CASE TEACHING NOTES
for
"Dr. Collins and the Case of the Mysterious Infection"
by
Paula P. Lemons and Sarah Huber
Biology Department
Duke University
INTRODUCTION / BACKGROUND
This case study was designed for the seminar component of Introductory Biology at Duke University. In
addition to seminar, students in this course also attend lecture and laboratory. Teaching assistant (TA)
mentors lead the 12-student seminars during which students engage in directed inquiry exercises.
Students work through the case study in the second week of the course. Topics covered in the lecture
and lab components of the course prior to this case include organisms and cells as units of life, proteins
and membranes, DNA structure, replication, and repair, and how to ask good questions in biology.
Objectives
After completing this case, students should be able to:
1. Describe some of the biochemical mechanisms by which antibiotics act against bacteria.
2. Evaluate which antibiotics would be effective against a given type of bacteria.
3. Apply their basic knowledge of DNA, genes, and proteins (and the relationship between them) to
a new question. That is, how do changes in DNA observed by pulsed-field gel electorophoresis
impact patterns of antibiotic resistance?
4. Synthesize the relationship between antibiotic consumption and antibiotic resistance based on
what they have learned about bacteria, antibiotics, and natural selection.
5. More adeptly use data from charts and gels to answer questions.
CLASSROOM MANAGEMENT
Students are expected to have read the background information about bacteria, antibiotics, and the
evolution of antibiotic resistance before coming to class. We assigned this reading in seminar the
previous week. The primary aim of the reading is to introduce students to the topics of bacteria,
antibiotics and antibiotic resistance.
This case is designed as an "interrupted case," that is, students are given one piece of information at a
time and asked to do something with that information. After they have completed one part, they receive
the next piece of information. There are four parts to the exercise.
Given that we use this case study for our 12-student seminar sections, we make the following
suggestions to TA mentors about how to teach the case although they are free to choose whichever
method they think will work best with their particular section.
1. Small group work. Each group of three or four students receives the first part of the exercise and
is given a specified amount of time to work through the questions in that part. At the end of the
designated time, the TA mentor surveys most or all of the groups for their answers, then hands out
the next part. Again, groups are given a specified amount of time to work through the problem and
are surveyed by the TA mentor about their answers. The exercise is written so that the TA mentor
needs to provide little information during the polling times; each new part provides "the answer"
to the previous part. However, this changes after Part IV when students are asked three questions
that challenge them to think about how antibiotics select for resistant strains and the molecular
events that lead to the emergence of a new strain of bacteria. Part IV offers a lead-in for TA
mentors to end seminar with a class-wide discussion about these more open-ended questions.
NOTE: Our TA mentors have found that assigning specific jobs (e.g., group leader; time keeper,
and recorder/reporter) to each student in a group helps to keep group work moving at a steady
pace.
3. Class-wide discussion. In this format, the TA mentor functions as the group leader and moves
students as a class through the exercise. The TA mentor hands out each part, gives students a
chance to read the information, then begins discussing the problem. This format allows the TA
mentor to exert more control over the classroom, but may prevent some of the quieter students
from becoming as involved in the discussion.
At the end of both the background reading and the case, a list of references is provided. Students are not
required to read these references. Rather, these are provided for students and instructors who may want
to do more reading about antibiotic resistance and, more specifically, cases like the one described in this
case study.
In terms of time requirements, in Introductory Biology, seminar lasts 50 minutes. Based on TA mentors'
experiences, we estimate only five to seven minutes for both Parts I and II since they are rather
straightforward. Part III is generally the most challenging part, and Part IV generates the most
interesting discussion. Because of this, we recommend allowing at least 15 minutes for each of these
parts. It might also be possible to expand this case study beyond a 50-minute class session so that there
would be time for more discussion among students and between students and the instructor during each
part of the exercise. Although the current constraints of Introductory Biology at Duke University prevent
us from taking this approach, individuals who use this case study should feel free to make such
adaptations.
ANSWER KEY
Answers to the questions posed in the case study are provided in a separate answer key to the case.
Those answers are password-protected. To access the answers for this case, go to the key. You will be
prompted for a username and password. For the username and password, contact the National Center
for Case Study Teaching in Science administrator at answerkey@sciencecases.org.
ASSESSMENT OF LEARNING GOALS
As explained in the background reading, we use this case in Introductory Biology as one of three
installments in a series on antibiotic resistance. After working through this case, students deal with the
topic again in lab by surveying for the prevalence of antibiotic resistant bacteria carried by them and
their peers and in nearby water supplies (Lemons et al.). Finally, students work in groups during seminar
and outside of seminar to collect their results from lab and analyze these results via a writing
assignment.
Due to this structure, we do not use the case study class period for assessment. Rather, we assess
whether they have achieved the specified learning objectives in three independent ways. First, in the lab
that follows this case, students design an experiment in which they select antibiotics that will be
effective against members of the genus Staphylococcus and members of the family Enterobacteriaceae.
Thus, TA mentors are able to assess how well students are achieving learning objective 2 as they
observe the quality of each student's experimental setup. Second, students are assessed by their
performance on a group writing assignment dealing with antibiotic resistance. As part of this
assignment, students must synthesize what they have learned from lab about the relationship between
antibiotic resistance and antibiotic consumption (learning objective 4) to address the concern that Dr.
Collins raises in Week 2, Day 10 of the case (i.e., that increased use of antibiotics will lead to the
emergence of new strains of resistant bacteria). Finally, on exams, students are responsible for concepts
that have been covered not only in lecture but also in lab and seminar. Therefore, we frequently use the
curriculum on antibiotic resistance as a source for exam questions.
ADDITIONAL COMMENTS AND RECOMMENDATIONS
This case study has been run in 26 sections of Introductory Biology. Here are some additional comments
and recommendations based upon the experience of our TA mentors:
! Many of our TA mentors report that students are highly engaged by and become emotionally
invested in the case. For example, they express frustration that Dr. Collins did not prescribe the
antibiotic that they recommended and feel that the outcome for Kayla Starnes would have been
different if she had.
! Some TA mentors caution that due to the large amount of reading involved in the case, managing
individual students and groups of students who read at different paces can be challenging. Perhaps
one solution is to predetermine groups such that differences in pace among individual students
and groups of students will be minimized. Some TA mentors have also found it helpful to have a
series of additional questions for each part prepared ahead of time for groups that work more
quickly than others.
! In response to TA mentors comments, Part III was rearranged to place the description of pulse-
field gel electrophoresis into an appendix-like subsection. While some TA mentors felt that it was
worthwhile for students to invest time in understanding this technique, others felt that it was not
essential. We hope that including it as a subsection to Part III gives you the leeway to either use or
ignore this portion of the exercise.
FOLLOW-UP WRITING ASSIGNMENT
The students are given the following assignment in connection with this case study:
For more than a week, you have focused on the problem of antibiotic resistance. You learned about
bacteria - "good" ones and "bad" ones - and some of the details about how antibiotics affect them.
From that you learned about some of the mechanisms bacteria use to evade antibiotics and
understand that changes in the antibiotic resistance patterns of bacteria occur first on the molecular
level. You carried out experiments that address the question of whether there is a positive
correlation between antibiotic resistance and antibiotic consumption. Finally, you've begun to think
about how bacterial transformation and the relative growth rates of different types of bacteria
inform our understanding of the antibiotic resistance problem. As a final synopsis of all you have
learned in this series of the course, address the following two sets of questions in writing as a
group.
1. We have posted the results from our class-wide correlational study (see lab p. 6) to the course
web site under "Course Documents." Look at the table showing the mean number of
antibiotic courses per student in the following categories: penicillin resistant and penicillin
susceptible. Also look at the graph plotting the number of antibiotic courses versus the
percent of students with penicillin resistant bacteria. Do our results support or refute Dr.
Collins' concern that increased use of antibiotics will lead to emergence of new strains of
resistant bacteria (as a reminder about Dr. Collins' concern see Part IV of the case study "Dr.
Collins and The Case of the Mysterious Infection")? What additional experimental results
would provide more information to support or refute this concern?
2. In the bacterial transformation that you carried out in lab, did the E. coli carrying the ampr
gene or those not carrying the ampr gene grow faster? What does this result suggest about
what would happen to antibiotic resistance if we curbed our antibiotic consumption? How is
the bacterial transformation that you performed in lab limited in answering this question?
What type of experiment might provide better information to answer this question?
Your paper should address these questions in a concise and coherent way. If you find it helpful to
include additional references - journal articles, web pages, literature reviews - please do so, citing
them appropriately. This assignment should be a collaborative effort by your group and should be 2
to 3 double-spaced typed pages in length.
REFERENCES
Lemons, P.P., Corliss, T., and Motten, A.F. 2000. Bacteria and antibiotics. Written for the
laboratory component of Introductory Biology at Duke University, unpublished.
Acknowledgements: This case study was developed with support from The Pew Charitable Trusts and
the National Science Foundation as part of the Case Studies in Science Workshop held at the State
University of New York at Buffalo on May 22-26, 2000.
Date Posted: 08/01/01 nas
BACKGROUND READING
for
Dr. Collins and the Case of the Mysterious Infection
by
Paula P. Lemons and Sarah Huber
Biology Department
Duke University
Bacteria, Antibiotics, and the Evolution of Antibiotic-Resistance
Although we can't see them, bacteria are everywhere - living on almost every surface, in the soil (one
gram of surface soil contains more than 100 million bacteria), and even in some of the harshest
environments on earth (e.g., sulfur pools and near-boiling undersea hydrothermal vents). They play a
critical role in our ecosystem, carrying out essential processes such as nitrogen fixation and participating
in symbiotic relationships with other organisms that cause no harm to the host. Disruption of the delicate
interplay between bacteria and their environment is potentially very dangerous to the health and well-
being of all organisms. Consider the bacteria Staphylococcus aureus. Normally, this species lives in the
human oropharynx, nose, large intestine, vagina, and on the skin without causing harm. However, if a
breach in the skin or mucosal barrier occurs, S. aureus gains access to nearby tissues or the bloodstream
where it can colonize and cause disease. The relationship between S. aureus and its human host, then, is
dynamic in nature, capable of quickly shifting from mutualistic or commensualistic to parasitic.
The search for ways to eliminate diseases caused by bacteria led to the discovery of antibiotics. These
drugs kill or inhibit the growth of susceptible bacteria. When antibiotics became widely available in the
1940s they were hailed as miracle drugs - able to cure diseases and not just reduce their symptoms.
However, as early as 1950 strains of bacteria emerged that were resistant to all standard antibiotics. This
problem, which has only intensified in severity since the 1950s, demonstrates the biological principle of
natural selection. As antibiotic use increases, natural variants of bacteria able to resist antibiotics survive
longer than antibiotic susceptible bacteria, their progeny become more numerous, and the pool of
antibiotic resistance genes grows. Since bacteria easily exchange genetic information with each other,
resistance genes are passed between species, genera, and families of bacteria. Today, several species of
bacteria capable of causing life-threatening diseases are able to withstand exposure to every available
antibiotic, creating an antibiotic resistance crisis.
Antibiotics: Range of Effectiveness and Mechanisms of Action
The effectiveness of antibiotics in ameliorating disease depends on two factors:
1. Spectrum. The spectrum of an antibiotic refers to the diversity of bacteria against which an
antibiotic acts. Bacteria are typically classified as gram positive (e.g., S. aureus) or gram negative
(e.g., Escherichia coli). Gram staining is a procedure in which bacteria are exposed to crystal violet
dye, washed, and then exposed to a counter-dye. Gram positive bacteria retain the crystal violet dye
while gram negative bacteria do not. Other bacteria do not fit neatly into the gram positive/gram
negative classification scheme. These include classes of bacteria like mycobacteria, rickettsia, and
chlamydia. Generally, narrow spectrum antibiotics act against one class of bacteria while broad
spectrum antibiotics act against more than one class of bacteria. Table 1 shows the spectrum of
several antibiotics.
2. Selective toxicity. Selective toxicity is a measure of the degree to which an antibiotic is harmful to
bacteria but not the bacterial host. Antibiotics with high selective toxicity disrupt enzymes or
structures that are unique to bacteria; antibiotics with low selective toxicity inhibit the same process
in bacteria as in host cells, or damage host cells in some other way. Table 1 shows the selective
toxicity of various antibiotics as well as the cellular processes they disrupt.
Table 1. Common antibiotics and some of their characteristics.
Antibiotic Spectrum Selective Mechanism of action Symptoms for which they
toxicity are the drug of choice
Methicillin and Narrow High Inhibit peptidoglycan Inflammation of the lungs,
penicillin formation by binding strep throat, pathogenic
to the enzyme toxins in the blood, skin
(gram +)
transpeptidase infections, gonorrhea
Cefazolin Broad High " Inflammation of the lungs,
strep throat, pathogenic
toxins in the blood, skin
(gram +,
infections, urinary tract
some gram -
infections
)
Tetracyclines Broad Moderate- Bind to the small Acute diarrhea, vomiting or
High ribosomal subunit and cramps, inflammation of the
interfere with lungs, muscular pains
(gram +/-,
aminoacyl tRNA combined with skin
rickettsia
and binding eruptions;
chlamydia)
Note – not usually
prescribed to children due
to side effects during
formative years
Vancomycin Narrow Low Inhibits the synthesis Inflammation of the lungs or
of peptidoglycan by brain meninges, ear
binding to the amino infections
(gram +)
acid polymer
Trimethoprim- Broad High Inhibit folic acid Urinary tract infections,
sulfamethoxazole synthesis (bacteria bronchitis, ear infections
(Bactrimâ„¢) must make, mammals
(gram +,-)
acquire in diet)
Cefazolin is an example of a broad spectrum antibiotic that functions by blocking bacterial cell wall
synthesis. Bacterial cell walls, which maintain osmotic balance and provide an added layer of protection
against toxic substances, are made up of a typical plasma membrane as well as additional specialized
structures like the peptidoglycan (Figure 1). In gram positive bacteria, a series of enzymes that includes
transglycosylase and transpeptidase catalyzes cell wall formation; cefazolin binds to and inhibits
transpeptidase (Figure 2). Since animal cells do not possess cell walls, cefazolin has a high selective
toxicity. Several strains of bacteria have emerged that are resistant to all cefazolin-related antibiotics
(including methicillin and penicillin) and are referred to as MRSA (Methicillin resistant S. aureus).
Resistance to cefazolin-related antibiotics is usually conferred by the chromosomal gene mecA. MecA
encodes a protein that binds to and sequesters these antibiotics, preventing them from inhibiting
transpeptidase (Figure 2). The most common MRSA strain is hospital-acquired MRSA which is also
resistant to many other common antibiotics (e.g. tetracycline, Bactrim).
Over the next week and a half you will be examining the problem of antibiotic resistance. In seminar, you
will walk in the steps of Dr. Jenna Collins as she attempts to prescribe the appropriate antibiotic to her
seriously ill pediatric patient. In lab, you will take an experimental approach to the problem by surveying
for the prevalence of antibiotic resistant bacteria carried by you and your peers and residing in nearby
water supplies. Finally, you will work with a group of students in your section to summarize the results
from your lab and discuss some of the insights you've gained about this global problem.
References
! Baquero, F. and Blazquez, J. 1997. Evolution of antibiotic resistance. Tree 12(12): 482-487.
! Coppoc, G. L. 10/10/99. Chemotherapy: Drug Groups.
http://www.vet.purdue.edu/bms/courses/mcmp611/chmrx/chmrxtit.htm.
! Clinical Pharmacology Online. http://www.cp.gsm.com/. Tampa: Gold Standard Multimedia Inc.
! Levy, S. B. 1998. The challenge of antibiotic resistance. Scientific American 278(3): 46-53.
! Prescott, L. M., Harley, J. P., and Klein, D. A. Microbiology. Dubuque: Wm. C. Brown Publishers,
1990.
! Walsh, C. Deconstructing vancomycin. 1999. Science 284: 442-443.