Chapter 13: Natural Selection and Adaptation

Chapter 13: Natural Selection and Adaptation

Bugs That Resist Drugs: Drug-resistant bacteria are on the rise. Can we stop them?

Drug-Resistant Bacteria and MRSA

• Definition: Drug-resistant bacteria are strains of bacteria that have developed the ability to resist the effects of antibiotics, making infections harder to treat.
• Key Question: What is MRSA, and how can bacteria resist the effects of antibiotics?
• MRSA: Stands for Methicillin-Resistant Staphylococcus Aureus.
• Historical Context:
  • For over 70 years, humans have relied on antibiotics to successfully treat bacterial infections.
  • In recent years, some bacteria, such as MRSA, have developed resistance to these treatments, resulting in a public health problem.
• Consequences:
  • Patients may require multiple rounds of different antibiotics to treat common bacterial infections, leading to potential deaths from diseases that were treatable two decades ago.

Vocabulary

• Antibiotic: A chemical that can slow or stop the growth of bacteria; many antibiotics are produced by living organisms.

Discussion Questions

• DQ1 LO1: Explain why MRSA and similar bacteria are becoming a public health problem.
  • Increased infections due to resistance.
  • High death rates from treatable diseases.
  • The evolution of bacteria contributing to widespread antibiotic resistance.

Characteristics of Staphylococcus aureus (S. aureus)

• Shape: Spherical and clusters resembling grapes when viewed under a microscope.
• Common Presence: Found on skin and in the nose; approximately one-third of the U.S. population carries it.
• Infections Caused: Can lead to skin infections, boils, and serious conditions in immunocompromised populations (elderly, newborns, ill).

Transmission of S. aureus

• Transmitted through skin-to-skin contact or contaminated objects.
• People can be colonized but not infected by S. aureus due to the body's defenses.

Immune Defense Against S. aureus

• A healthy immune system prevents infection. Open wounds increase susceptibility.

Basic Features of Bacterial Genetic Diversity

• DQ2: How can populations of bacteria become genetically diverse?
  • Bacteria reproduce asexually through binary fission.
  • Variability is a result of mutations during DNA replication and gene transfer.
  • The rapid reproduction rate allows accumulation of mutations and genetic diversity.

Genetic Mechanisms in Bacteria

• Antibiotic resistance arises from genetic mutations and horizontal gene transfer, which can rapidly disseminate resistance traits through populations.

Mechanism of Action of Beta-lactam Antibiotics

• How Beta-lactams Work: Interfere with bacterial cell wall synthesis, essential for maintaining internal pressure. This leads to cell rupture and bacterial death.

Steps in Bacterial Reproduction (Binary Fission)

1. DNA replication occurs.
2. The cell wall begins to form around the divided DNA.
3. The cell wall and plasma membrane start to divide.
4. Daughter cells separate, each containing its own DNA and structures.

Accumulation of Mutations Over Time

• Binary fission results in exponential growth, making bacteria accumulate mutations more quickly.
  • Formula: At the end of one hour, assuming a replication time of 20 minutes, there would be $2^{3} = 8$ bacteria.
• Daily Calculation: If uninterrupted, bacterial replication results in $2^{72}$ bacteria in one day (calculated over 24 hours).
• Mutant strains can significantly increase in prevalence due to rapid replication rates.

Gene Transfer Mechanisms in Bacteria

• Genes can also be transferred between bacteria via a pilus, which facilitates the exchange of DNA sequences.

Interaction of Mutation, Binary Fission, and Gene Transfer

• These processes interact to form diverse populations that may include antibiotic-resistant strains, resulting in rapid adaptation under antibiotic pressure.

Concept of Evolution and Fitness

• Evolution in Bacteria: Facilitated by natural selection, which leads to the accumulation of traits needed for survival.
• Fitness: Refers to an organism's ability to survive and reproduce in its environment, influenced by its genetic traits and environmental pressures.

Natural Selection Scenarios

• Natural selection can rapidly shift populations toward traits that enhance survival, exemplified by antibiotic resistance in bacteria like MRSA.
• Scenario Examples:
  • Larger birds with stronger bills are favored during food shortages, showing directional selection.
  • A population of beetles undergoes changes not through selection pressures but through random reproduction, indicating genetic drift.

Mechanisms to Reduce Infection Risk

1. Hand hygiene: Washing hands with soap reduces transmission.
2. Vaccination: Prevents illness from specific antibiotic-resistant strains.
3. Antibiotic stewardship: Avoid unnecessary use of antibiotics to protect commensal bacteria and prevent engagement in selective pressures.
4. Awareness and education: Understanding the risks of antibiotic misuse.

Conclusion

• Rising rates of antibiotic-resistant bacteria pose a significant problem for public health. Understanding natural selection and bacterial adaptation is crucial in mitigating this issue and improving healthcare outcomes.

Review Questions

1. Natural selection does not cause individuals to change allele frequencies directly; it acts on populations.
2. For fitness changes to occur, genetic mutation or environmental change needs to exist.