8.6: Exercises

Review Questions for Chapter 8

Multiple Choice

1) Which of the following methods would be used to measure the concentration of bacterial contamination in processed peanut butter?

1. turbidity measurement
2. total plate count
3. dry weight measurement
4. direct counting of bacteria on a calibrated slide under the microscope

2) In which phase would you expect to observe the most endospores in a Bacillus cell culture?

1. death phase
2. lag phase
3. log phase
4. log, lag, and death phases would all have roughly the same number of endospores.

3) During which phase would penicillin, an antibiotic that inhibits cell-wall synthesis, be most effective?

1. death phase
2. lag phase
3. log phase
4. stationary phase

4) Which of the following is the best definition of generation time in a bacterium?

1. the length of time it takes to reach the log phase
2. the length of time it takes for a population of cells to double
3. the time it takes to reach stationary phase
4. the length of time of the exponential phase

5) What is the function of the Z ring in binary fission?

1. It controls the replication of DNA.
2. It forms a contractile ring at the septum.
3. It separates the newly synthesized DNA molecules.
4. It mediates the addition of new peptidoglycan subunits.

6) If a culture starts with 50 cells, how many cells will be present after five generations with no cell death?

1. 200
2. 400
3. 1600
4. 3200

7) Filamentous cyanobacteria often divide by which of the following?

1. budding
2. mitosis
3. fragmentation
4. formation of endospores

8) Which is a reason for antimicrobial resistance being higher in a biofilm than in free-floating bacterial cells?

1. The EPS allows faster diffusion of chemicals in the biofilm.
2. Cells are more metabolically active at the base of a biofilm.
3. Cells are metabolically inactive at the base of a biofilm.
4. The structure of a biofilm favors the survival of antibiotic resistant cells.

9) Quorum sensing is used by bacterial cells to determine which of the following?

1. the size of the population
2. the availability of nutrients
3. the speed of water flow
4. the density of the population

10) Which of the following statements about autoinducers is incorrect?

1. They bind directly to DNA to activate transcription.
2. They can activate the cell that secreted them.
3. N-acylated homoserine lactones are autoinducers in gram-negative cells.
4. Autoinducers may stimulate the production of virulence factors.

11) An inoculated thioglycolate medium culture tube shows dense growth at the surface and turbidity throughout the rest of the tube. What is your conclusion?

1. The organisms die in the presence of oxygen
2. The organisms are facultative anaerobes.
3. The organisms should be grown in an anaerobic chamber.
4. The organisms are obligate aerobes.

12) An inoculated thioglycolate medium culture tube is clear throughout the tube except for dense growth at the bottom of the tube. What is your conclusion?

1. The organisms are obligate anaerobes.
2. The organisms are facultative anaerobes.
3. The organisms are aerotolerant.
4. The organisms are obligate aerobes.

13) Pseudomonas aeruginosa is a common pathogen that infects the airways of patients with cystic fibrosis. It does not grow in the absence of oxygen. The bacterium is probably which of the following?

1. an aerotolerant anaerobe
2. an obligate aerobe
3. an obligate anaerobe
4. a facultative anaerobe

14) Streptococcus mutans is a major cause of cavities. It resides in the gum pockets, does not have catalase activity, and can be grown outside of an anaerobic chamber. The bacterium is probably which of the following?

1. a facultative anaerobe
2. an obligate aerobe
3. an obligate anaerobe
4. an aerotolerant anaerobe

15) Why do the instructions for the growth of Neisseria gonorrhoeae recommend a CO₂-enriched atmosphere?

1. It uses CO₂ as a final electron acceptor in respiration.
2. It is an obligate anaerobe.
3. It is a capnophile.
4. It fixes CO₂ through photosynthesis.

16) Bacteria that grow in mine drainage at pH 1–2 are probably which of the following?

1. alkaliphiles
2. acidophiles
3. neutrophiles
4. obligate anaerobes

17) Bacteria isolated from Lake Natron, where the water pH is close to 10, are which of the following?

1. alkaliphiles
2. facultative anaerobes
3. neutrophiles
4. obligate anaerobes

18) In which environment are you most likely to encounter an acidophile?

1. human blood at pH 7.2
2. a hot vent at pH 1.5
3. human intestine at pH 8.5
4. milk at pH 6.5

19) A soup container was forgotten in the refrigerator and shows contamination. The contaminants are probably which of the following?

1. thermophiles
2. acidophiles
3. mesophiles
4. psychrotrophs

20) Bacteria isolated from a hot tub at 39 °C are probably which of the following?

1. thermophiles
2. psychrotrophs
3. mesophiles
4. hyperthermophiles

21) In which environment are you most likely to encounter a hyperthermophile?

1. hot tub
2. warm ocean water in Florida
3. hydrothermal vent at the bottom of the ocean
4. human body

22) Which of the following environments would harbor psychrophiles?

1. mountain lake with a water temperature of 12 °C
2. contaminated plates left in a 35 °C incubator
3. yogurt cultured at room temperature
4. salt pond in the desert with a daytime temperature of 34 °C

23) Which of the following is the reason jams and dried meats often do not require refrigeration to prevent spoilage?

1. low pH
2. toxic alkaline chemicals
3. naturally occurring antibiotics
4. low water activity

24) Bacteria living in salt marshes are most likely which of the following?

1. acidophiles
2. barophiles
3. halotolerant
4. thermophiles

25) Which of the following refers to the type of interaction between two prokaryotic populations in which one population benefits and the other is not affected?

1. mutualism
2. commensalism
3. parasitism
4. neutralism

Fill-in-the-Blanks

26) Direct count of total cells can be performed using a ________ or a ________.

27) The ________ method allows direct count of total cells growing on solid medium.

28) A statistical estimate of the number of live cells in a liquid is usually done by ________.

29) For this indirect method of estimating the growth of a culture, you measure ________ using a spectrophotometer.

30) Active growth of a culture may be estimated indirectly by measuring the following products of cell metabolism: ________ or ________.

31) A bacterium that thrives in a soda lake where the average pH is 10.5 can be classified as a(n) ________.

32) Lactobacillus acidophilus grows best at pH 4.5. It is considered a(n) ________.

33) A bacterium that thrives in the Great Salt Lake but not in fresh water is probably a ________.

34) Bacteria isolated from the bottom of the ocean need high atmospheric pressures to survive. They are ________.

35) Staphylococcus aureus can be grown on multipurpose growth medium or on mannitol salt agar that contains 7.5% NaCl. The bacterium is ________.

36) When prokaryotes live as interacting communities in which one population benefits to the harm of the other, the type of symbiosis is called ________.

Short Answers

37) What is the direction of water flow for a bacterial cell living in a hypotonic environment? How do cell walls help bacteria living in such environments?

38) Why is it important to measure the transmission of light through a control tube with only broth in it when making turbidity measures of bacterial cultures?

39) In terms of counting cells, what does a plating method accomplish that an electronic cell counting method does not?

40) Order the following stages of the development of a biofilm from the earliest to the last step.

1. secretion of EPS
2. reversible attachment
3. dispersal
4. formation of water channels
5. irreversible attachment

41) Infections among hospitalized patients are often related to the presence of a medical device in the patient. Which conditions favor the formation of biofilms on in-dwelling catheters and prostheses?

42) Why are some obligate anaerobes able to grow in tissues (e.g., gum pockets) that are not completely free of oxygen?

43) Why should Haemophilus influenzae be grown in a candle jar?

44) In terms of oxygen requirements, what type of organism would most likely be responsible for a foodborne illness associated with canned foods?

45) Which macromolecule in the cell is most sensitive to changes in pH?

46) Which metabolic process in the bacterial cell is particularly challenging at high pH?

47) How are hyperthermophile’s proteins adapted to the high temperatures of their environment?

48) Why would NASA be funding microbiology research in Antarctica?

49) Fish sauce is a salty condiment produced using fermentation. What type of organism is likely responsible for the fermentation of the fish sauce?

50) Compare commensalism and amensalism.

Critical Thinking

51) A patient in the hospital has an intravenous catheter inserted to allow for the delivery of medications, fluids, and electrolytes. Four days after the catheter is inserted, the patient develops a fever and an infection in the skin around the catheter. Blood cultures reveal that the patient has a blood-borne infection. Tests in the clinical laboratory identify the blood-borne pathogen as Staphylococcus epidermidis, and antibiotic susceptibility tests are performed to provide doctors with essential information for selecting the best drug for treatment of the infection. Antibacterial chemotherapy is initiated and delivered through the intravenous catheter that was originally inserted into the patient. Within 7 days, the skin infection is gone, blood cultures are negative for S. epidermidis, and the antibacterial chemotherapy is discontinued. However, 2 days after discontinuing the antibacterial chemotherapy, the patient develops another fever and skin infection and the blood cultures are positive for the same strain of S. epidermidis that had been isolated the previous week. This time, doctors remove the intravenous catheter and administer oral antibiotics, which successfully treat both the skin and blood-borne infection caused by S. epidermidis. Furthermore, the infection does not return after discontinuing the oral antibacterial chemotherapy. What are some possible reasons why intravenous chemotherapy failed to completely cure the patient despite laboratory tests showing the bacterial strain was susceptible to the prescribed antibiotic? Why might the second round of antibiotic therapy have been more successful? Justify your answers.

52) Why are autoinducers small molecules?

53) If the results from a pond water sample were recorded as 3, 2, 1, what would be the MPN of bacteria in 100 mL of pond water?

54) Why does turbidity lose reliability at high cell concentrations when the culture reaches the stationary phase?

55) A microbiology instructor prepares cultures for a gram-staining practical laboratory by inoculating growth medium with a gram-positive coccus (nonmotile) and a gram-negative rod (motile). The goal is to demonstrate staining of a mixed culture. The flask is incubated at 35 °C for 24 hours without aeration. A sample is stained and reveals only gram-negative rods. Both cultures are known facultative anaerobes. Give a likely reason for success of the gram-negative rod. Assume that the cultures have comparable intrinsic growth rates.

56) People who use proton pumps inhibitors or antacids are more prone to infections of the gastrointestinal tract. Can you explain the observation in light of what you have learned?

57) The bacterium that causes Hansen’s disease (leprosy), Mycobacterium leprae, infects mostly the extremities of the body: hands, feet, and nose. Can you make an educated guess as to its optimum temperature of growth?

58) Some hyperthermophiles can survive autoclaving temperatures. Are they a concern in health care?

59) Haemophilus, influenzae grows best at 35–37 °C with ~5% CO₂ (or in a candle-jar) and requires hemin (X factor) and nicotinamide-adenine-dinucleotide (NAD, also known as V factor) for growth. Using the vocabulary learned in this chapter, describe H. influenzae.¹

Footnotes

1. Centers for Disease Control and Prevention, World Health Organization. “CDC Laboratory Methods for the Diagnosis of Meningitis Caused by Neisseria meningitidis, Streptococcus pneumoniae, and Haemophilus influenza. WHO Manual, 2nd edition.” 2011. http://www.cdc.gov/meningitis/lab-ma...ull-manual.pdf)