Variation and Evolution » Antibiotic Resistance

What you'll learn this session

Study time: 30 minutes

The concept of antibiotic resistance and how it develops
Natural selection and its role in antibiotic resistance
How bacteria become resistant to antibiotics
The global impact of antibiotic resistance
Strategies to reduce antibiotic resistance
Real-world examples of antibiotic-resistant bacteria

Introduction to Antibiotic Resistance

Antibiotics have been one of medicine's greatest success stories, saving countless lives since their discovery. However, their effectiveness is now under threat due to a growing problem: antibiotic resistance. This occurs when bacteria change in ways that reduce or eliminate the effectiveness of antibiotics.

Key Definitions:

Antibiotics: Medicines used to prevent and treat bacterial infections.
Antibiotic resistance: The ability of bacteria to survive exposure to antibiotics that would normally kill them or stop their growth.
Natural selection: The process where organisms with favourable traits are more likely to survive and reproduce, passing these traits to the next generation.

💊 How Antibiotics Work

Antibiotics work by either killing bacteria (bactericidal) or preventing them from multiplying (bacteriostatic). They target specific parts of bacterial cells that are different from human cells, such as:

Cell walls (which human cells don't have)
Protein synthesis machinery
DNA replication systems

🦠 Bacteria Basics

Bacteria are single-celled organisms that reproduce rapidly through binary fission. A single bacterium can become millions in just hours. This rapid reproduction rate means that beneficial mutations can spread quickly through a population, including those that provide resistance to antibiotics.

How Antibiotic Resistance Develops

Antibiotic resistance is a perfect example of evolution through natural selection happening in real time. When bacteria are exposed to antibiotics, most die, but some may have random mutations that allow them to survive. These survivors then reproduce, creating more resistant bacteria.

The Process of Developing Resistance

💧 Step 1: Variation

A population of bacteria contains natural genetic variation. Some bacteria may have mutations that give them slight resistance to an antibiotic.

⚖ Step 2: Selection

When exposed to antibiotics, susceptible bacteria die, while those with resistance survive - this is natural selection in action.

👪 Step 3: Reproduction

The surviving resistant bacteria reproduce, passing their resistance genes to their offspring, creating a new resistant population.

Mechanisms of Antibiotic Resistance

Bacteria have evolved several clever ways to resist antibiotics:

🛡 Defensive Strategies

Enzyme production: Bacteria produce enzymes that break down antibiotics (e.g., beta-lactamases that destroy penicillin)
Altered target sites: Changing the structure of the target site so antibiotics can't bind to it
Reduced permeability: Changing cell membranes to prevent antibiotics from entering
Efflux pumps: Developing pumps that remove antibiotics from the cell before they can work

🔍 Genetic Transfer

Bacteria can acquire resistance genes through:

Vertical gene transfer: Passing resistance genes to offspring during reproduction
Horizontal gene transfer: Sharing resistance genes with other bacteria (even different species) through:

Conjugation (direct cell-to-cell contact)
Transformation (uptake of DNA from environment)
Transduction (via viruses that infect bacteria)

Case Study Focus: MRSA

Methicillin-resistant Staphylococcus aureus (MRSA) is one of the most well-known antibiotic-resistant bacteria. It first appeared in the 1960s shortly after methicillin was introduced to treat penicillin-resistant Staphylococcus aureus infections.

MRSA has developed resistance to multiple antibiotics and can cause serious infections, particularly in hospitals. It has the mecA gene, which produces a modified penicillin-binding protein that has a low affinity for beta-lactam antibiotics (including penicillins and cephalosporins).

In the UK, MRSA infections have decreased due to improved hospital hygiene practices, but they remain a significant concern worldwide.

Factors Accelerating Antibiotic Resistance

Several human practices have sped up the development of antibiotic resistance:

💊 Misuse in Healthcare

Prescribing antibiotics for viral infections (which antibiotics can't treat)
Patients not completing the full course of antibiotics
Using broad-spectrum antibiotics when narrow-spectrum would suffice
Poor infection control in hospitals

🐄 Agricultural Use

Using antibiotics to promote growth in livestock
Preventative use in healthy animals
Environmental contamination from farm runoff
Creating reservoirs of resistant bacteria that can spread to humans

The Global Impact of Antibiotic Resistance

Antibiotic resistance is not just a medical problem; it's a global health crisis with far-reaching consequences:

Healthcare challenges: Infections become harder to treat, requiring longer hospital stays, more expensive medicines and resulting in more deaths
Economic burden: Estimated to cost billions in healthcare expenses and lost productivity
Modern medicine threatened: Many procedures (surgeries, cancer treatments, organ transplants) rely on effective antibiotics to prevent or treat infections

Alarming Statistics

According to the World Health Organization:

At least 700,000 people die each year due to drug-resistant diseases
By 2050, antibiotic resistance could kill 10 million people annually if no action is taken
The development of new antibiotics has slowed dramatically, with few new classes discovered in recent decades

Combating Antibiotic Resistance

Tackling antibiotic resistance requires a coordinated approach at individual, community and global levels:

👤 Individual Actions

Only take antibiotics when prescribed
Complete the full course as directed
Never share antibiotics
Prevent infections through good hygiene

🏥 Healthcare Approaches

Antibiotic stewardship programs
Better diagnostic tests
Infection prevention and control
Surveillance of resistance patterns

🌎 Global Strategies

Developing new antibiotics
Reducing antibiotic use in agriculture
International cooperation
Research into alternatives

Examining Evolution Through Antibiotic Resistance

Antibiotic resistance provides one of the clearest and most observable examples of evolution through natural selection:

Rapid timescale: We can observe evolution happening over days or weeks rather than millennia
Clear selection pressure: Antibiotics create a strong selection pressure favoring resistant bacteria
Measurable outcomes: Scientists can track the spread of resistance genes through bacterial populations
Predictable patterns: The development of resistance follows predictable evolutionary principles

The Luria-Delbrück Experiment

In 1943, Salvador Luria and Max Delbrück conducted a famous experiment that demonstrated that bacterial resistance to viruses (bacteriophages) arose from random mutations that occurred before exposure, not as a direct response to the virus. This experiment provided strong evidence for Darwin's theory of natural selection and helped us understand how antibiotic resistance develops.

The same principle applies to antibiotic resistance - the mutations that confer resistance occur randomly and are then selected for when antibiotics are present.

Summary: Key Points About Antibiotic Resistance

Antibiotic resistance is a natural process accelerated by human antibiotic use
It develops through natural selection - resistant bacteria survive and reproduce
Bacteria can become resistant through mutations or by acquiring resistance genes from other bacteria
Misuse of antibiotics in healthcare and agriculture has accelerated the problem
Antibiotic resistance threatens modern medicine and poses a global health challenge
Combating resistance requires action at individual, community and global levels
Studying antibiotic resistance provides clear evidence of evolution through natural selection

Remember: Antibiotics are precious resources that we must use wisely to preserve their effectiveness for future generations. Every time we misuse antibiotics, we give bacteria another opportunity to evolve resistance.

🧠 Test Your Knowledge!