OpenStax Microbiology Test Bank Chapter 9: Microbial Growth A+

OpenStax Microbiology Test Bank Chapter 9: Microbial Growth Page 1 of 17 Chapter 9: Microbial Growth * = Correct answer Multiple Choice 1. Bacteria most commonly reproduce using which of the following methods? A. binary fission* B. budding C. meiosis D. mitosis Difficulty: Easy ASM Standard: N/A 2. Which structure separates new daughter cells in binary fission? A. the crescent B. the division septum* C. the FtsZ D. the spindle apparatus Difficulty: Easy ASM Standard: N/A 3. Which of the following is another term for generation time? A. doubling time* B. growth rate C. mitotic index D. replication index Difficulty: Easy ASM Standard: N/A 4. Which of the following best describes culture density? A. another term for a batch culture B. the number of cells per unit volume* C. the reproducible pattern of growth D. the total number of cells Difficulty: Easy ASM Standard: 36 5. Which of the following is the correct order of phases of growth in the growth curve of a batch culture? A. death/decline phase  lag phase  log phase  stationary phase B. lag phase  log phase  stationary phase  death/decline phase* OpenStax Microbiology Test Bank Chapter 9: Microbial Growth Page 2 of 17 C. log phase  lag phase  stationary phase  death/decline phase D. stationary phase  log phase  lag phase  death/decline phase Difficulty: Easy ASM Standard: 36 6. Which of the following is not a common way bacteria divide? A. binary fission B. budding C. fragmentation D. sporulation* Difficulty: Moderate ASM Standard: N/A 7. Which of the following is the term for free-living bacteria? A. biofilms B. pathogenic C. planktonic* D. sessile Difficulty: Easy ASM Standard: 21 8. Which of the following is not a reactive oxygen species? A. atmospheric oxygen* B. peroxides C. singlet oxygen D. superoxide Difficulty: Easy ASM Standard: 14 9. Which of the following best describes FtsZ? A. a cytoskeletal protein in eukaryotic cells B. the protein that assembles the Z ring, which forms a divisome before division by binary fission* C. the protein that forms within the central structure of the new cell wall and septum that separates the daughter cells during binary fission D. the protein that gives structure to spirochetes, contributing to their unusual shape and motility Difficulty: Moderate ASM Standard: N/A 10. Which microbial pathogen has an unusually long doubling time? OpenStax Microbiology Test Bank Chapter 9: Microbial Growth Page 3 of 17 A. Bacillus subtilis B. Escherichia coli C. Mycobacterium leprae* D. Salmonella typhimurium Difficulty: Moderate ASM Standard: N/A 11. The initial cells added to a batch culture are called which of the following? A. the batch B. the culture C. the inoculum* D. the lagging cells Difficulty: Moderate ASM Standard: 36 12. Which best describes bacterial growth during the log phase of the growth curve? A. exponential* B. linear C. S-shaped D. stationary Difficulty: Moderate ASM Standard: N/A 13. Which best defines the term “persisters?” A. cells that form endospores rapidly when environmental conditions become unfavorable, allowing them to survive in a dormant state without causing disease symptoms B. cells that persist in the log phase for much longer than other cells C. cells that rapidly colonize new environments by releasing virulence factors and moving through tissues, allowing them to contribute to severe systemic infections. D. cells with relatively slow metabolic rates that survive longer than others during the death phase and that can cause chronic infections* Difficulty: Moderate ASM Standard: 14, 23 14. Which type of system is used to maintain bacteria in a relatively constant environment, allowing them to stay in the log phase of growth? A. bacteriostat B. chemostat* C. maintenance state D. stable state Difficulty: Moderate OpenStax Microbiology Test Bank Chapter 9: Microbial Growth Page 4 of 17 ASM Standard: 36 15. Which of the following is an indirect method of measuring numbers of bacterial cells? A. a Coulter counter B. a Petroff-Hausser chamber C. microscopic cell count D. turbidity measurements* Difficulty: Moderate ASM Standard: 35, 36 16. Which of the following is not a common step in preparing a pour plate to make a count of microbial numbers? A. A serial dilution is performed. B. Bacteria are spread across the agar, using a sterile spreader.* C. Bacteria are mixed with warm agar and poured onto a Petri plate, then swirled. D. The sample is allowed to grow, then colonies are counted. Difficulty: Moderate ASM Standard: 33, 35, 36 17. Which of the following best describes why extracellular polymeric substances are important? A. They are chemicals produced as part of the immune response to microbial invasion. B. They are components of a biofilm secreted by the host cells as an immune response to wall off the pathogens. C. They are components of a biofilm secreted by the microbes present.* D. They are secreted by free-living bacteria as virulence factors to allow them to penetrate the bloodstream. Difficulty: Moderate ASM Standard: 20, 21, 22 18. Which of the following media is commonly used to determine the oxygen requirements of a bacterial species? A. chocolate agar medium B. Mueller-Hinton agar medium C. thioglycolate medium* D. tryptic soy medium Difficulty: Moderate ASM Standard: 11, 35, 36 19. Which of the following best describes how a microaerophile would grow in a thioglycolate tube culture? A. a short distance below the top of the medium, but not at the top* B. at the bottom of the medium only, with no growth in the middle or top OpenStax Microbiology Test Bank Chapter 9: Microbial Growth Page 5 of 17 C. near and at the top of the medium, with no growth at the bottom D. throughout the medium Difficulty: Moderate ASM Standard: 20, 22 20. Which of the following is not an example of an enzyme used to detoxify reactive oxygen species? A. catalase B. permeases* C. peroxidases D. superoxide dismutases Difficulty: Moderate ASM Standard: 11, 20, 22 21. A candle jar is often used to grow which type of bacteria? A. capnophiles* B. microaerophiles C. obligate aerobes D. obligate anaerobes Difficulty: Moderate ASM Standard: 14, 20, 22, 36 22. Which of the following is the most common category of bacteria? A. acidophiles B. alkaliphiles C. extreme alkaliphiles D. neutrophiles* Difficulty: Easy ASM Standard: 13, 14, 20, 22, 36 23. Human pathogens are generally which type of microbe? A. mesophiles* B. psychrophiles C. psychrotrophs D. thermophiles Difficulty: Moderate ASM Standard: 14, 20, 22, 26 24. Food spoilage is often caused by which type of microbe? A. mesophiles B. psychrophiles OpenStax Microbiology Test Bank Chapter 9: Microbial Growth Page 6 of 17 C. psychrotrophs* D. thermophiles Difficulty: Moderate ASM Standard: 14, 20, 22, 36 25. Which type of microbes thrive in the Great Salt Lake? A. barophilic B. barotolerant C. extreme halophiles* D. halotolerant Difficulty: Easy ASM Standard: 11, 20, 22 26. Which type of organism can survive at great depths in the ocean? A. acidophilic B. barophilic C. barotolerant* D. halophilic Difficulty: Moderate ASM Standard: 11, 20, 22 27. Which range of the electromagnetic spectrum is considered photosynthetically active radiation? A. from ultraviolet to near infrared B. from visible light to near infrared* C. in near infrared only D. in visible light only Difficulty: Difficult ASM Standard: 11, 20, 22 28. In batch culture, how does the log phase of the bacterial growth curve appear? A. exponential on a semi-log scale B. linear on a semi-log scale* C. logarithmic on an arithmetic scale D. logarithmic on a semi-log scale Difficulty: Difficult ASM Standard: N/A 29. Which of the following best describes the enzymes of extreme alkaliphiles? A. They have many acidic amino acids. B. They have relatively high isoelectric points.* OpenStax Microbiology Test Bank Chapter 9: Microbial Growth Page 7 of 17 C. They have relatively high numbers of negatively charged surface amino acids. D. They have the ability to pump protons out of cells. Difficulty: Difficult ASM Standard: 11, 20, 22 30. Capnophiles are bacteria that grow best under which of the following conditions? A. relatively high CO2 and relatively low oxygen concentrations* B. relatively high oxygen and relatively low CO2 concentrations C. relatively low CO2 and oxygen concentrations D. relatively low oxygen and CO2 concentrations Difficulty: Difficult ASM Standard: 11, 20, 22 31. Which of the following best describes the cell membranes of microbes that thrive at low temperatures? A. They have only one layer. B. They have relatively even amounts of saturated and unsaturated fatty acids. C. They have relatively high amounts of saturated fatty acids. D. They have relatively high amounts of unsaturated fatty acids.* Difficulty: Difficult ASM Standard: 11, 20, 22 32. If you grow two species of bacteria on mannitol salt agar to determine which can ferment mannitol, and you know both species can grow on the medium, which type of medium is the agar? A. differential* B. selective C. both a differential medium and a selective medium D. neither a differential medium nor a selective medium Difficulty: Difficult ASM Standard: 20, 22, 33, 34 33. Which of the following pigments are especially useful at great depths, where less light penetrates? A. bacteriorhodopsin and carotenoids B. carotenoids and chlorophylls C. fucoxanthin and carotenoids D. fucoxanthin and phycobilins* Difficulty: Difficult ASM Standard: 11, 20, 22 OpenStax Microbiology Test Bank Chapter 9: Microbial Growth Page 8 of 17 34. Which of the following microbes uses light energy to pump hydrogen and sodium? A. Bacteriodetes B. Campylobacter C. Chlamydomonas D. Halobacterium* Difficulty: Difficult ASM Standard: 34 35. Which of the following is true of using a Petroff-Hausser chamber to do a direct cell count? A. Identical squares, all the same size, are etched into the slide. B. If the sample was diluted, then the concentration must be corrected accordingly.* C. It is not necessary to know the volume of the sample added to the slide. D. The chamber does not resemble a hemocytometer. Difficulty: Moderate ASM Standard: 35, 36 36. Which of the following would be the best choice for distinguishing living and dead bacteria in a cell count? A. using a Coulter counter or other electronic counting method B. using a streak plate, spread plate, or pour plate C. using gram staining or a similar method D. primary and secondary fluorescent stains* Difficulty: Moderate ASM Standard: 35, 36 37. What is the goal number of CFUs on a spread plate if a serial dilution is done correctly? A. 1–10 per plate B. 30–300 per plate* C. 400–900 per plate D. 1000 or more per plate Difficulty: Easy ASM Standard: 35, 36 True/False 38. The largest component of the dry weight of a biofilm is usually extracellular polymeric substances. Answer: True Difficulty: Easy ASM Standard: 21 OpenStax Microbiology Test Bank Chapter 9: Microbial Growth Page 9 of 17 39. Generation time in prokaryotes refers to the amount of time required for the doubling of the population through a single round of binary fission. Answer: True Difficulty: Easy ASM Standard: N/A 40. Estimates of coliforms in water are often made using most probable number techniques. Answer: True Difficulty: Moderate ASM Standard: 36 41. Quorum sensing always occurs between members of the same species. Answer: False Difficulty: Moderate ASM Standard: 12 42. Halotolerant microbes cannot grow without high concentrations of salt. Answer: False Difficulty: Moderate ASM Standard: 11, 13, 20, 22 43. Fungi can generally thrive in drier environments than can bacteria. Answer: True Difficulty: Moderate ASM Standard: 11, 20, 22 44. MacConkey agar can be used to select for the growth of gram-positive bacteria, making it especially useful to study Enterobacteriaceae. Answer: False Difficulty: Difficult ASM Standard: 11, 20, 22, 33, 34, 36 45. Some halotolerant microbes are important causes of food poisoning. Answer: True Difficulty: Difficult ASM Standard: 11, 20, 22, 23 Matching OpenStax Microbiology Test Bank Chapter 9: Microbial Growth Page 10 of 17 46. Match each type of organism with the best description of its oxygen requirements. A. aerotolerant anaerobes i. an organism that requires an oxygen-free environment B. facultative anaerobe ii. an organism that requires oxygen to survive C. microaerophile iii. an organism that requires a low concentration of oxygen (less than the oxygen concentration in the atmosphere) D. obligate aerobe iv. an organism that grows equally well with or without oxygen E. obligate anaerobe v. an organism that can grow without oxygen but that grows best with oxygen Answers: A. iv., B. v., C. iii., D. ii., E. i. Difficulty: Easy ASM Standard: 11, 20, 22 47. Match each type of microbe with the best example. A. aerotolerant i. Streptococcus B. facultative anaerobe ii. Clostridium difficile C. microaerophile iii. Campylobacter pylori D. obligate anaerobe iv. Staphylococcus Answers: A. i., B. iv., C. iii., D. ii. Difficulty: Difficult ASM Standard: 11, 14, 20, 22 48. Match each type of microbe with the best description. A. hyperthermophile i. temperature range for growth: approximately 0–15 °C B. mesophile ii. temperature range for growth: approximately 80–110 °C C. psychrophile iii. temperature range for growth: approximately 50–80 °C D. psychrotroph iv. optimal temperature range for growth: approximately 20–40 °C E. thermophile v. grows well between 4 °C and 25 °C Answers: A. ii., B. iv., C. i., D. v., E. iii. Difficulty: Moderate ASM Standard: 11, 14, 20, 22 49. Match each type of medium with the best description. A. chemically defined medium i. a medium with growth factors added to promote the growth of fastidious organisms B. complex medium ii. a medium made from the digests of yeasts, meats, or plants, meaning that the exact composition is unknown C. differential medium iii. a medium that favors the growth of some organisms over others D. enriched medium iv. a medium for which the entire composition is exactly known E. selective media v. a medium that allows colonies of bacteria to be distinguished based on one or more characteristics OpenStax Microbiology Test Bank Chapter 9: Microbial Growth Page 11 of 17 Answers: A. iv., B. ii., C. v., D. i., E. iii. Difficulty: Moderate ASM Standard: 11, 20, 22, 33, 34 50. Match each microbe with the best description. A. Bacillus firmus i. acidophile B. Ferroplasma ii. alkaliphile C. Geobacillus iii. halotolerant D. Halomonas iv. thermophile Answers: A. ii., B. i., C. iv., D. iii. Difficulty: Difficult ASM Standard: 11, 20, 22, 33, 34 Fill in the Blank 51. The process of division by which cells split from a parent filament is called ________. Answer: fragmentation Difficulty: Easy ASM Standard: 22 52. The generation time for a particular bacterial species in a particular batch culture is called the ________. Answer: intrinsic growth rate Difficulty: Easy ASM Standard: N/A 53. Turbidity is measured using a ________. Answer: spectrophotometer Difficulty: Easy ASM Standard: 36 54. When a spread plate is used to estimate bacterial numbers, the numbers are reported as ________. Answer: colony forming units per milliliter (CFU/mL) Difficulty: Moderate ASM Standard: 36 55. ________ organisms do not require high salt concentrations for growth but can grow in salty environments. Answer: Halotolerant OpenStax Microbiology Test Bank Chapter 9: Microbial Growth Page 12 of 17 Difficulty: Moderate ASM Standard: 11, 20, 22 56. ________ is a measure of available moisture. Answer: Water activity Difficulty: Moderate ASM Standard: N/A 57. ________is when the cell membrane moves away from the cell wall due to osmosis occurring within the cytoplasm. Answer: Plasmolysis Difficulty: Moderate ASM Standard: N/A 58. At low temperatures, membranes become ________ fluid unless they are able to modify their composition. Answer: less Difficulty: Moderate ASM Standard: 11, 20, 22 59. The pigment used by Halobacteria to absorb light is called ________. Answer: bacteriorhodopsin Difficulty: Moderate ASM Standard: 11, 20, 22 60. The process of freeze-drying microbes so they can be stored until needed is called ________. Answer: lyophilization Difficulty: Difficult ASM Standard: 36 61. ________ is an important accessory pigment used by brown algae to aid photosynthesis at low light levels. Answer: Fucoxanthin Difficulty: Difficult ASM Standard: 11, 20, 22 Short Answer 62. Which group of bacteria commonly divides using fragmentation? OpenStax Microbiology Test Bank Chapter 9: Microbial Growth Page 13 of 17 Sample Answer: The Actinomycetes commonly divide using fragmentation. Difficulty: Easy ASM Standard: 11, 20, 22 63. Where are barophilic organisms found? Sample Answer: Barophilic organisms require high pressures to grow, so they are found deep in the ocean. Difficulty: Easy ASM Standard: 11, 20, 22 64. What happens during plasmolysis and what causes it? Sample Answer: When a cell is exposed to a high osmotic pressure, the protoplasm moves away from the cell wall. (More detailed answer students may give if they remember the information from Chapter 3: Plasmolysis occurs when an organism with a cell wall is placed in a hypertonic solution. Water leaves the cell through osmosis, causing the plasma membrane to pull away from the cell wall, containing the reduced contents of the cell.) Difficulty: Easy ASM Standard: 11, 20, 22 65. What is the difference between the optimum oxygen concentration and the maximum permissive oxygen concentration? Sample Answer: The optimum oxygen concentration is the concentration at which a microbe grows best. In contrast, the maximum permissive oxygen concentration is the maximum concentration at which a microbe can grow. Difficulty: Easy ASM Standard: 11, 14, 20, 22 66. What is the following equation used for and what do the variables n, Nn, and N0 refer to? Nn = N02n Sample Answer: This equation is used to calculate the number of cells in a population undergoing binary fission for a particular starting population size. The initial number of cells is N0; n represents the number of generations; and Nn represents the number of cells after n generations. Difficulty: Moderate ASM Standard: 35 67. Why do researchers often work with bacteria in the log phase of bacterial growth in a batch culture, rather than in other phases? Sample Answer: Bacteria in the log stage are most commonly used because of their relatively consistent characteristics (i.e., growth and metabolism) and susceptibility to disinfectants and antibiotics. OpenStax Microbiology Test Bank Chapter 9: Microbial Growth Page 14 of 17 Difficulty: Moderate ASM Standard: 11, 20, 22, 36 68. What are the most important signaling molecules for quorum sensing in gram-negative bacteria? How does this compare with the signaling molecules used by gram-positive bacteria? Sample Answer: N-acetylated lactones are the most important signaling molecules used for quorum sensing by gram-negative bacteria. In contrast, the most important signaling molecules used by gram-positive bacteria are small peptides. Difficulty: Moderate ASM Standard: 12 69. What is the standard way to grow obligate anaerobes in the laboratory? Sample Answer: Obligate anaerobes are killed by oxygen, so they need to be maintained in an oxygen-free environment. This can be accomplished using an anaerobic jar or anaerobic chamber. A candle jar is generally used for capnophiles, specifically because it contains a high CO2 concentration. The oxygen concentration decreases as the candle burns, so it is not completely devoid of oxygen, which is the condition needed for strict anaerobes. Difficulty: Moderate ASM Standard: 11, 20, 22, 36 70. Why is MacConkey agar considered both a selective and a differential medium? Sample Answer: MacConkey agar is selective because the bile salts and crystal violet included in the medium favor the growth of gram-negative bacteria while interfering with the growth of gram-positive bacteria. It is differential because bacteria that are able to grow cause the medium to turn red if they ferment lactose, whereas nonfermenters do not cause a color change. In this way, fermenters can be distinguished from nonfermenters. Difficulty: Moderate ASM Standard: 11, 34, 36 71. Why might you need to enrich media? Sample Answer: Fastidious microbes need particular specialized vitamins, growth factors, or other nutrients added to media. When these are added, the medium is called an enriched medium. Difficulty: Moderate ASM Standard: 11, 34, 36 72. Give an example of a halotolerant species that can cause food poisoning. Sample Answer: Staphylococcus aureus and Bacillus cereus are halotolerant and can cause food poisoning. Vibrio cholerae, the causative agent of cholera, is also halotolerant. OpenStax Microbiology Test Bank Chapter 9: Microbial Growth Page 15 of 17 Difficulty: Difficult ASM Standard: 11, 34 73. Mixed bacterial infections can occur that include both facultative anaerobes and obligate anaerobes. What happens when these species coexist? Sample Answer: When facultative anaerobes and obligate anaerobes coexist, the facultative anaerobes use up any oxygen present, which allows the obligate anaerobes to thrive. Difficulty: Moderate ASM Standard: 11, 14, 20, 22 74. Staphylococci thrive on human skin. What does this tell you about their environmental requirements? Sample Answer: Organisms that thrive on human skin are generally mesophiles, because the optimal temperature range for these species includes human body temperature. They also need to be able to tolerate salt from sweat, so they are generally halotolerant. Water availability varies on different parts of the skin, as do other environmental factors. Difficulty: Moderate ASM Standard: 11, 20, 22, 23 75. Explain how you would do a serial dilution beginning with 1 mL of your bacterial culture. Sample Answer: If you have 1 mL of culture, then you would add 9 mL to have 10 mL total (a 1:10 dilution). You then take 1 mL of the first dilution and add 9 mL of water to get a 1:100 dilution. You repeat this process as many times as needed to obtain the dilution required. Difficulty: Moderate ASM Standard: 35, 36 76. Why are microbes in the stationary phase of growth less susceptible to antibiotics? Sample Answer: Microbes at this stage have slower metabolic activities, so they are less susceptible to antibiotics that target those activities. Difficulty: Difficult ASM Standard: 7, 14 77. Which enzymes that detoxify reactive oxygen species are present or absent in obligate anaerobes and aerotolerant organisms? Sample Answer: The three major types of enzymes used to detoxify reactive oxygen species are catalases, peroxidases, and superoxide dismutases. Aerotolerant organisms usually have superoxide dismutase, but obligate anaerobes usually lack all these enzymes. Difficulty: Difficult ASM Standard: 11, 14, 20, 22 OpenStax Microbiology Test Bank Chapter 9: Microbial Growth Page 16 of 17 78. Why is an enzyme from Thermus aquaticus especially useful for PCR? Sample Answer: Thermus aquaticus is a thermophile; its enzymes function well at high temperatures, making it useful for PCR. Difficulty: Difficult ASM Standard: 11, 20, 22, 26 Brief Essay Essay Question Rubric RATING Failing Below Average Competent Advanced Criteria for evaluation Answer does not provide an argument. Answer contains inaccuracies. Writing is poor and contains numerous grammatical mistakes and misspellings. Answer fails to provide examples to support an argument. Writing is poor and grammatical errors are common. Answer is somewhat incoherent. Answer provides an argument with one or two examples that support it. Writing is acceptable for the college level but may contain one or two grammatical mistakes or misspellings. Answer clearly provides an argument with two or more excellent examples that support it; student makes the argument clearly and eloquently. Answer is well organized and free of grammatical errors and misspellings. POINT VALUE 0 1 2 3 Assume rating/grading scale for the question ranges from 0 to 3 points. 79. Using the standard equation to calculate number of cells at each generation, how many cells would be present after four generations in a population undergoing exponential growth that started with a population of two cells? How does this compare with the size after four generations of a similar population that started with eight cells? Sample Answer: Student answers will vary but should include components of the following. After four generations, a population of two cells would have grown to 32 cells. After four generations, a population of eight cells would have grown to 128 cells. Difficulty: Moderate ASM Standard: 35 80. Explain what factors influence the length of the lag phase in a batch culture, and why. Sample Answer: Student answers will vary but should include the following. The species present, health of the cells, type of medium, and other factors influence the length of the lag phase. When microbes are first placed into a new culture, they must adjust and repair damage before beginning the log phase of growth. They must produce any enzymes necessary to grow in the new medium, although this will be more rapid if the medium and growth OpenStax Microbiology Test Bank Chapter 9: Microbial Growth Page 17 of 17 conditions are very similar to their previous growth medium. Damaged cells need to undergo repair before rapid growth can begin. Difficulty: Moderate ASM Standard: 36 81. Why is it helpful for some bacteria, such as Staphylococcus aureus, to produce virulence factors during the stationary phase of growth in a batch culture? Sample Answer: Student answers will vary but should include the following. Cells enter the stationary phase because their environment has become less favorable for growth because of increased concentrations of waste products and/or reduced availability of nutrients and oxygen. By producing virulence factors, bacteria may increase their ability to spread to new locations. For example, S. aureus produces enzymes that help it break down tissues to more easily move to new locations. Difficulty: Difficult ASM Standard: 13 82. There has been considerable concern about finding good ways to treat infections involving biofilms, such as those that develop on medical devices. What characteristics of biofilms complicate the treatment of infections involving biofilms? Sample Answer: Student answers will vary but should include components of the following. The proposed hypotheses include at least three possibilities. First, microbes in biofilms may be less metabolically active and bacteria that are less metabolically active are generally affected less by medications. Second, the extracellular polymeric substances may inhibit the ability of medications and other chemicals to penetrate the biofilm to affect the deeper cells. In addition, microbes in biofilms may have metabolic abilities that help to counteract medications. For example, they may be able to pump out antibiotics using efflux pumps. Finally, microbes in biofilms can exchange resistance genes through horizontal gene transfer. Difficulty: Difficult ASM Standard: 2, 14, 21 83. What are some ways in which the structures of thermophiles and hyperthermophiles differ in order to help them thrive at high temperatures? Sample Answer: Student answers will vary but should include components of the following. These organisms have greater proportions of unsaturated fatty acids in their membranes, which reduces membrane fluidity. They also have higher proportions of guanine–cytosine nitrogenous bases in their DNA to strengthen hydrogen bonding between the two strands, as well as additional bonds (ionic and covalent) to stabilize their protein structure. Difficulty: Difficult ASM Standard: 11, 14, 20, 22 This file is copyright 2017, Rice University. 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