Exam 3

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.

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).

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.

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.

The term "MRSA" as it is used today refers to

S. aureus bacteria that are resistant to many antibiotics.

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.

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.

The response of a population to environmental pressure so that advantageous traits become more common in the population over time is called:

adaptation

How do beta-lactam antibiotics kill sensitive bacteria?

by destabilizing the cell wall

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?

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

Which of the following is NOT involved in bacterial reproduction?

daughter cells stay attached

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?

digest beta-lactam antibiotics

The evolution of antibiotic resistance is an example of

directional selection.

What are the two major mechanisms by which bacterial populations generate genetic diversity?

gene transfer and mutation

MRSA is most likely to be problematic if found

in the bloodstream.

The most severe disease that can result from MRSA is:

necrotizing fasciitis.

Which of the following hypothetical antibiotics would treat bacterial infections by interfering with bacterial reproduction?

one that decreases cell membrane synthesis

Which of the following is a harmful bacterium?

salmonella

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?

stabilizing selection

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?

stabilizing selection

What is the environmental pressure in the case of antibiotic resistance?

the presence or absence of antibiotics in the environment

Can S. aureus be present in or on a person who has no evidence of an infection?

yes; S. aureus is a common skin bacterium