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Microbiology

1. Introduction

2. Names

3. Microorganisms

4. Lab Procedures

5. Resources

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Microbiology - Microorganisms


3 MICROORGANISMS

3.1 Bacteria (singular: bacterium)

The bacteria are not at all simple, but appear less complex than other microorganisms. They are relatively small. The diameter is typically about 1 - 2 mm, (but can be as little as about 0.1 mm) and if they are not spherical, the length may be about 2 - 10 mm. The spherical ones are called cocci (singular: coccus). Rods are bacilli (singular bacillus) (but remember that Bacillus is also a genus). A rod with some spiral form is a spirillum (while spirochetes are much smaller and flexible). Bacteria increase in numbers by dividing, approximately in half. This can happen in a very short time C as little as 15 minutes C which means that it is possible to grow many generations of bacteria from one day to the next.

When bacterial cultures are grown, there may be very little growth for 3 - 4 hours after inoculation. This period is called the lag phase. Then growth increases rapidly, and a graph of the logarithm of the number of living bacteria versus time becomes a straight line. This is called the log phase (because of the logarithmic relation). Eventually the culture becomes crowded and nutrients are used up, and the growth rate decreases to zero in the stationary phase. That is followed by a death phase as most of the bacteria die. The generation time, or doubling time, is the average time between divisions.

The major structures in bacteria are:

Cell wall A relatively rigid outer layer which supports the cell membrane against osmotic pressure. Some bacteria have a more or less thick “capsule”, usually polysaccharide, outside the cell wall.
Cell membrane
(cytoplasmic membrane)
A complex structure just inside the cell wall. It is a barrier against diffusion, and many important enzyme systems are structurally part of the membrane. Because it is a diffusion barrier, osmotic pressure develops across the membrane, which breaks unless it is supported by the cell wall.
Cytoplasm Everything inside the cell membrane, including nuclear material and many enzymes.
Chromosome The name was initially applied to structures in plant and animal cells because they stained intensely with certain dyes, but the bacterial chromosome is, at best, very difficult to visualize (there is no separate nucleus). There is only one chromosome (i. e., bacteria are haploid), and it is typically circular.
Protoplast In some species, the cell wall can be dissolved by enzymes (notably lysozyme). That leaves the thin flexible cell membrane holding the cytoplasm. Because the cytoplasm is a concentrated solution, water diffuses inward and the protoplast swells and bursts unless the enzyme treatment is done in a solution with high osmotic pressure, such as 20% sucrose. If the protoplast does burst, it leaves a solution of the cytoplasm and a suspension of small structures such as ribosomes, and the empty membrane, which has various names, including “ghost”. Many of the enzymes important for the cell biochemistry are built into the membrane.
Flagellum
(plural: flagella)
Some bacteria have these tiny filaments which propel them through liquids. They appear to be spiral structures which are spun, like propellers, by little power units inside the cell. They may be monotrichous (one filament at one end of the cell), amphitrichous (one or more filaments at both ends), lophotrichous (multiple filaments at the ends) or peritrichous (filaments all around the cell).
Pilus (plural pili) Some bacteria can produce short tubes of protein through which nuclear material can pass between cells (conjugation). They also function to attach bacteria to surfaces. Similar but shorter tubes are “fimbriae” (singular: fimbria).
Spores Some species can produce spores (endospores) which are extremely resistant to drying, heat, and other hostile conditions. Some have been reported to have survived for 1300 years. This is a survival mechanism, not reproduction, as one cell becomes one spore, with no surviving cell. (Molds can produce spores for reproduction; they are not resistant.)
Plasmids relatively short lengths of nucleic acid, in circles or loops, which are found outside the chromosome, but which can also become part of the chromosome. They are very important in genetics and genetic engineering. They are designated by code names such as pBR322 which obviously cannot be translated.


The various bacterial species are capable of a very wide range of activities. One can convert solid sulfur into sulfuric acid (up to about 0.1 N concentration, about pH 1). Another can use cyanide (CN-) as its sole source of carbon, nitrogen, and energy. Still another can grow on substances remaining in good-quality distilled water.

Some bacteria grow well in the presence of air (oxygen). Those are aerobes. Some require oxygen, and are obligate aerobes. Others cannot tolerate oxygen, and are anaerobic. Some facultative organisms can grow with or without oxygen. Still others tolerate oxygen, but grow much better at oxygen concentrations much lower than the concentration in air. Those are microaerophilic. Then there are temperature relations: psychrophiles grow best (though still relatively slowly) at refrigerator temperatures. Mesophiles grow at “ordinary” temperatures (about 20 - 40 C). Thermophiles grow well only at high temperatures (65 - 100 C).

Heterotrophic bacteria require relatively complex materials for growth, while the autotrophic bacteria produce their own complex materials from very simple sources. The organisms which can grow on cyanide as their only source of carbon, nitrogen, and energy are autotrophs. Organisms which require only inorganic materials may be called lithoautotrophs.

One rather vague term, eubacteriales, indicates “true” bacteria, as distinguished from something else. At present, the primary “something else” is represented by the archaeobacteria (or Archaebacteria). Those are found most commonly near ocean thermal vents, at depths about 1500 meters with very high pressures and temperatures up to 250C. They are still very poorly known. Because of their growth conditions, they are extremophiles.

3.2 Yeasts

Like all the organisms other than bacteria, yeasts are eucaryotes (they have nuclei separated inside the nuclear membrane) and have pairs of chromosomes (they are diploid). The cells also contain mitochondria and other organelles. The mitochondria are small structures primarily important for producing most of the energy used by cells. They have their own nuclear materials and reproduce independently of the cells which contain them. They strongly resemble bacterial protoplasts, and may have developed from them. Organelles are various subcellular structures with specific functions.

Yeasts reproduce by fission, but by “budding” rather than by dividing in half. Some species of yeast have been used since ancient times in brewing and baking. Some have been grown for food, but that is not (yet) a major industry. When grown for food, yeast cultures are supplied with abundant air, and they convert their energy source (typically glucose) to carbon dioxide and water. In bread dough, the carbon dioxide produces bubbles which make the bread rise. In the absence of oxygen, yeast grows less well because it can convert glucose only to ethyl alcohol. That provides much less energy than the complete oxidation of alcohol to carbon dioxide. While alcohol production is fermentation, the same term is often applied to any anaerobic culture, and sometimes to microbial culture in general.

3.3 Molds

These are rather like yeasts biochemically, but they divide more like bacteria, usually with the rod-shaped cells remaining connected to form filaments (mycelia; singular: mycelium). They also produce reproductive spores which are not particularly resistant. Molds are strictly (obligately) aerobic. They were the original source, and remain a major source, of antibiotics. One mold, Neurospora crassa, was a major source of information about genetics.

3.4 Algae (singular: alga)

These are microscopic plants (although some large seaweeds are also considered to be algae). They contain chlorophyll and can carry out photosynthesis, using the energy from light for their metabolism rather than getting all their energy by oxidizing glucose (which they do in the dark). Most algae perform photosynthesis in organelles called chloroplasts which might have originated as simpler algae trapped within other cells. The blue-green algae lack the separate chloroplast structure, and contain not only chlorophyll but also a water-soluble blue “phycocyanin”.

Large-scale culture of algae has generally been limited to applied research on closed ecological systems for space travel, in which the algae were to convert the carbon dioxide exhaled by the crew back to oxygen, using light for energy. That work, mostly done in the late 1950s and 1960s failed partly because of the difficulty of providing light efficiently, and largely because the crew would have had to eat all the algae. More recently, algae, especially the blue-green Spirulina, have been grown profitably as food supplements.


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