Micro-organisms PDF

Micro-organisms Viruses, bacteria and fungi  Big fleas have little fleas upon their backs to bite ‘em And little fleas have lesser ones and so ad infinitum  The viruses Virus is a Latin word meaning poison These are subcellular, nonliving particles tha...

Micro-organisms Viruses, bacteria and fungi  Big fleas have little fleas upon their backs to bite ‘em And little fleas have lesser ones and so ad infinitum  The viruses Virus is a Latin word meaning poison These are subcellular, nonliving particles that multiply only when they are inside living cells. They appear in different shapes, but have many features in common Louis Pasteur in 1884 suggested that something smaller than bacteria was the cause of rabies. In 1892, Dimitri Ivanowsky observed that the cause of tobacco mosaic disease could be filtered through a fine- pore filter, enhancing Pasteur’s suggestion Following the development of the electron microscope in the 20th century, viruses were seen for the first time.  Viral structure The size of a virus is comparable to that of a large protein Viruses can be purified and stored in crystalline form If given a chance to invade a living cell, they become infectious. A virus structure is usually made of an outer coat, called the capsid surrounding either DNA or RNA The coat may or may not be surrounded by an envelope Human Immunodeficiency Virus. A schematic diagram of HIV reveals its proteins and nucleic acid components. The membrane-envelope glyco-proteins are shown in dark and light green. The viral RNA is shown in red, and molecules of reverse transcriptase are shown in blue The influenza virus A model showing the structure of part of the tobacco mosaic virus, TMV. The blue strand is the single strand of RNA and the rest is protein  Classification Viruses may be classified according to various criteria, but the most important one is the type of nucleic acid Accordingly, we recognize DNA viruses and RNA viruses RNA viruses (also called retroviruses) have a reverse transcriptase enzyme capable of synthesizing DNA from their genomic RNA Human immunodeficiency virus (HIV), which causes AIDS, is a retrovirus  Lytic and lysogenic life cycles Viral infection may cause cell lysis when the virus goes through a cycle of replications, producing many new virus particles, (i.e. lytic life cycle). With other infections, the genetic material of the virus becomes integrated with the host cell DNA. The host cell remains alive and continues to release virus particles at a slow rate. This is known as the lysogenic life cycle. A virus that infects a bacterium is called a bacteriophage or simply a phage During the lysogenic life cycle, the host cell remains alive and continues to release virus particles at a slow rate (not shown on the diagram)  Bacteria (singular, bacterium) Bacteria are prokaryotic single celled organisms that lack nuclei (and other internal organelles) but are capable of independent existence. Some of these bacteria are parasitic on humans and cause many diseases The major characteristic of bacterial cells is the cell wall that is made of peptidoglycan Depending on the thickness of the peptidoglycan in the cell wall bacteria are either Gram+ or Gram- Gram Positive and Gram Negative bacteria The Gram stain was named after the Danish bacteriologist Hans Christian Gram who discovered the procedure in the late 1880s Bacteria with a thick peptidoglycan layer will retain the dye and appear purple under the light microscope While Gram negative bacteria, with a thin layer of peptidoglycan, do not retain the dye and appear pink Gram positive cocci Singular, coccus The third bacterial shape (not shown here) is the rod- shaped or Gram negative bacillus. spirillum Plural, bacilli Plural, spirilla  Pathogenic bacteria Most bacteria are free-living, but a few cause human diseases that can often be cured by antibiotic therapy To be a pathogen, a bacterium must: have an ability to pass from one host to the next penetrate into the host’s tissues withstand the host’s defense mechanisms induce illness in the host  Examples of diseases caused by bacteria Mycobacterium tuberculosis causes TB Mycobacterium leprae causes leprosy Helicobacter pylori causes peptic ulcer Cholera is caused by the bacterium Vibrio cholerae Salmonella enterica causes typhoid fever Dental caries (tooth decay) results from bacteria metabolizing sugars on teeth surfaces. Streptoccoccus mutans is one of the main cause of dental caries.  Antibiotics An antibiotic is a drug that kills or stops the growth of bacteria, without harming the cells of the infected organism. Antibiotics are derived from living organisms, although they are often made more effective by various chemical processes. There are a wide range of antibiotics to treat bacterial infections. Other antimicrobial drugs such as isoniazid (used for the treatment of TB) are synthetic  How Antibiotics Work? Antibiotics interfere with some aspect of growth or metabolism of the target bacterium. These include: Synthesis of bacterial cell walls Activity of proteins in the cell membrane Enzyme action (by enzyme or coenzyme inhibition) DNA synthesis (replication) RNA synthesis (transcription) Protein synthesis (translation)  Bacterial Growth Culture Media – A medium is a solid or liquid preparation containing nutrients for the culture (growth) of micro-organisms, animal or plant cells. e.g. nutrient agar – A culture is a collection of “microbial” cells growing on or in a medium.  Some types of media Enrichment media: – These are media in which substances are added to meet the requirements of certain micro-organisms in preference to others. Selective media: – A selective medium is one in which substances are added which inhibit the growth of all but one or a few micro-organisms  Some techniques Aseptic Technique – This refers to using sterilized equipment and solutions and preventing their contamination while in use; as by flame heating or the use of filtered sterile air. Inoculation – The introduction of a small number of micro- organisms into a nutrient medium is known as inoculation. Bacterial Growth in Culture Media In a nutrient medium at optimal growth conditions, bacteria divide (by binary fission) every 30 minutes. A single cell would produce some ten million cells in 8 hours – exponential growth! Bacterial Growth curve shows four phases: – Lag, log (logarithmic), stationary and decline phases  3 2 4 1  The Phases Lag phase: bacteria adapting to medium; it needs time to secrete enzymes, digest food.. Log phase: growth at maximal rate Stationary phase: zero growth rate, due to competition for resources: growth = death Decline phase: Growth > Death due to: – Exhaustion of nutrients, accumulation of toxic waste, change in pH, ….. etc  Importance of Bacteria Medical products: eg insulin by genetic engineering Foods and drinks: dairy products Environmental: biogas production Agriculture: Nitrogen fixation The expansion in genetic engineering or Molecular biology makes the potential of micro-organisms unlimited  Fungi Fungi have some similarities with plants, but none of them is able to photosynthesize. They are all heterotrophic, obtaining energy and carbon from dead and decaying matter or by feeding as parasites on living organisms. There is a vast range in size from the microscopic yeasts to what may be the world’s largest organism.  Fungi A specimen of the honey fungus, Armillaria bulbosa, grows in a forest in Wisconsin, USA and spreads over 160 000 m2. Not only is it possibly the largest organism in the world, but it may also be the oldest at 1500 to 10 000 years old; its estimated mass is 100 tonnes. Characteristic features of fungi are: Eukaryotic Do not have chlorophyll and do not photosynthesize Heterotrophic nutrition – they use organic compounds made by other organisms as their source of energy and source of molecules for metabolism Reproduce by means of spores Characteristic features of fungi are: Simple body form, which may be unicellular or made up of long threads called hyphae (with or without crosswalls); Large fungi such as mushrooms produce large compacted masses of hyphae known as ‘fruiting bodies’ to release spores Cells have cell walls made of chitin or other substances, not cellulose Never have cilia or flagella The bread mould fungus, Rhizopus nigricans, and a detail of the end of one hypha. THANKS