Assignment Problem:
Problem 1: What is the Theory of Spontaneous Generation? How did Louis Pasteur disprove this theory? What did his observations provide the basis for, and how has this impacted microbiology?
Problem 2: List and explain one beneficial effect and one detrimental effect of bacteria.
Problem 3: Column A lists bacterial cell structures. Column B lists the function of these structures. For each item listed in Column A, indicate the correct description from Column B.
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Column A
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Column B
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Cell wall
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a. energy reserves in cell
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Capsule
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b. filamentous appendage for movement
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Flagella
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c. contains the cell's DNA
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Plasma membrane
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d. substance inside the cell that contains enzymes and cell structures
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Ribosomes
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e. polysaccharide attached to the cell wall that resists phagocytosis
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Fimbriae
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f. sites of protein synthesis
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LPS
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g. is composed of lipids and proteins, and serves as a selective barrier
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Inclusion bodies
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h. is composed of lipid A, which is an end toxin
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Nucleoid
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i. maintains cell shape
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Cytoplasm
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j. external protein structures for adherence
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Problem 4: Describe each of the following stains, and indicate when it is appropriate to use each one:
a. simple stain
b. differential stain
c. negative stain
d. acid-fast stain
e. flagella stain
Problem 5: Give an example of an organism that can survive best in the following conditions:
a. acidic
b. basic
c. neutral pH
Problem 6: Distinguish between respiration and fermentation. Which process is anaerobic? Which process is aerobic? What are the substrates and products of lactic acid fermentation and alcohol fermentation?
Problem 7: Define the following terms, and identify which of these four nutritional groups most of the medically important microbes would belong to.
a. photoautotroph
b. photoheterotroph
c. chemoautotroph
d. chemoheterotroph
Problem 8: A buffet restaurant uses heat lamps at a temperature of 50oC to keep the food warm. If the restaurant opens for service at 4 pm and closes at 10 pm, some of the food items that are not refilled frequently may sit for 6 hours at this temperature.
The following experiment was conducted to determine whether this temperature is suitable to maintain the food and prevent bacterial growth:
Beef cubes were inoculated with 500,000 bacterial cells and incubated at 43-53oC to establish temperature limits for bacterial growth. Table 1 contains the numbers of bacteria after incubation for 6 hours, and plating on nutrient agar given in the numbers of colonies for a particular dilution.
Calculate the cfu/ml for each organism at each temperature using the formula
cfu/ml = # colonies x dilution factor x plating factor (10)
and draw a line or bar graph to indicate the amount of growth after 6 hours for each temperature.
What holding temperature would you recommend to maintain the food with minimal contamination? Is 50oC sufficient? Assuming that cooking kills bacteria in food, how could these bacteria contaminate the cooked foods? What disease does each organism cause?
TABLE 1: Numbers of colony-forming units per ml of bacteria in beef cubes after incubation at different temperatures.
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Organism
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Temp. (oC)
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# Colonies
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Dilution
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Staphylococcus aureus
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43
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15
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1:1000000
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51
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8
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1:10000
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53
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6
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0.111111111
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Salmonella typhimurium
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43
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32
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1:100000
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51
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9
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1:10000
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53
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2
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0.736111111
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Clostridium perfringens
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43
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17
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1:100000
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51
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12
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0.736111111
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53
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4
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0.111111111
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Problem 9: Table 2 summarizes results obtained from an experiment of microbial sampling onto media that can help detect Enterobacter spp. from a freshwater spring.
Based on these results, answer the questions below. You may wish to consult the Photographic Atlas for supplemental information.
a. Compare the numbers of colony-forming units between nutrient agar and EMB agar for each temperature.
b. Why is there a difference in the number of organisms between the two types of media?
c. Describe the purpose of EMB agar in this experiment, and describe how this purpose is achieved.
d. Does Enterobacter spp. cause disease?
e. What can you deduce about the temperature requirements of these organisms?
TABLE 2. Numbers of colony-forming units per ml of sample obtained from spring water subjected to different temperatures.
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Incubation temperature (oC)
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Nutrient agar Eosin Methylene
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Blue agar (EMB)
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4
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3.0 x 101
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0
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20
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7.2 x 102
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20
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34
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2.1 x 106
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1.7 x 104
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42
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1.5 x 102
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9
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Problem 10: Fill in the table below.
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Chemical Agent
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Mechanism of Action
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Preferred Use
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Phenol
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Chlorhexadine
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Iodine
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Alcohol
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Silver nitrate
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Soap
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Quaternary Ammonium Compounds
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Nitrates
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Ethylene Oxide
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Aldehydes
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Problem 11: Is the lac operon a repressible or an inducible operon? What happens in the absence of lactose? What happens in the presence of lactose? Is the trp operon a repressible or an inducible operon? What happens in the absence of tryptophan? What happens in the presence of tryptophan?
Problem 12: A researcher is interested in isolating an auxotrophic mutant in the laboratory that is unable to synthesize the amino acid histadine. Explain how this researcher would have to use negative selection to isolate and identify a histadine mutant.
Problem 13: Differentiate between vertical and horizontal gene transfer. List and describe the three types of horizontal gene transfer in bacteria.
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