• Home
  • Textbooks
  • Genetics: From Genes to Genomes
  • Bacterial Genetics

Genetics: From Genes to Genomes

Leland Hartwell, Michael L. Goldberg, Janice Fischer

Chapter 14

Bacterial Genetics - all with Video Answers

Educators

Chapter Questions

View

Problem 1

Choose the phrase from the right column that best fits the term in the left column.
a. transformation
b. conjugation
c. transduction
d. lytic cycle
e. lysogeny
f. episome
g. auxotroph
h. pangenome
i. gene targeting

1. requires supplements in medium for growth
2. a method for mutagenizing genes in bacterial genomes
3. small circular DNA molecule that can
integrate into the chromosome
4. the core genes that define a bacterial species plus all of the genes unique to individual strains
5. transfer of DNA requiring direct physical contact
6. integration of phage DNA into the chromosome
7. infection by phages in which lysis of cells releases new virus particles
8. transfer of naked DNA
9. transfer of DNA between bacteria via virus particles

Farhan Anwar
Farhan Anwar
Numerade Educator
View

Problem 2

The unicellular, rod-shaped bacterium $E$. coli is $\sim 2 \mu \mathrm{m}$ long and $0.8 \mu \mathrm{m}$ wide, and has a genome consisting of a single $4.6 \mathrm{Mb}$ circular DNA molecule. The unicellular archaean Methanosarcina acetivorans is spherical (coccus-shaped) with a diameter of $3 \mu \mathrm{m}$ and has a $5.7 \mathrm{Mb}$ circular genome. The unicellular eukaryote Saccharomyces cerevisiae is roughly spherical, with a diameter of $5-10 \mu \mathrm{m}$ It has a haploid genome of $12 \mathrm{Mb}$ divided among 16 linear chromosomes. Given these descriptions, how could you determine whether a new, uncharacterized microorganism was a bacterium, an archaean, or a eukaryote?

Farhan Anwar
Farhan Anwar
Numerade Educator
06:20

Problem 3

Now that the sequence of the entire $E$. coli $\mathrm{K} 12$ strain genome (roughly $5 \mathrm{Mb}$ ) is known, you can determine exactly where a cloned fragment of DNA came from in the genome by sequencing a few bases and matching that data with genomic information. a. About how many nucleotides of sequence information would you need to determine exactly where a fragment is from?
b. If you had purified a protein from $E .$ coli cells, roughly how many amino acids of that protein would you need to know to establish which gene encoded the protein?
c. You determine 100 nucleotides of sequence of genomic DNA from a different $E .$ coli strain, but you cannot find a match in the $E$. coli K 12 genome sequence. How is this possible?

Niamat Khuda
Niamat Khuda
Numerade Educator
View

Problem 4

Bacterial genomes such as that of $E .$ coli typically have only a single origin of replication, from which replication proceeds bidirectionally. Pol III, the DNA polymerase responsible for replicating the $E$. coli chromosome, synthesizes DNA at a rate of about 1000 nucleotides per second.
a. From this information, estimate the minimum generation time of $E .$ coli
b. Under optimal conditions, $E .$ coli have been observed to divide in as little as 17 minutes. Speculate how this might be possible, given your answer to part (a).

Farhan Anwar
Farhan Anwar
Numerade Educator
View

Problem 5

List at least three features of eukaryotic genomes that are not found in bacterial genomes.

Farhan Anwar
Farhan Anwar
Numerade Educator
View

Problem 6

Describe a mechanism by which a gene could move from the bacterial genome to a plasmid in the same cell, or vice versa.

Farhan Anwar
Farhan Anwar
Numerade Educator
View

Problem 7

High salt concentrations tend to cause protein aggregation. Suggest a way to identify proteins normally expressed in particular bacterial species that can retain their solubility despite high salt conditions.

Farhan Anwar
Farhan Anwar
Numerade Educator
View

Problem 8

Recently, scientists tested the possibility that human gut bacteria may play a role in determining body weight. The study subjects were four sets of twins (one set of identical twins and three sets of fraternal twins), where one twin was of normal weight and the other was obese. Samples of their gut bacteria were collected and transplanted into bacteria-free mice. Mice with the different bacterial transplants were all fed the same diet and monitored over the course of about one month. For each of the four twin pairs, the mice with the bacteria from the obese twin gained significantly more weight and fat than the mice transplanted with the bacteria from the normal twin.
a. What would you conclude about the relationship between the human gut microbiome and body weight?
b. Why were twins used in the study?
c. Do the results of this study mean that human genes (genes in the nuclei of human cells) do not play a role in body weight and fat content? Explain.
d. Mice are coprophagic, meaning that they eat feces. How could you test whether a certain bacterial species associated with leanness or obesity could successfully invade the gut microbiome of an animal in which that bacterial species was not previously found?
e. One problem with using bacteria-free mice in experiments such as this is that the mouse gut is not equivalent to the human gut as a bacterial host:
Different bacterial species thrive in mice and humans. Explain how this fact could affect the experiment discussed in this problem.

Farhan Anwar
Farhan Anwar
Numerade Educator
View

Problem 9

A recent metagenomic study analyzed the microorganisms present on surfaces within the entire subway system of New York City. The researchers found hundreds of bacterial species in the subway, most of them nonpathogenic. Interestingly, almost half of all DNA found in the subway matches no known organism.
a. The scientists found that different subway stations had characteristic microbiomes. How might this observation be useful to the police?
b. Because the majority of the subway DNA that could be identified was bacterial, the researchers presume that most of the DNA fragments that could not be matched to a known organism are bacterial. Why do you think that so many bacterial species are unknown to us? What feature of these unknown bacteria might prevent us from studying them?

Farhan Anwar
Farhan Anwar
Numerade Educator
02:21

Problem 10

Linezolid is a new type of antibiotic that inhibits protein synthesis in several bacterial species by binding to the 50 S subunit of the ribosome and inhibiting its ability to participate in the formation of translational initiation complexes. Physicians are particularly interested in this antibiotic for treating pneumonia caused by penicillin-resistant Streptococcus pneumoniae (also called pneumococci). To explore the mechanisms by which pneumococci can develop resistance to linezolid, you first want to identify linezolid-resistant strains. Next, using one of these strains as starting material, you want to identify derivatives of these mutants that are no longer tolerant of linezolid.
a. Outline the techniques you would use to identify linezolid-resistant mutant pneumococci and linezolid-sensitive derivatives of these mutants. In each case, would your techniques involve direct selection, screening, replica plating, treating with mutagens, or testing for a visible phenotype?
b. Suggest possible mutations that could be responsible for the two kinds of phenotypes you will identify. What types of events in the bacterial cells would be altered by the mutations? Do you think that these mutations would be loss-of-function or gain-offunction? Explain.

Alexander Cheng
Alexander Cheng
Numerade Educator
View

Problem 11

A liquid culture of $E .$ coli at a concentration of $2 \times 10^{8}$ cells/ml was diluted serially, as shown in the following diagram, and $0.1 \mathrm{ml}$ of cells from the final two test tubes were spread on agar plates containing rich medium. How many colonies do you expect to grow on each of the two plates?

Farhan Anwar
Farhan Anwar
Numerade Educator
06:10

Problem 12

Pick out the medium (i, ii, iii, or iv) onto which you would spread cells from a Lac $^{-}$ Met $^{-} E .$ coli culture to:
a. select for Lac $^{+}$ cells
b. screen for $\mathrm{Lac}^{+}$ cells
c. select for Met $^{+}$ cells
i. minimal medium + glucose + methionine
ii. minimal medium + glucose (no methionine)
iii. rich medium $+\mathrm{X}$ -gal
iv. minimal medium $+$ lactose $+$ methionine

Jessica Wooten
Jessica Wooten
Numerade Educator
View

Problem 13

This problem concerns Fig. 14.14 , which illustrates the experiment performed by Lederberg and Tatum that first indicated the existence of bacterial conjugation.
a. Strain A had mutations in two genes, while strain B had three mutations. The reason is that Lederberg and Tatum wanted to ensure that the phenomenon they were examining did not involve reversion of mutations. Explain the logic behind this aspect of their experimental design, assuming that the rate of reversion of a single gene is one in 10 million $(1$ in $\left.10^{7}\right)$ cells. How did these investigators know that the cells they found after mixing the two cultures were indeed not due to reversion?
b. The experiment shown in Fig. 14.14 did not inform the investigators which strain was the donor and which was the recipient. Describe a way in which they could modify this experiment to answer this question.

Farhan Anwar
Farhan Anwar
Numerade Educator
View

Problem 14

In two isolates (one is resistant to ampicillin and the other is sensitive to ampicillin) of a new bacterium, you found that genes encoding ampicillin resistance are being transferred into the sensitive strain.
a. How would you know that gene transfer is taking place?
b. To determine if the gene transfer is transformation or transduction, you treat the mixed culture of cells with DNase. Why would this treatment distinguish between these two modes of gene transfer? Describe the results predicted if the gene transfer is transformation versus transduction.
c. To determine if the gene transfer involves transformation, conjugation, or transduction, you separate the ampicillin-resistant and ampicillin-sensitive strains by a membrane with pores that are smaller than the size of a bacterium, but larger than the sizes of bacteriophage or DNA fragments. If gene transfer is still observed, what mechanisms are possibly involved and which are excluded?

Farhan Anwar
Farhan Anwar
Numerade Educator
01:03

Problem 15

$E .$ coli cells usually have only one copy of the Fplasmid per cell. You have isolated a cell in which a mutation increases the copy number of $F$ to three to four per cell. How could you distinguish between the possibility that the copy number change was due to a mutation in the F plasmid versus a mutation in a chromosomal gene?

Sana Riaz
Sana Riaz
Numerade Educator
View

Problem 16

In $E .$ coli, the genes $p u r C$ and $p y r B$ are located halfway around the chromosome from each other. These genes are never cotransformed. Why not?

Farhan Anwar
Farhan Anwar
Numerade Educator
05:17

Problem 17

DNA sequencing of the entire $H$. influenzae genome was completed in $1995 .$ When DNA from the nonpathogenic strain $H$. influenzae $R d$ was compared to that of the pathogenic $b$ strain, eight genes of the fimbrial gene cluster (located between the $p u r E$ and pep$N$ genes involved in adhesion of bacteria to host cells were completely missing from the nonpathogenic strain. What effect would this deletion have on cotransformation of $p u r E$ and $p e p N$ genes using DNA isolated from the nonpathogenic versus the pathogenic strain? Genes encoding toxins are often located on plasmids. A recent outbreak has just occurred in which a bacterium that is usually nonpathogenic is producing a toxin. Plasmid DNA can be isolated from this newly pathogenic bacterial strain and separated from the chromosomal DNA. To establish whether the plasmid DNA contains a gene encoding the toxin, you could determine the sequence of the entire plasmid and search for a sequence that looks like other toxin genes previously identified. An easier way exists to determine whether the plasmid DNA carries the gene(s) for the toxin; this strategy does not involve DNA sequence analysis. Describe this easier method.

Eric Tran
Eric Tran
Numerade Educator
View

Problem 18

Genes encoding toxins are often located on plasmids. A recent outbreak has just occurred in which a bacterium that is usually nonpathogenic is producing a toxin. Plasmid DNA can be isolated from this newly pathogenic bacterial strain and separated from the chromosomal DNA. To establish whether the plasmid DNA contains a gene encoding the toxin, you could determine the sequence of the entire plasmid and search for a sequence that looks like other toxin genes previously identified. An easier way exists to determine whether the plasmid DNA carries the gene(s) for the toxin; this strategy does not involve DNA sequence analysis. Describe this easier method.

Farhan Anwar
Farhan Anwar
Numerade Educator
02:19

Problem 19

a. You want to perform an interrupted-mating mapping with an $E .$ coli Hfr strain that is Pyr $^{+},$ Met $^{+}, \mathrm{Xyl}^{+}$ Tyr', Arg^ $+$ His $^{+}$, Mal' , and Stres. Describe an appropriate bacterial strain to be used as the other partner in this mating.
b. In an Hfr $\times \mathrm{F}^{-}$ cross, the $p$ yr $E$ gene enters the recipient in 5 minutes, but at this time point there are no exconjugants that are Met $^{+}, \mathrm{Xyl}^{+}, \mathrm{Tyr}^{+}$ $\mathrm{Arg}^{+}, \mathrm{His}^{+},$ or $\mathrm{Mal}^{+}$. The mating is now allowed to proceed for 30 minutes and $\mathrm{Pyr}^{+}$ exconjugants are selected. Of the Pyr $^{+}$ cells, $32 \%$ are Met $^{+}, 94 \%$ are $\mathrm{Xyl}^{+}, 7 \%$ are $\mathrm{Tyr}^{+}, 59 \%$ are $\mathrm{Arg}^{+}, 0 \%$ are $\mathrm{His}^{+},$ and
$71 \%$ are $\mathrm{Mal}^{+} .$ What can you conclude about the order of the genes?

Patina Herring
Patina Herring
Numerade Educator
00:33

Problem 20

Required to diagram recombination events that can replace specific genes on the chromosome of a recipient cell with copies of those genes introduced from a donor cell. As seen in the solution to Solved Problem I, only an even number of crossovers can produce viable recombinant chromosomes. Gene mapping is simplified if you remember that progeny classes that result from four crossovers are found much less often than progeny classes that require two crossovers.
In Problem $19,$ do you think that most of the Pyr $^{+}$ Arg $^{+}$ exconjugants are also $\mathrm{Xyl}^{+}$ and $\mathrm{Mal}^{+}$, or not? Explain your answer by considering the recombination events that would be required to generate colonies that are $\mathrm{Pyr}^{+}$ Arg $^{+} \mathrm{Xyl}^{+}$ Mal $^{+}$ and those required to make Pyr $^{+}$ Arg $^{+}$ Xyl $^{-}$ Mal $^{-}$ colonies.

Sam Limsuwannarot
Sam Limsuwannarot
Numerade Educator
01:30

Problem 21

Required to diagram recombination events that can replace specific genes on the chromosome of a recipient cell with copies of those genes introduced from a donor cell. As seen in the solution to Solved Problem I, only an even number of crossovers can produce viable recombinant chromosomes. Gene mapping is simplified if you remember that progeny classes that result from four crossovers are found much less often than progeny classes that require two crossovers.
One issue with interrupted-mating experiments such as that in Problem 19 is that gene order may be ambiguous if the genes are close together. Another shortcoming is that such experiments do not provide accurate map distances. The reason is that researchers select for the first Hfr marker transferred into the recipient, but the recovery of $\mathrm{F}^{-}$ exconjugants with a later Hfr marker is complex, depending both on transfer of the marker into the cell and on crossovers that transfer the marker into the recipient chromosome. To make more accurate maps, bacterial geneticists often do Hfr $\times \mathrm{F}^{-}$ crosses in a different way: They select for exconjugants that contain a late Hfr marker, and then screen for the presence of the earlier markers. This method ensures that all of the markers have entered the $\mathrm{F}^{-}$ cell, so relative gene distances are now accounted for solely by crossover frequencies. Furthermore, gene order is clarified by considering the crossovers responsible for each class of exconjugants. As an example, suppose you performed the same cross as in Problem $19,$ but you selected for $\mathrm{Arg}^{+}$ exconjugants, and then screened them for the earlier Hfr markers $\mathrm{Mal}^{+} \mathrm{Xyl}^{+}$ and $\mathrm{Pyr}^{+} .$ You obtained the following data:
a. Explain why four of the exconjugant types are much more frequent than the other two.
b. What can you conclude about the relative distances between the four genes?
c. The data allow you to estimate one other relevant genetic distance. Explain.

James Kiss
James Kiss
Numerade Educator
01:30

Problem 22

Required to diagram recombination events that can replace specific genes on the chromosome of a recipient cell with copies of those genes introduced from a donor cell. As seen in the solution to Solved Problem I, only an even number of crossovers can produce viable recombinant chromosomes. Gene mapping is simplified if you remember that progeny classes that result from four crossovers are found much less often than progeny classes that require two crossovers.
Suppose you have two Hfr strains of $E .$ coli (Hfra and HfrB $),$ derived from a fully prototrophic streptomycinsensitive (wild-type) $F^{-}$ strain. In separate experiments you allow these two Hfr strains to conjugate with an $\mathrm{F}^{-}$ recipient strain (Rcp) that is streptomycin resistant and auxotrophic for glycine (Gly $^{-}$ ), lysine (Lys $^{7}$ ), nicotinic acid (Nic $^{-}$ ), phenylalanine (Phe $^{-}$ ), tyrosine (Tyr), and uracil (Ura $7 .$ By using an interruptedmating protocol you determine the earliest time after mating at which each of the markers can be detected in the streptomycin-resistant recipient strain, as shown here.
a. Draw the best map you can from these data, showing the relative locations of the markers and the origins of transfer in strains HfrA and HfrB. Show distances where possible.
b. To resolve ambiguities in the preceding map, you studied cotransduction of the markers by the generalized transducing phage P1. You grew phage $\mathrm{P} 1$ on strain HfrB and then used the lysate to infect strain Rcp. You selected 1000 Phe $^{+}$ clones and tested them for the presence of unselected markers, with the following results:
Draw the order of the genes as best you can based on the preceding cotransduction data.
c. Suppose you wanted to use generalized transduction to map the gly gene relative to at least some of the other markers. How would you modify the cotransduction experiment just described to increase your chances of success? Describe the composition of the medium you would use.

James Kiss
James Kiss
Numerade Educator
01:30

Problem 23

Required to diagram recombination events that can replace specific genes on the chromosome of a recipient cell with copies of those genes introduced from a donor cell. As seen in the solution to Solved Problem I, only an even number of crossovers can produce viable recombinant chromosomes. Gene mapping is simplified if you remember that progeny classes that result from four crossovers are found much less often than progeny classes that require two crossovers.
Starting with an $\mathrm{F}^{-}$ strain that was prototrophic (that
is, had no auxotrophic mutations) and $\operatorname{Str}^{s}$, several independent Hfr strains were isolated. These Hfr strains were mated to an $\mathrm{F}^{-}$ strain that was $\mathrm{Str}^{\mathrm{r}} \mathrm{Arg}^{-} \mathrm{Cys}^{-}$
His $^{-}$ Ilv $^{-}$ Lys $^{-}$ Met $^{-}$ Nic $^{-}$ Pab $^{-}$ Pyr $^{-}$ Trp $^{-}$. Interruptedmating experiments showed that the Hfr strains transferred the wild-type alleles in the order listed in the following table as a function of time. The time of entry for the markers within parentheses could not be distinguished from one another.
a. From these data, derive a map of the relative position of these markers on the bacterial chromosome. Indicate with labeled arrows the position and orientation of the integrated $\mathrm{F}$ plasmid for each Hfr strain.
b. To determine the relative order of the $t r p, p y r,$ and cys markers and the distances between them, HfrB
was mated with the $\mathrm{F}^{-}$ strain long enough to allow transfer of the nic marker, after which Trp $^{+}$ recombinants were selected. The unselected markers pyr and $c y s$ were then scored in the Trp $^{+}$ recombinants, yielding the following results:
Draw a map of the $t r p, p y r,$ and $c y s$ markers relative to each other. (Note that you cannot determine the order relative to the nic or his genes using these data.) Express map distances between adjacent genes as the frequency of crossing-over between them.

James Kiss
James Kiss
Numerade Educator
01:26

Problem 24

You can carry out matings between an Hfr and $\mathrm{F}^{-}$ strain by mixing the two cell types in a small patch on a plate and then replica plating to selective medium. This methodology was used to screen hundreds of different cells for a recombination-deficient $r e c A^{-} \mathrm{mu}-$ tant. Why is this an assay for RecA function? Would you be screening for a recA $^{-}$ mutation in the $\mathrm{F}^{-}$ or Hfr strain using this protocol? Explain.

Jorge Villanueva
Jorge Villanueva
Numerade Educator
View

Problem 25

Genome sequences show that some pathogenic bacteria contain virulence genes that promote disease next to genes that originally came from bacteriophage. Why does this result suggest horizontal gene transfer, and what would the mechanism of transfer have been?

Farhan Anwar
Farhan Anwar
Numerade Educator
View

Problem 26

Generalized and specialized transduction both involve bacteriophages. What are the differences between these two types of transduction?

Farhan Anwar
Farhan Anwar
Numerade Educator
04:24

Problem 27

This problem highlights some useful variations of the gene identification by plasmid transformation procedure shown in Fig. 14.28
a. Suppose you have obtained a new bacterial mutant strain with a phenotype of interest. To determine the affected gene, you sequence the entire genome of the mutant strain and compare it with that of a wild-type strain. One of the differences found is a nonsense mutation that seems to be a good candidate. How would you use a plasmid library to verify that this nonsense mutation is responsible for the mutant phenotype?
b. Figure 14.28 showed how plasmid libraries could be used to identify genes with loss-of-function mutations that are responsible for a given aberrant phenotype. How could you use a plasmid library to identify a gene affected by a gain-of-function mutation?

James Kiss
James Kiss
Numerade Educator
02:14

Problem 28

A researcher has a Trp $^{-}$ auxotrophic strain of $E .$ coli with a mutation in a single gene. To identify that mutant gene, she uses a genomic library made from a wild-type version of that same strain to find plasmids that rescue the mutant phenotype. The result is surprising. She recovers 10 plasmids that provide a Trp $^{+}$ phenotype, but six of the plasmids contain gene
$X,$ while the other four contain gene $Y$. Our scientist has encountered a phenomenon called multicopy suppression, related to the fact that plasmids are usu-
ally present in several copies per bacterium. Because the genes in the plasmids are present in more than their usual single copy in the bacterial chromosome, more than the usual amount of Protein $X$ or Protein $Y$ is being produced from the plasmids. Sometimes, over expression of one protein can rescue the mutant phenotype caused by loss of a different protein. Suggest at least two ways that our scientist could determine which of the two genes, gene $X$ or gene $Y$, actually corresponds to the mutant gene causing the Trp $^{-}$ phenotype.

James Kiss
James Kiss
Numerade Educator
01:32

Problem 29

Streptococcus parasanguis is a bacterial species that initiates dental plaque formation by adhering to teeth. To investigate ways to climinate plaque, rescarchers constructed a plasmid, depicted in the figure shown, to mutagenize $S$. parasanguis. The key features of this plasmid include rep$A^{\text {ss}}$ a temperature-sensitive origin of replication $), k a n^{r}$ (a gene for resistance to the antibiotic kanamycin), and the transposon IS256. This transposon contains the $e r m^{r}$ gene for resistance to the antibiotic erythromycin. IS256 transposes in $S$. parasanguis
a. How could the researchers use this plasmid as a mutagen? Consider how they could get the transposon's into the bacteria, and how they could identify strains that had new insertions of $I S 256$ into $S$. parasanguis genes. Your answer should explain why the plasmid has two different antibiotic resistance genes as well as a temperature-sensitive origin of replication. b. Why would the researchers use this plasmid as a mutagen?
c. If the investigators found a mutant strain of $S$. parasanguis that was defective in plaque formation,
how could they identify the affected gene?

Josee Pacheco
Josee Pacheco
Numerade Educator
04:19

Problem 30

The sequence at one end of one strand of the Drosophila transposon Mariner is shown below (dots indicate sequences within the transposon): $5^{\prime}$ TTAGTT TGGCAAATATCTCCCTTCCGCCTTTTTGATCTTATGT.... 3'
You obtain a mutant bacterial strain tagged with an engineered Mariner transposon, cut the genomic DNA from this strain with the restriction enzyme MboI (whose recognition site is $\left.^{\wedge} \text { GATC }\right)$, and circularize the resultant DNA fragments by diluting the restriction enzyme digest and adding DNA ligase.
a. Design two 17 bp PCR primers that you could use to identify (by inverse PCR) the gene into which the transposon inserted.
b. What DNA sequence will be amplified from the circularized fragments of the mutant genome? Show the extent of this DNA sequence on a map of the genome of the mutant strain, indicating the locations of the transposon insertion and any relevant sites for the enzyme $M b o \mathrm{I}$.

Khalida Dawar
Khalida Dawar
Numerade Educator
11:02

Problem 31

Scientists can use gene targeting not just to knock out genes (as was shown in Fig. 14.31 ), but also to introduce nonbacterial genes into bacterial chromosomes. One such gene in wide use is a gene from jellyfish encoding Green Fluorescent Protein. In one example of this strategy, suppose you want to make $E$. coli into a biosensor to detect the highly toxic metal cadmium. The $E .$ coli genome has a gene called yodA that is only transcribed (and its mRNA translated) in the presence of cadmium. You want to use gene targeting to make a strain of $E .$ coli that will fluoresce brightly in green when cadmium is present in the environment.
a. Draw a DNA construct that you could use to exchange the yod $A$ coding sequence with that for jellyfish Green Fluorescent Protein. Would you obtain the jellyfish DNA from genomic DNA or from a cDNA clone?
b. Explain why yod $A$ is no longer functional in bacteria that glow green in the presence of cadmium.
c. Can you think of a way to alter the approach so that yod $A$ might remain functional?

Jenny Wu
Jenny Wu
Numerade Educator
03:59

Problem 32

Scientists who study amino acid biosynthesis pathways want to isolate auxotrophic bacteria. A technique called penicillin enrichment makes this task easier. This procedure starts by exposing a liquid culture of wild-type (prototrophic) bacteria growing in rich (complete) medium to a chemical mutagen. After this treatment, the cells are centrifuged to remove the liquid and the mutagen. The pellet of cells at the bottom of the centrifuge tube is now resuspended in medium that lacks one amino acid (in this example, cysteine) but contains penicillin. Subsequently, the bacteria are poured onto a filter that concentrates them and allows them to be washed free of the penicillin. The living bacteria retained on the filter are highly enriched for cysteine auxotrophs. a. Given what you know about the action of pencillin, explain why this enrichment occurs.
b. Penicillin enrichment is not a selection, because the drug does not kill $100 \%$ of the prototrophs. The cells on the filter thus need to be screened for cysteine auxotrophy. How would the scientists perform this screen?
c. If the starting strain contained a pen $^{r}$ gene on a plasmid, would this scheme still enrich for auxotrophs? Explain.

James Kiss
James Kiss
Numerade Educator
04:19

Problem 33

Suppose that you could obtain radioactively labeled penicillin. How could you use this compound to distinguish whether a penicillin-resistant bacterium harbors a gene encoding penicillinase or whether the bacterium has acquired a mutation in penA , penB, or mtr?

Rabeya Zahid
Rabeya Zahid
Numerade Educator
02:40

Problem 34

Scientists are using metagenomics to tackle one of the most significant problems affecting human beings: the resistance of many pathogenic bacteria to currently available antibiotics. One aspect of solving this problem is developing different bactericidal drugs. To do so, researchers are taking advantage of the discovery that several bacterial species synthesize toxins that enable them to prey on other bacterial species. Remarkably, these scientists have discovered that the enzymes that work in synthesis pathways to make such toxins often have particular structures and therefore characteristic patterns within their amino acid sequences.

Describe how metagenomic analysis of the microbiomes of soil, the ocean, or the human body could enable researchers to discover new antibiotics that could be effective on human pathogens.

James Kiss
James Kiss
Numerade Educator
04:01

Problem 35

Some scientists are trying to engineer bacteriophage to treat bacterial infections in humans when the infections do not respond to chemical antibiotics.
a. What possible advantages might phage therapy have over antibiotic therapy?
b. Describe potential difficulties that would need to be overcome for phage therapy to succeed.
c. How might researchers best confront the issue that bacterial cells could become resistant to bacteriophage just as they could to antibiotics?

Bryan Valdivia
Bryan Valdivia
Numerade Educator