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Honors Biology Unit 6 – Part 1: Bacteria and Viruses
Exploring Bacterial Evolution: The Antibiotics Controversy
Part 1: Background Information: Antibiotic Resistance
Function and Origin of Antibiotics
Antibiotics, also known as antibacterials, are types of medications that destroy or slow
down the growth of bacteria, thus they are used to treat infections caused by bacteria. While
many bacteria are either harmless or beneficial, some cause serious illness and disease in
plants, humans, and other animals.
The immune response systems of plants and animals are typically able to destroy foreign
bacteria before is can multiply and cause symptoms. In humans, even if symptoms do
occur, our immune system can often fight off the invasion; we have specialized white blood
cells that attack the bacteria cells. However, sometimes the bacterial infection is too
aggressive and cannot be fought off without the assistance of antibiotics.
In 1944, Alexander Fleming discovered the first antibiotic: penicillin. This antibiotic was
hailed as a miracle drug that killed bacteria but left the body unharmed. Unfortunately,
because penicillin was the only antibiotic available, it was heavily used during the late
1940's. By 1949 (only 5 years after we began using it), 59% of the bacteria in sick patients
were resistant to penicillin. How did this happen?
Developing Antibiotic Resistance
The overuse/improper use of antibiotics acts as a natural selection pressure that favors the
survival and reproduction of resistant bacteria. Widespread use of antibiotics increases the
proportion of resistant bacteria in a population. Over 100 antibiotics are currently available
for use in infected patients today, but without stricter regulations, many of these will
become ineffective in the next few years.
Bacteria resist antibiotics through minor changes in their biochemistry. First, bacteria may
possess enzymes that actually degrade antibiotics. Originally, only one group of bacteria
had the genes for these enzymes (Streptomyces sp.), but due to genetic recombination, this
enzyme is now found in a large number of bacteria groups. Second, bacteria can adapt
mechanisms that alter how an antibiotic enters the cell, making it difficult or impossible for
the antibiotic to disrupt the bacterial cell. They can also evolve to have modified target sites
for the antibiotic (the specific structures within the bacteria that the antibiotic attacks or
disrupts, such as ribosomes).
Spreading Antibiotic Resistance
These methods of resistance require relatively simple changes, in the DNA of the bacteria
cells and are passed on to not only the offspring of those cells but also other species of
bacteria that can easily incorporate them into their own genome.
Bacteria have special DNA elements called R plasmids that often contain resistance genes
and are easily passed to other bacteria (through conjugation). Bacteria may also take up
random sequences of DNA encountered in their environment (through transformation). If
this DNA contains resistance genes, the bacteria could become resistant.
Viruses can also serve as a means of DNA transfer (through transduction). When
bacteriophage viruses infect a host bacteria cell, DNA with resistance genes may accidently
be incorporated into the new viruses that are produced. If those new daughter viruses infect
other bacteria, the DNA for antibiotic resistance is spread to the other cells.
Finally, bacteria can reproduce as rapidly as once every 15 minutes. If a cell contains
resistance genes, all daughter cells will also have these genes.
Misuse and Overuse of Antibiotics: Promoting Antibiotic Resistance
In humans, antibiotics are often used when unnecessary, and when they are necessary, often
the wrong antibiotic is prescribed, selecting for bacteria that have developed or received a
resistance mechanism. Antibiotics are necessary when an infection cannot be eliminated
through the body’s natural defenses; they are unnecessary when incorrectly prescribed for a
viral infection like the "common cold." According to a study done by the Center for
Disease Control, of the 150 million outpatient prescriptions each year in the United States,
50 million were estimated to be unnecessary.
During hospital stays, patients are often prescribed broad-spectrum antibiotics that kill all
types of bacteria, rather than targeting a specific population. Since broad-spectrum
antibiotics affect all bacteria, they select for resistance in harmless bacteria, giving them the
potential to become pathogenic.
Antibiotics are also misused as growth enhancing supplements in food animals. Many of
these animals are given antibiotics to reduce inflammation in the intestine and allow more
efficient uptake of nutrients, enhancing growth. The large-scale use of antibiotics in
livestock acts as a tremendous selection pressure encouraging the spread of resistance
genes. Moreover, there is evidence to suggest that antibiotic resistant bacteria in food
animals are able to pass their resistance to bacteria that naturally live in humans and other
species.
Part 2: Organizing Our Understanding
Complete the table below using what you learned in class (notes), the information you
recorded during the documentary clip, and the article from Part 1.
How do bacteria reproduce and exchange
genetic information (genetic
recombination)?
How do these methods of genetic
recombination contribute to bacterial
evolution?
Part 3: Connecting Concepts: Bacterial Evolution and Antibiotic Resistance
1. What benefits do antibiotics provide to humans and other organisms (e.g. livestock)?
2. What human actions/practices lead to increases antibiotic resistance?
3. Because the use of antibiotics actually promotes the spread of bacterial resistance,
many scientists believe that the use of antibiotics must be more heavily regulated.
Given what you know about how antibiotic resistance spreads and the
actions/practices that contribute to this spread, what steps do you think we should
take to “regulate” antibiotic use? (Think: What does it mean to “regulate” our use of
antibiotics? What would this look like?)