Chapter 14: Test Your Knowledge

In the presence of penicillin: What happens to a resistant strain of S. aureus?

A resistant strain of S. aureus will not be affected by penicillin and will continue to grow.

In the presence of penicillin: What happens to a sensitive strain of S. aureus?

A sensitive strain of S. aureus will eventually burst (lyse) and die because of its weakened cell wall.

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.

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.

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

S. aureus bacteria that are resistant to many antibiotics.

What is the evolutionary meaning of the term "fitness"?

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.

How do beta-lactam antibiotics kill sensitive bacteria?

by destabilizing the cell wall

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.

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?

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

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