Lab Exam 2

• What is the purpose of a streak plate?

  Pure Culture Bacteria
• Shown is a mixed culture. How can you tell?

  Colonies with different morphologies.

  
• What is different from these 5 streak plates? Enriched, selective, and/or differential? Is growth present on all plates? Does growth look different or same?

  They are selective and differential

  
• Is medium X (the agar) enriched, selective, and/or differential?

  Selective and differential

  
• What is the purpose of aseptic technique?

  Reduce likelihood of contamination when transferring organisms
• Steps to smear preparation?


  One: adding water to plate and apply thin layer of cells

  two: Air dry

  three: Heat fix (pass through flame briefly to kill cells)

  four: begin staining procedure
• Simple Stain: What are the reagents and their functions?


  Crystal violet: Stains the bacterial cells Iodine: Acts as a mordant to enhance crystal violet retention Alcohol/acetone: Decolorizes the cells Safranin: Counterstains the cells
• What are the clear ovals?

  Endospores

  
• What do cells look like under the microscope after each step?


  Stained with either pink or purple
• What causes a False Negative (G+ stains pink)?


  Cells too old

  Cell wall no longer intact

  Decolorization step too long
• What causes a False Positive (G- stains purple)?


  Smear of cells too thick

  Decolorization step too short
• What is the gram reaction and shape of these cells?

  Gram negative (pink) bacillus

  
• What is the gram stain and shape of these cells? Although they are Gram Positive. What could have caused them to stain gram negative?


  Gram stain: Purple Shape: Bacillus Cause of Gram-negative staining: Over-decolorization or old culture or cell culture no longer intact

  
• What is the gram stain and shape of these cells? These cells are actually Gram Negative. What could have caused them to stain Gram Positive


  Gram Stain: Pink

  Shape: cocci

  Cause of gram staining positive: smear too thick or decolorization step too short

  
• Which slide, A or B, is a positive catalase test shown?

  B

  
• What is the substrate of the catalase test? What is the products of the catalase test?


  Substrate: 2H2O2

  Products: O2 + 2H20
• What is the pH indicator for glucose, lactose, and mannitol?


  Phenol Red

  pH of pink: 7.0

  pH of yellow: 6.8 or below
• What is responsible for the color change to yellow

  acid has been produced from breakdown of sugar

  
• What does the methyl red test detect?

  Acid production (the relative amount of acid)
• What color is a positive methyl red test? Does a yellow color indicate a higher or lower pH?

  Positive methyl red test: red (pH below 4.5)

  Yellow color indicates: pH 4.5 higher

  
• What are the four phases of growth?


  Lag Phase

  Exponential Phase

  Stationary Phase

  Death Phase
• What effects of temperature have on bacterial growth?


  Most microbes grow optimally within a certain temperature range dictated by the ability of proteins within the cell to function. In general, at low temperatures, microbes grow slower. At higher temperatures, microbes grow more quickly.

  
• How does media affect bacterial growth?


  allow some types of bacteria to grow, while inhibiting the growth of other types (environment)
• Which culture has the fastest growth rate? The shortest g? Identify the TSB+ glucose and M-9+ glucose.

  fastest Growth Rate: A

  Shortest g: B

  
• What is the lowest concentration of salt that inhibits bacterial growth?

  8%

  
• how do you determine if the salt concentration is bacteriostatic or bactericidal?

  bacteriostatic means that the agent prevents the growth of bacteria (keeps them in the stationary phase), and bactericidal means that it kills bacteria
• Which tube indicates 8% salt concentration is bacteriostatic? Bacteriocidal?


  Bacteriostatic: B (Stationary Phase: cloudy & turbid) Bacteriocidal: A

  
• Bacteriophage: Why is it easily found in sewage?

  That is where lots of bacteria is present.
• What is a plaque

  A clearing in what is a lawn of cells that results from lysis and death of bacterial cells.
• What is a coliform?

  its a gram negative bacteria that ferments lactose to produce acid and gas at 35-37C.
• Indicator organism?

  When a person is infected with a pathogen, will excrete many more million times of indicator bacteria than pathogens.
• Identify.

  Blue w/ gas bubbles: E.coli

  Red w/ gas bubbles: Non-coli coliform

  Red w/ no bubbles: Non-coliform

  
• Given the following antibiogram, which antibiotic should be prescribed?

  Only S (Gentamycin & Erthromycin)

  
• What is the equation for PFU/mL?


  # of plaques/volume plated x 1/dilution factor
• What is the calculation for mutation frequency?

  colonies/cells

  example: 222 colonies/10^10 cells = 2.22 x 10 ^-8
• Which organism likely shows S. aureus? S. epidermis?

  Yellow: S. aureus

  Red: S. epidermis