NATURE OF MICROBES WORKBOOK

Transcription

1 NATURE OF MICROBES WORKBOOK Name: Tutor Group: 1

2 Microbes and Mankind 4. NATURE OF MICROBES 1. OBJECTIVES: What are microbes and are there different types? How are they seen? How can they be grown? How were their effects first proved? By the end of this topic you should: (a) Know that microbes include bacteria, viruses and fungi. A bacterial cell consists of cytoplasm, cell membrane and a cell wall. There is no distinct nucleus. Bacteria reproduce by dividing into two. (b) Know that viruses are smaller than bacteria. They consist of a protein coat surrounding a small number of genes and can only reproduce inside a host cell. The production of new viruses results in the destruction of the host cell and the release of new viruses which can then attack new cells. (c) Know that yeasts are fungi. Yeast cells are larger than bacteria and have a nucleus, cytoplasm and a membrane surrounded by a cell wall. Yeasts reproduce by budding. (d) Understand the safe use of basic aseptic techniques involved in inoculating, plating and incubating microbes. (e) Investigate the presence of bacteria in milk using agar plates. (f) Explore information about the effect of temperature on the growth of bacteria and understand its application in food storage. (g) Understand how Pasteur used scientific method to devise experiments and make deductions. This resolved the controversy about the, traditionally held, theory of 'spontaneous generation', whereby non-living substances could give rise to life, and proved the role of micro organisms as the source of food spoilage. 2. Look at the video, OR MSS presentation OR PowerPoint presentation, OR read your textbook on GROUPS OF MICROBES. Discuss the different groups of microbes with your teacher. a. Make a list of the 3 main groups of microbe. 3. Construct a BACTERIA FACTFILE, including a LABELLED DRAWING showing the structure of a bacterium; 2

3 where bacteria grow; anywhere warm, moist & with food (not salty!); e.g. in living things (parasitic); soil, freshwater, rotting organic matter, sewage, etc. how they reproduce; binary fission: asexual; cell just divides into 2, grows then divides again. Can happen every 20 minutes! examples of useful bacteria; cheese & yoghurt manufacture ( good bacteria Yakult!); sewage bacteria; decay bacteria; common diseases caused by pathogenic bacteria. Tetanus, Typhoid, Tuberculosis (TB), Cholera, Salmonella, Bacterial Meningitis. 3

4 4. Construct a VIRUSES FACTFILE including a LABELLED DRAWING showing the structure of a virus; where viruses grow; ONLY in other living cells they are all PARASITES! how they reproduce; How viruses reproduce. 1. Viral DNA is injected into host cell. 2. Viral DNA becomes incorporated into host cell DNA, in the nucleus. 3. The viral DNA instructs enzymes in the host cell to make new virus protein coats. 4. The viral DNA also multiplies inside the host cell. 5. The new viral DNA & protein coats join together to make new viruses. 6. The host cell bursts releasing the new viruses. 7. The new viruses are now free to infect other cells. the effect of viruses on host cells; They usually kill them! common diseases caused by pathogenic viruses. Cold, flu, AIDS, measles, rubella, viral meningitis, mumps. 4

5 5. Construct a YEAST FACTFILE, including a LABELLED DRAWING showing how the size and the structure of yeast differs from that of a bacterium; yeast cells are much larger than bacteria (4 12 µm = 4/ /1000 mm) similar in size to a red blood cell. Fungal cells are similar to plant cells BUT lack chloroplasts & chlorophyll AND they store glycogen, not starch. where yeasts grow naturally; Anywhere warm, moist & with a good supply of carbohydrate mouldy bread, fruit, etc. Some yeasts are parasitic & grow in living things. how they reproduce; Asexually, by BUDDING (b4u, P 154) a kind of mitosis. Also by sexual reproduction, where 2 cells join & exchange genes. examples of uses made by yeast. Brewing yeast feeds on sugar & converts it to ALCOHOL (ETHANOL), & releases CARBON DIOXIDE (an example of ANAEROBIC RESPIRATION). Bread-making the carbon dioxide released causes bread dough to RISE ( leavened bread). 5

6 6. Look at the video, OR MSS presentation OR PowerPoint presentation, OR read your textbook on GROWING MICROBES. Discuss with your teacher how bacteria can be grown on agar plates. If possible, watch the demonstration of inoculating an agar plate. Answer the following questions carefully: a. What is agar? A jelly (made of seaweed) which bacteria (& fungi) grow well on. b. What is nutrient agar? As above, plus nutrients to encourage healthy growth of bacteria. c. What is a Petri dish? A round (glass or plastic) dish with a lid. Transparent, for observing growth of bacteria. d. What is a nutrient agar plate? A petri dish filled with solidified nutrient agar. e. Explain the meaning of the term aseptic techniques. Carrying out a procedure which ensures no bacteria are transmitted EXCEPT those being studied. f. Suggest 3 ways of maintaining aseptic conditions when culturing bacteria. 1. Never remove lid of petri dish, 2. Always sterilise equipment being used, e.g. flame the wire loop, by heating it in a blue Bunsen flame for a few seconds. 3. Wash hands or wear gloves while inoculating agar plates. g. Explain clearly how to inoculate bacteria onto a nutrient agar plate. 1. Flame the wire loop, & allow to cool for a few seconds. 2. Dip the loop into the sample medium (e.g. soil, water, milk). 3. Lift the lid of the dish and inoculate the plate in a square/diamond, each side consisting of 3 parallel lines. 6

7 4. Seal the plate AROUND THE EDGE, leaving a 1 cm air gap. h. Explain how bacteria are cultured on a nutrient agar plate. 1. Leave the plate in a warm, dark place for 24 hours. Optimum temperature is 37 C. 2. Examine the plate and count the colonies that have grown. Each colony has developed from ONE bacterium. i. Draw a typical agar plate which has been inoculated and cultured. Label the small circles and explain how each circle develops. One colony of bacteria Each circle is a colony of thousands of bacteria which have grown from the binary fission (division) of a single bacterium. 7. Investigating the presence of bacteria in milk using agar plates. You are provided with samples of milk of different freshness. It has been suggested that milk turns sour because of large numbers of bacteria growing in the milk. Plan and carry out an experiment to find out whether there are more bacteria in sour milk. a. Outline of plan. 1. Inoculate a small sample of milk onto separate agar plates using a sterile loop. 2. Seal the plates with sellotape & label clearly on the edge. 3. Incubate a t room temperature for hours. 7

8 b. Independent variable. Age of milk c. Dependent variable. Number of colonies growing on the plate. d. Control variables. Type of agar used Type of milk used Method of inoculation Temperature of incubation Time of incubation Once you have had your plan approved, set up your agar plates and leave them to incubate for 24 hours at 37 C. Once your plates are set up, continue with section 8. Once your plates have incubated, examine each one (N.B. DO NOT TAKE OFF THE LIDS!), and draw or describe the appearance of each plate in the results section. Results. Sample Age (weeks) No. of colonies (circles) 1 4 Dozens! 2 3 Hundreds! 3 2 Hundreds! 4 1 Hundreds! 5 0 Very few (1 or 2 mould colonies) Conclusion. Explain whether your results support the original idea. Our hypothesis was correct EXCEPT for the oldest milk. This might be because the bacteria are running out of food in the milk, or even being poisoned by their own waste. Pasteurised milk has been treated to kill MOST bacteria, but not all. This is shown in sample 5 a few bacteria were present, which then multiply rapidly, producing the large numbers shown in samples

9 Evaluation. Suggest limitations of the techniques used and possible improvements you could make. 1. Use milk days old, rather than weeks old, as this would have shown a gradual increase in the number of bacteria present, rather than the huge numbers shown in samples Some samples were contaminated. We need to improve our aseptic techniques, e.g. wash hands, sterile gloves, face masks, sterilise bench, etc. 3. The plates were incubated for 48 hours which was too long. 36 hours would have produced better results. 8. The effect of temperature on the growth of bacteria In common with all living organisms, the activity of bacteria is controlled by ENZYMES. This activity includes GROWTH and REPRODUCTION. In an investigation, harmless bacteria were inoculated onto nutrient agar plates and then incubated at different temperatures for 24 hours. The number of colonies of bacteria which grew were counted and recorded. The results are shown in the table below: Temperature ( C) No of bacterial colonies after 24 hrs a. Draw a line graph in the space below: 9

10 b. EXPLAIN fully the shape of the graph, at: (i) Low temperatures Too cold for bacterial reproduction. As temperature increases, more kinetic energy is supplied, which increases ENZYME activity, so bacteria reproduce more quickly. (ii) Medium temperatures Optimum temperature for bacterial reproduction. Enzyme activity is at a maximum. (iii) High temperatures Increasingly, bacterial enzymes are DENATURED by the high temperatures, eventually only a few resistant strains can survive. c. How have the results of this investigation been put to good use by people in the food industry attempting to find ways of preventing food being spoiled by bacterial decay? List as many different examples as you can think of to support your answer. Keep food cold (fridge or freezer) to prevent bacterial reproduction. Heat food - Boil / sterilise / cook / pasteurise. This kills bacteria & denatures their enzymes. 10

11 9. Independent research: Louis Pasteur. Produce 1 or 2 A4 pages highlighting some of the important discoveries made by Louis Pasteur in the 19 th Century. One of these MUST be the way he proved that microbes (bacteria) did not arise by 'spontaneous generation'. Another MUST be how he proved the role of micro organisms as the source of food spoilage. Ensure you include information about how Pasteur used the scientific method to devise experiments and make deductions. 10. Well done! You have now completed the Nature of Microbes topic. Now you need to revise your work in preparation for the Nature of Microbes test. 11. Useful web links: (general revision site) 12. Glossary. (Muddled sort them out!) 1 Agar Outer layer of bacterial, fungal and plant cells 5 2 Aseptic Method of asexual reproduction in yeast 4 3 Bacteria Techniques which prevent the spread of unwanted microbes 2 4 Budding Jelly-like material made from seaweed which bacteria grow on 1 5 Cell wall Group of simple, single-celled microbes, some of which cause disease 3 6 Colony Breakdown of sugars to alcohol & CO 2, e.g. by yeast enzymes 9 7 Culture Varied group, all with microscopic threads or cells e.g. yeast 10 8 Decay Grow microbes (e.g. bacteria or fungi) on a nutrient agar plate 7 9 Fermentation Visible circle containing millions of bacteria, all grown from one 6 10 Fungi Rotting. Large molecules digested to small ones by bacterial or fungal enzymes 8 11 Inoculate Famous French scientist who proved that some microbes cause 13 disease 12 Incubate Petri dish containing agar with nutrients for bacterial growth Louis Pasteur Microscopic organism, e.g. bacteria, fungi, viruses Microbe Introduce microbes onto an area e.g. put bacteria on an agar 11 plate 15 Nutrient agar plate Grow microbes at a fixed temperature, usually 37 C Pathogen Smallest of all microbes. Can only grow inside other living cells Petri dish Completely free from microbes e.g. by boiling or disinfectant Protein coat Any microbe capable of causing disease Spontaneous generation Group of single-celled fungi important in brewing and baking Sterile (False) Idea than living cells can arise from non-living materials Virus Outer layer of viruses, surrounding a length of genes Yeast Glass or plastic container into which agar is poured to form a plate 17 11

12 13. Questions. 12

13 GCSE - WJEC