infectious diseases. By 1946, however, approximately 14 percent of
Staphylococcus aureus strains isolated at a London hospital were resistant
to penicillin. Today, scientists estimate that more than 95 percent of all
S. aureus strains are penicillin-resistant.
After the introduction of penicillin, additional antibiotics were rapidly
isolated and developed, including streptomycin and the tetracylines.
Today, more than 100 antibiotics are available. Nevertheless, some strains
of at least three bacterial species ( Enterococcus faecium, Mycobacterium
tuberculosis, and Pseudomonas aeruginosa) are resistant to all the antibiotics 71
Emerging and Re-emerging Infectious Diseases
available to treat these species, and healthcare workers fear that the time is
rapidly approaching when more deadly organisms escape the effects of all
known antibiotics.
The primary reason for the increase in antibiotic resistance is the excessive
use of antibiotics. When mutant genes arise that make a bacterium less
sensitive to an antibiotic, that bacterium survives and produces descendants
in an environment rich in antibiotics. That is, the process of natural
selection operates. Multiple mutations may be necessary for fully resistant
bacteria. However, once resistant genes appear, bacteria have a variety
of mechanisms for exchanging those (and other) genes both within and
across species. These mechanisms include conjugation, transformation,
transduction, and transposon-mediated exchange. This exchange allows
for “accelerated evolution” of bacterial species (accelerated in the sense that
random mutations that result in antibiotic resistance need not occur in
every individual bacterium, or even in every species of pathogen, but can
simply be acquired from another organism).
This lesson invites students to explore one reason for the re-emergence
of some infectious diseases: the evolution of antibiotic resistance among
pathogens. In Lesson 4, Protecting the Herd, students explore another reason
for the re-emergence of infectious diseases.
In Advance
Photocopies and Transparencies
Equipment and Materials
• 1 copy per student of Masters
• (Optional) Computers with
3.1, 3.2, 3.4
access to the Internet
• 1 copy per student for the print-
• All items needed for the lab
based version only of Master 3.3
(see page 80)
• 1 copy per group of Master 3.5
Students complete this lesson during several (five to seven) class periods.
You will need to prepare the materials for the laboratory exercise. Ordering
information and preparation directions are on pages 80–81, immediately
following the lesson.
For classes with access to the Internet:
Information about the safe use of microorganisms in
classrooms, including lists of organisms considered safe
for students at various levels of school, can be found at
http://www.science-projects.com/safemicrobes.htm. Leaders in infectious disease research, including scientists from NIH, contributed to the
Web site. Pseudomonas fluorescens, the organism used in the laboratory
exercise in this lesson, is included on the list of microorganisms
considered appropriate for students in grade 9 or higher. Nevertheless,
experts acknowledge that people who are immunocompromised may be
at risk for infection by organisms that do not affect healthy individuals.
72
We recommend that you read a statement such as the following to your
classes before beginning the lesson:
Pseudomonas fluorescens, the bacterium used in the laboratory exercise
you will begin soon, does not cause disease in healthy people. However,
people who have weakened immune systems should not have contact with
most microorganisms or with people who handle those organisms. Your
immune system may be weakened if you are undergoing antibiotic therapy,
if you are taking immunosuppressive drugs or drugs for cancer treatment,
or if you have AIDS or are HIV-positive. If you have a weakened immune
system for these or any other reasons, let me know, and I will give you an
alternative experience that is safer for you.
Students who should not participate in the laboratory exercise can view
a video demonstration of it on the Web site, as described in the following
paragraphs. They can rejoin the class on Day 3 of the lesson, after the other
students have recorded their results and discarded their bacterial cultures.
If you do not have the time or facilities to conduct the laboratory exercise,
you will need only one day to complete this lesson. Complete Steps 1 to 3,
Day 1, and then have students view a video demonstration of the laboratory
exercise, Bacterial Growth Experiment, on the Emerging and Re-emerging
Infectious Diseases Web site. Students will need copies of Master 3.1 to help
them follow the steps in the demonstration. Then, move to Day 3 of the
lesson.
To set up computers, go to http://science.education.nih.gov/supplements/
diseases and choose “Web Portion of Student Activities.”
Note to teachers: If you don’t have enough computers equipped with
Internet access to conduct Steps 4 and 5 on Day 3, you can use the
print-based alternative (page 78).
DAY 1 (5 to 7 days before Day 3 of the lesson)
Procedure
1. Remind students of the theory of evolution. Explain that theories in
science are well-accepted explanations about some natural phenomenon
and are backed up with a great deal of scientific evidence. The greater
the evidence and the more diverse the evidence, the stronger the theory.
The evidence comes from scientists who generate hypotheses and
conduct experiments to test their hypotheses.
Students should be able to state the basic elements of the theory
of evolution: 1) there is variation among the individuals in a
population; 2) some of these differences can be inherited; 3) some
individuals will be better adapted to their environment than others;
4) the better-adapted individuals will reproduce more successfully; and
5) thus, the heritable characteristics that make individuals better
adapted will increase in frequency in the population.
73
Student Lesson 3
Emerging and Re-emerging Infectious Diseases
2. Organize students into groups of three and challenge the groups
to use their understanding of evolution by natural selection to write
a hypothesis about what will happen in a population of bacteria after
growing for several generations in the presence of an antibiotic.
If students have difficulty with this, stimulate their thinking by asking
questions such as, “What effect does an antibiotic usually have on
bacteria? Do you know of cases in which that effect did not occur?
What does that suggest about variations that exist in the bacteria
population? Which bacteria survived? What trait did they pass on
to other progeny?”
3. Convene a class discussion in which you ask several groups to share
the hypotheses they developed. Challenge the class to work together
to refine them into one hypothesis similar to the following:
If a bacterial culture is grown in a medium containing an antibiotic,
then after several generations, all the bacteria in the culture will
be resistant to the antibiotic.
4. Tell students that they will conduct an experiment to test this
hypothesis, and explain that they will also consider the implications
of their results for controlling infectious diseases in an activity
the following week. Then, distribute Master 3.1, Bacterial Growth
Experiment, and instruct students to complete Steps 1 through 4
with their group members.
Emphasize that for safety reasons as well as the success of their
experiments, students must use aseptic techniques. If students are not
familiar with aseptic techniques for handling bacterial cultures,
you will need to demonstrate them.
Alternatively, you can have your students view the “Day 1”
video segment of Bacterial Growth Experiment online, which
shows students using aseptic techniques as they prepare the
initial cultures in the experiment ( http://science.education.
nih.gov/supplements/nih1/diseases/activities/activity3.htm).
DAY 2 (2 to 3 days before Day 3 of the lesson)
1. Direct groups to complete Steps 5–8 on Master 3.1 .
DAY 3
1. Tell students that today they will analyze the results of the bacterial
growth experiment they have been running and will use those
results to help explain what happened to a high school student
who had tuberculosis.
74
2. Organize students into groups and instruct them to collect their
bacterial growth plates. While they do this, give each student a
copy of Master 3.2, Discussion Questions for the Bacterial Growth
Experiment. Tell the groups to draw (or describe) their results on
the flow chart on Master 3.1c first, then refer to those results
as they discuss and write answers to the discussion questions
on Master 3.2.
Depending on students’ microbiology background, you may need to
explain that when a single, microscopic bacterium is placed on an agar
plate, it will grow and divide into two progeny cells. Each progeny
cell will grow and divide, and so on, until thousands and thousands
of individual bacteria are growing right in that spot. At this point, the
growth becomes visible to us as a colony of bacteria. Each colony came
from a single original bacterium on the plate. When approximately
10,000 or more bacteria are plated, each individual bacterium is close
enough to a neighboring bacterium that the colonies they produce
merge together, and we observe confluent growth, or a “lawn,” of
bacteria across the plate.
Move among the groups as they discuss each question and help lead
students to the following understandings.
Question 1. Compare the bacterial growth on the two plates from
the parental culture (Plates 1 and 2). Which has more growth?
Explain why. How do you explain the presence of bacteria on
the plate containing kanamycin?
The nutrient agar plate (Plate 1) should show a lawn of bacteria, or
confluent growth, whereas the plate containing kanamycin should
show only 50 to 100 colonies. Students should explain that the
antibiotic prevented the growth of most of the bacteria on Plate 2.
A simple, straightforward answer is all students need to provide for
the last question: The bacteria that grew on Plate 2 were resistant
to the antibiotic.
Question 2. Compare the growth on Plates 3 and 4, which you prepared
from culture A (without kanamycin). How does the growth on the plates
with and without kanamycin appear? What does this tell you about the
bacteria grown in culture A?
The plate without kanamycin (Plate 3) should show a lawn of bacterial
growth, whereas the plate with kanamycin (Plate 4) should show 50 to
100 colonies. The results on Plate 3 indicate that a lot of bacteria were
growing in the sample plated from culture A. Comparing the results
on that plate with the results on Plate 4 indicates that some of the
bacteria in the culture (for example, 50 out of 10,000 or more) were
resistant to the antibiotic, but most were not.
75
Student Lesson 3
Emerging and Re-emerging Infectious Diseases
Question 3. Compare the growth on Plates 5 and 6, which you prepared
from culture B (with kanamycin). How does the growth on the plates
with and without kanamycin appear? What does this tell you about
the bacteria grown in culture B?
Both plates should show a lawn of bacterial growth. This indicates
that most or all of the bacteria growing in this culture were resistant
to kanamycin.
Question 4. Compare the growth of cultures A and B on Plates 4 and
6 (with kanamycin). Explain how culture B could have so many more
resistant bacteria than culture A, even though they both came from the
same parental culture.
If, after a minute or two of discussion, students cannot offer an
explanation, suggest that they use their understanding of natural
selection to explain the difference in the results on the plates for the
two cultures. They should be able to explain that the environment
in culture B (which contained kanamycin) selected for the growth of
those bacteria that were resistant to kanamycin. By the time students
plated a sample from that culture, all of the bacteria in the sample
were resistant, so they all grew on the plate with kanamycin, resulting
in a lawn of bacterial growth (Plate 6). Culture A did not contain
kanamycin, so there was no selection for kanamycin resistance,
and most of the bacteria students plated from that culture were
not resistant. Thus, most did not grow on the plate with kanamycin
(Plate 4).
Question 5. How do you explain the presence of some resistant bacteria
in the parental culture and culture A?
To answer this question, students must recognize that bacteria
become resistant (for example, through mutation) before natural
selection operates. In other words, the bacteria in the parental strain
did not “know” that some of them would be placed in growth medium
with kanamycin and “respond” by becoming resistant. Instead, in
the parental strain, a few bacteria were already present that were
resistant to kanamycin, even though no kanamycin was present.
Similarly, a few bacteria in culture A were resistant to kanamycin,
even though no antibiotic was present. When the resistant and
nonresistant bacteria from the parental culture were placed in medium
containing kanamycin (culture B), only the resistant bacteria survived
and reproduced, passing their kanamycin resistance trait on to their
progeny. Soon, virtually all the bacteria in the culture—the progeny
of the original resistant bacteria—were resistant to kanamycin, as
observed on the students’ plates.
76
3. Convene a brief class discussion in which you clarify any confusion
you noted as you circulated among the groups and/or invite students
to ask questions about the results of their experiments.
Steps 4 and 5 for classes with access to the Internet:
4. Tell students that they will watch a young woman
named Debi French discuss her battle with
tuberculosis. Then, they will use the results of their bacterial
growth experiments to help explain what happened in her struggle
with the disease. Ask groups to take their copies of the flow chart
and Discussion Questions with them to the computer stations.
Emphasize that the bacterium in their experiment ( P. fluorescens) is
not the kind that causes tuberculosis ( M. tuberculosis). P. fluorescens
does not cause disease in healthy people. Furthermore, the antibiotic
kanamycin is not used clinically, so the resistant bacteria cultured
As they use the results of
in this exercise do not compromise medical treatments. Emphasize,
however, that all bacterial cultures in your class are decontaminated
their bacterial growth
before disposal and that aseptic conditions must be followed in all
experiment to explain
work with microorganisms.
what happened to Debi
French, students will
5. Distribute a copy of Master 3.4, Debi’s Story: Explaining What
experience how basic
Happened, to each student and tell them to click on Debi’s Story
research leads to expla-
to start the video. Indicate that students have 20 minutes to answer
nations for disease and
the questions on Debi’s Story.
for the success or failure
You may want to emphasize to students that this is a true story, and
of disease treatment. This
that Debi herself tells her story on the video.
understanding leads
scientists to pro pose
Organizing student groups at individual computer stations to view
further research and
Debi French’s story will allow students to complete this part of the
policies directed at
lesson at their own pace. An alternative, if you have the equipment
to project the video from the Web site onto a large screen for whole-
improving public health.
class viewing, is to show the first part of the video to the class,
then reorganize students into their groups. After the groups have
discussed and written answers to the first set of questions on
Master 3.4a, reconvene the class to watch the second part of the
video. Instruct students to return to their groups to answer the
second set of questions on the handout. Follow this process until
students have completed their study of Debi’s story.
You may need to remind students of the information they learned
about tuberculosis in Lesson 1.
77
Student Lesson 3
Emerging and Re-emerging Infectious Diseases
Steps 4 and 5 for classes using the print version of the lesson:
4. Tell students that they will learn about a young woman
named Debi French and her battle with tuberculosis. They
will use the results of their bacterial growth experiments to help
explain what happened in her struggle with the disease.
Emphasize that the bacteria in their experiment ( P. fluorescens) is
not the kind that causes tuberculosis ( M. tuberculosis). P. fluorescens
does not cause disease in healthy people. Furthermore, the antibiotic
kanamycin is not used clinically, so the resistant bacteria cultured
in this exercise do not compromise medical treatments. Emphasize,
however, that all bacterial cultures in your class are decontaminated
before disposal and that aseptic conditions must be followed in all
work with microorganisms.
5. Give each student one copy of Masters 3.3, Debi’s Story, and 3.4, Debi’s
Story: Explaining What Happened. Indicate that students have 20
minutes to read about Debi and answer the questions on Debi’s Story.
You may want to emphasize to students that this is a true story.
You may want to remind students of the information they learned
about tuberculosis in Lesson 1.
6. Convene a whole-class discussion in which you ask several groups
to share their responses to the questions on Master 3.4. Invite the
other groups to add information and disagree with these responses.
Then, ask students, “What explanation does the Debi French example
The Debi French example
suggest for the re-emergence of diseases like tuberculosis?”
reminds students of the
major concept of the
Students should be able to provide answers such as the following:
activity: One explanation
Sentence 1
for the re-emergence of
• Debi contracted tuberculosis (TB) from a student in one of her
infectious diseases is
classes who had an active, misdiagnosed case of TB. Debi did not
resistance of the causative
know this student.
agent to the treatment that
• The symptoms Debi had were fatigue, weight loss, and a severe,
once cured infections of
persistent cough.
that agent. The important
Sentence 2
public health issue is
• The treatment to cure TB is a combination of several antibiotics. Debi
avoiding inappropriate
named standard drugs used for TB such as isoniazid and rifampin.
use of antibiotics as a way
• When Debi started the treatment, she initially got better.
to minimize, or at least
delay, the evolution of
Sentence 3
resistant pathogens.
• Debi’s health began improving when she started the drug therapy
for TB because the bacteria that caused her tuberculosis were killed
(or their growth was inhibited) by the drugs she was taking.
78
Sentence 4
• On Valentine’s Day 1994, Debi learned that her tuberculosis was
active again.
• The drugs Debi took to cure her TB were not working because the
bacteria that caused her TB had become resistant to the drugs.
Sentence 5
• Debi had a relapse (developed an active case of TB again), even