid, butyric acid, acetone, and other mixed acids can be formed. #149 | | |
Respiration is used with the aerobic and facultative anaerobic organisms. Respiration includes glycolysis, Krebs tricarboxylic-acid cycle, and the electron trans port chain coupled with oxidative phosphorylation. These pathways combine to produce ATP. #150 | | |
