Post by Rae on Apr 14, 2009 12:05:30 GMT
Microbiology and Biotechnology
Using Multimedia To Revise:
Module 1 - Microbiology
Module 1 - Microbiology
Kingdom Prokaryotae
Contains all prokaryotic organisms, such as bacteria and cyanobacteria.
Bacteria
Bactera can be found in a wide range of habitats. Soil, air, water, as well as on the surface of many plant or animal species. They range from 0.1-10µm in length. One gram of soil may contain 100 million individual bacteria.
- Bacteria with an optimum temperature over 45°c are thermophillic bacteria.
- Bacteria with an optimum temperature below 20°c are psychrophilic bacteria.
Bacteria are important for recycling organic waste. Some cause disease, but most are harmless and are of increasing importance in the field of biotechnology.
Bacteria have no nucleus, but instead have a circular piece of double-stranded DNA often known as the bacterial chromosome. It is different from the DNA of eukaryotes as it is naked (has no protien complex).
- Gram-Staining
Gram-positive bacteria have a plasma membrane surrounded by a rigid cell wall around 20-80 nm thick. This layer is comprised of a peptidoglycan, murein, which has a complex 3D structure.
Gram-negative bacteria also have a rigid cell wall, but it is only around 2-3 nm thick. On the otuside of this there is an outer membrane containing lipopolysaccharides instead of phospholipids. This forms an extra physical barrier to antibiotics and enzymes like lysozyme, which would destroy or inhibit a Gram-positive bacterium.
During Gram staining, the crystal violet/iodine complex becomes trapped within the Gram-positive cell wall, and is more easily washed out of the Gram-negative cell wall.
- Bacterial Shape
When viewed under a microscope, bacteria show several distinct shapes which may help with identification.
- COCCI OR STAPHYLOCOCCI - eg Staphylococcus aureus, causes boils and food poisoning.
- DIPLOCOCCI - eg Diplococcus pneumoniae, causes pneumonia.
- STREPTOCOCCI - eg Streptococcus pyrogenes, causes sore throats, Streptococcus thermophilus, used in yoghurt-making.
- RODS (BACILLI) - eg Salmonella typhi, causes typhoid fever, Escherichia Coli common symbiont of gut.
- CHAINS OF RODS - eg Azotobacter, a free-living nitrogen-fixing bacterium, Bacillius anthracis, causes anthrax.
- SPIRILLA - eg Spirillium rubrum, saprotroph of fresh water, Treponema pallidum, causes the sexually transmitted disease syphilis.
- VIBRIO - eg Vibro cholerae, causes cholera.
- Reproduction
Bacteria grow quickly in favourable conditions, as much as one generation in 20 minutes, though for many species it is 15-20 hours. Division is usually by binary fission.
1. The circular bacterial chromosome divides but there is no mitotic spindle. The chromosome attaches itself to the plasma membrane or the mesosome.
2. A septum starts to be synthesised to divide the cell. This often starts where there are already mesosomes.
3. The septum grows and splits the original cell into two daughter cells.
The cytoplasm of certain bactera contains one or more small loops of DNA called plasmids, which can replicate independant of the bacterial chromosome. They are known to carry genes that help the bacterium survive in adverse conditions. For example, R-factors cause resistance to anitbiotics, viral infection and UV radiation. Plasmids can be transfered to another bacterium in conjugation, transformation or transduction.
Some bacteria have 'mating' cells, which come together joined by pilli in what is known as conjugation. The donor passes a plasmid called the F-factor. This provides the bacterial cell with everything it needs to become a donor, including the synthesis of the sex pilius. The F-factor can exist freely in the cytoplasm, or as part of the bacterial chromosome.
Transformation occurs when one bacterium releases DNA which is then absorbed by another bacterium, which then obtains the same characteristics.
Transduction is where a bacteriophage virus inserts new genetic information into the bacterial chromosome.
Conjugation, transformation and transduction are NOT forms of sexual reproduction since there is no fertilisation.
- Economic Importance
Some genera of bacteria contain species which are commercially useful and some that are harmful to humans.
Rhizobium, the nitrogen-fixing bacterium that lives in soil, is a useful bacteria when fertilising leguminous plants.
Staphylococcus aureus causes boils, but there is a methicillin resistant strain, MRSA. It has developed a resistance to most antibiotics through overuse and now poses a major problem for hospitals worldwide. It is what can cause septicaemia after surgery.
Cyanobacteria
Cyanobacteria a similar to true bacteria, they are photosynthetic, but are not true algae as they have no membrane-bound nucleus, and are considered to be one of the oldest life-forms on Earth. They grow on the surface of fresh or salt water, as slime on rocks and wood or on living organisms such as the sloth. Their photosynthetic pigment is what gives them their green-blue colour.
- Structure
Cyanobacteria have a typical prokayotic cell structure. The cell wall is of similar structure to Gram-negative bacteria. They have chlorophyll and carotenoid pigments in the inforldings of the plasma membrane, called lamellae. They also have photosynthetic pigments such as phycocyanin and phycoerythrin, which are present in phycobilisomes. These pigments are what gives the cell it's distinctive colouration.
- Nitrogen Fixation
Cells able to fix nitrogen contain the enzyme nitrogenase. This is inactivated in the presence of oxygen so conditions inside the cell must be anaerobic. Some cyanobacteria, eg Anabaema, have thick-walled cells called heterocysts. Anabaema have filaments made of many normal photosynthetic cells. Nitrogen-fixing heterocysts are scattered along the filament.
Many filamentous cyanobacteria are able to produce akinetes or spores, able to survive in adverse conditions such as periods of overpopulation (algal 'bloom'). They develop a vegetative wall near to a heterocyst. The cell increases in size and can accumulate large food reserves. Photosynthesis reduces and gas vacuoles in the akinete disappear. This means the akinete slowly sinks to the bottom of the water where it may survive for several years, and will germinate when conditions are favourable.
- Economic Importance
Spirulina platensis and Nostoc which are native in parts of Africa and South America, and are dried and used as food.
Nitrogen fixing cyanobacteria are used on a large scale as organic fertilisers in China, India, Indonesia and the Philippines, particularly in paddy fields where rice is grown. This has been shown to increase the yield of rice by 15-20%
