CHAPTER 14 Test Your Knowledge
Driving Question 1
What is staph, and can it be present in the absence of an infection?
By answering the questions below and studying Infographic 14.1, you should be able to generate an answer for the broader Driving Question above.
KNOW IT
Can S. aureus be present in or on a person who has no evidence of an infection?
a. no; S. aureus is associated only with infections
b. yes, but only non-MRSA strains are present in the absence of an infection
c. yes, but only for very short periods of time (between touching a contaminated surface and washing your hands)
d. yes; S. aureus is a common skin bacterium
e. yes; S. aureus is a common bacterium found in the bloodstream
d
The term “MRSA” as it is used today refers to
a. S. aureus bacteria that are resistant to many antibiotics.
b. a collection of skin and other infections caused by a type of bacteria.
c. S. aureus bacteria that are found only in humans with certain types of skin infections.
d. S. aureus bacteria that are normal residents of human skin in the vast majority of the human population.
e. all bacteria that are resistant to antibiotics.
a
What is the difference between an S. aureus colonization and an S. aureus infection?
In colonization, the bacteria are growing on or in the body without causing disease. Infections are associated with disease.
MRSA is most likely to be problematic if found
a. on the surface of the skin.
b. in nasal passages.
c. in the bloodstream.
d. on the fingernails.
e. The presence of MRSA in any of those locations indicates a serious infection.
c
USE IT
A young athlete has a nasty skin infection caused by MRSA. How might this infection have been contracted?
The athlete could have been exposed by direct skin contact with another athlete who has MRSA, or by coming into contact with a contaminated item (e.g., towels, shared sports equipment), particularly if the contact occurs at sites where the skin is broken, as by a scrape or cut.
For the patient in Question 5, which general kinds of antibiotics would you choose (or avoid) in treating the infection? What other measures would you recommend to prevent spread of MRSA to the athlete’s teammates and family? Explain your answer.
Once the infection is confirmed as MRSA, it would be important not to treat with beta-lactam antibiotics, as MRSA strains are resistant to beta-lactams in general (not just methicillin). Ideally, the bacteria causing the infection would be tested against a range of antibiotics in the lab, to determine the bacterium’s precise profile of sensitivity and resistance. If the results of such tests are not available, then one or more non–beta-lactam antibiotics should be tried first. If that is not successful, the treatment of last resort would be vancomycin. Teammates and relatives should be reminded of the importance of washing their hands frequently and of not sharing personal hygiene items such as towels or razors. All showers, locker rooms, and door handles should be thoroughly cleaned regularly, to avoid having MRSA linger on surfaces and transferred by contact with contaminated surfaces.
Driving Question 2
How do bacteria resist the effects of antibiotics?
By answering the questions below and studying Infographics 14.2, 14.3, and 14.4, you should be able to generate an answer for the broader Driving Question above.
KNOW IT
In the presence of penicillin:
a. What happens to a sensitive strain of S. aureus?
b. What happens to a resistant strain of S. aureus?
a: A sensitive strain of S. aureus will eventually burst (lyse) and die because of its weakened cell wall. b: A resistant strain of S. aureus will not be affected by penicillin and will continue to grow.
How do beta-lactam antibiotics kill sensitive bacteria?
a. by attracting water into cells
b. by destabilizing the cell membrane
c. by preventing DNA replication during bacterial reproduction
d. by destabilizing the cell wall
e. all of the above, depending on the specific strain of bacteria
d
USE IT
Why do the beta-lactam antibiotics affect sensitive bacterial cells but not eukaryotic cells? (You may want to review cell structure, discussed in Chapter 3, to answer this question.)
Beta-lactam antibiotics interfere with the synthesis of peptidoglycan-containing cell walls. Humans (and other animals) do not have cell walls at all, and while plant cells do have a cell wall, it is made of cellulose and not peptidoglycan. Eukaryotic cells therefore do not have the structure that is the target of penicillin (and other beta-lactam antibiotics).
A sensitive S. aureus bacterium acquires a new gene that allows it to resist the effects of beta-lactam antibiotics (that is, the bacterium is now resistant). What might the protein encoded by that gene do?
a. synthesize beta-lactam antibiotics
b. digest beta-lactam antibiotics
c. produce a toxin
d. enhance colonization of human skin
e. enhance entry into the bloodstream
b
Driving Question 3
How do populations evolve, and what is the role of evolution in antibiotic resistance?
By answering the questions below and studying Infographics 14.5, 14.6, and 14.7, you should be able to generate an answer for the broader Driving Question above.
KNOW IT
What are the two major mechanisms by which bacterial populations generate genetic diversity?
a. mutation and meiosis
b. binary fission and evolution by natural selection
c. gene transfer and mutation
d. mutation and binary fission
e. gene transfer and replication
c
What is the environmental pressure in the case of antibiotic resistance?
a. the growth rate of the bacteria
b. how strong or weak the bacterial cell walls are
c. the relative fitness of different bacteria
d. the presence or absence of antibiotics in the environment
e. the temperature of the environment
d
What is the evolutionary meaning of the term “fitness”?
Fitness, in an evolutionary context, describes the ability of an individual to survive and reproduce in a given environment. Individuals that are more fit leave more offspring and more of their alleles in the next generation relative to individuals that are less fit.
The evolution of antibiotic resistance is an example of
a. directional selection.
b. diversifying selection.
c. stabilizing selection.
d. random selection.
e. steady selection.
a
In humans, very-large-birth-weight babies and very tiny babies do not survive as well as midrange babies. What kind of selection is acting on human birth weight?
a. directional selection
b. diversifying selection
c. stabilizing selection
d. random selection
e. steady selection
c
USE IT
Binary fission is asexual. What does this mean? How could two daughter cells end up with different genomes at the end of one round of binary fission?
Asexual reproduction involves only one parent. That parent passes on a copy of all of its genetic material to its offspring. This means that the offspring are copies of the parent. The two daughter cells that are the products of binary fission could be genetically different if a mutation occurred during replication of the parent genome before division. One copy of the genome would contain the mutation and one copy would not, and each daughter cell would inherit one of the two different copies.
In what sense do bacteria “evolve faster” than other species?
Because bacteria divide very rapidly, they have a very short generation time. As evolution is a change in allele frequency over time, it doesn’t take long for bacterial populations to accumulate many generations. As there is a chance of one or more mutations to arise with every generation, there is a wealth of genetic diversity for natural selection to act on.
INTERPRETING DATA
A single S. aureus cell gets into a wound on your foot. S. aureus divides by binary fission approximately once every 30 minutes.
a. Thirty minutes after the initial infection, how many S. aureus cells will be present?
b. In 1 hour, how many S. aureus cells will be present?
c. In 12 hours, how many S. aureus cells will be present? (Hint: The general formula is 2number of generations; you need to figure out how many generations occurred in 12 hours.)
d. Mutations occur at a rate of 1 per 1010 base pairs per generation. S. aureus has 2.8 × 106 base pairs in its genome. Therefore, approximately 0.0028 mutations will occur per cell in the population. At the end of 12 hours, how many mutations will be present in the population of S. aureus in the wound in your foot? What are the implications of this genetic diversity in the context of treating a possible infection?
a: There will be two cells after 30 minutes. b: There will be four cells after 1 hour. c: 12 hours represents 24 generations. 224 = 16,777,216, the number of cells after 12 hours. d: At a rate of 0.0028 mutations per cell, there will be (0.0028 × 16,777,216) or approximately 46,976 mutations in the population. This means that there is a great deal of genetic diversity in the population. With all those mutations, it is quite possible that there are several (or more) cells in the population that have a mutation conferring resistance to antibiotics. This means that if an infection worsens and further antibiotic treatment is given, it may not be successful, as some cells will survive and continue to divide even in the presence of the antibiotic.
If we take the most fit bacterium from one environment—one in which the antibiotic amoxicillin is abundant, for example—and place it in an environment in which a different antibiotic is abundant, will it retain its high degree of fitness?
a. yes; fitness is fitness, regardless of the environment
b. yes; once a bacterium is resistant to one antibiotic it is resistant to all antibiotics
c. not necessarily; fitness depends on the ability of an organism to survive and reproduce, and it may not do this as well in a different environment
d. no; what is fit in one environment will never be fit in another environment
c
If a single bacterial cell that is sensitive to an antibiotic—for example, vancomycin—is placed in a growth medium that contains vancomycin, it will die. Now consider another single bacterial cell, also sensitive to vancomycin, that is allowed to divide for many generations to become a larger population. If this population is placed into vancomycin-containing growth medium, some bacteria will grow. Why do you see growth in this case, but not with the transferred single cell?
The single cell is sensitive, so it will die in the presence of vancomycin. In the case of a single sensitive cell allowed to divide in the absence of vancomycin, the daughter cells in the population will begin to acquire mutations. Some of these will, by chance, render those cells resistant to vancomycin. When the entire population is transferred to vancomycin-containing medium, the sensitive cells will die, but the resistant cells will continue to divide.
Imagine that a genetically diverse population of garden snails occupies your backyard, in which the vegetation is many shades of green with some brown patches of dry grass.
a. If birds like to eat snails, but they can see only the snails that stand out from their background and don’t blend in, what do you think the population of snails in your backyard will look like after a period of time? Explain your answer.
b. Suppose you move the population of snails to a new environment, one with patches of dark brown pebbles and patches of yellow ground cover. Will individual snails mutate to change their color immediately? As the population evolves and adapts to the new environment, what do you predict will happen to the phenotypes in your population of snails after several generations in this new environment? How did this occur? Include the terms gametes, mutation, fitness, phenotype, and environmental selective pressure in your answer.
a: After a period of time, there would be snails that are shades of green and snails that are brown. The green snails will blend in to the areas with green grass, and the brown snails will blend in to the brown areas of dead grass. b: In the new environment, snails with a green phenotype will be visible to birds on both the brown pebbles and the yellow ground cover. The snails with the brown phenotype will be able to blend with the brown pebbles. The green snails have lower fitness in this new environment, so will leave fewer green alleles in subsequent generations. Thus the proportion of green snails will decrease in the population over time because of the different selective pressure exerted by the new environment. In some cases, mutations will occur as snail gametes are produced. By chance, some of these mutations will change the color of the snail. Any offspring that have a yellow coloration will be able to blend into the new environment. Over time, the population will be composed of brown and yellow snails, which will have the highest fitness in this environment.
MINI CASE
Your friend has had a virus-caused cold for 3 days and is still so stuffy and hoarse that he is hard to understand. He seems to be telling you that his doctor called in a prescription for an antibiotic for him to pick up at his pharmacy. You hope that you misunderstood him, but you realize that you heard him perfectly well.
a. Will the antibiotic help your friend’s cold?
b. What are the risks to your friend if he takes the antibiotic? (Think about what might happen if he should develop a wound infection in the future.)
a: Antibiotics are effective only against sensitive bacteria. They are not effective against viruses, the causative agents of the common cold. b: By taking an antibiotic, your friend is applying a selective pressure to the normal bacteria colonizing his body. In the presence of the antibiotic, only bacteria which are resistant to the antibiotic will survive and grow. If your friend later has a skin wound, some of the bacteria in or on his skin may be able to enter the body and cause an infection. Because of the earlier selection by antibiotics, those infecting bacteria are likely to be resistant to antibiotics, making the infection harder to treat.
BRING IT HOME
Your roommate has been prescribed an antibiotic for bacterial pneumonia. She is feeling better and stops taking her antibiotic before finishing the prescribed dose, telling you that she will save the remainder to take the next time she becomes sick. What can you tell your roommate to convince her that this is not a good plan?
Although your friend may be feeling better, if she hasn’t taken the full dose of the antibiotic, the pneumonia bacteria may not have been fully wiped out. Instead, the bacteria in the population at this point may be able to survive a certain concentration of antibiotic, having adapted to the current amounts of antibiotic. Removing the antibiotic will allow these bacteria to continue to grow, potentially causing a recurrence of the pneumonia. Additionally, by subjecting her normal bacteria to lower than normal doses of antibiotics, she may have selected for bacteria that can survive in that concentration of antibiotic. Essentially, she is setting herself up for a recurrence of the pneumonia—but this time with bacteria that can survive in the presence of the antibiotic. She is also setting herself up for a situation similar to the one in Question 22—normal bacteria are being selected for at least partial resistance to an antibiotic, and these bacteria can be problematic in the future.
