Biology (9700) | Antibiotics

Introduction: The Biology of Antibiotics

Welcome to the fascinating world of antibiotics! This chapter is vital because it connects cell biology, infectious disease, and evolution. Antibiotics are arguably one of the most important medical discoveries ever made, saving millions of lives since the 1940s.

In these notes, we will look at how these tiny molecules wage war against bacteria, why they are useless against viruses, and the serious global threat of drug resistance. Don't worry if this seems tricky at first—we'll break down the concepts into clear, simple steps!

1. What Are Antibiotics?

An antibiotic is a chemical substance produced by microorganisms (or synthetic means) that kills or inhibits the growth of bacteria.

Key Terms to Remember

• Bactericidal: Kills bacteria directly (like Penicillin).
• Bacteriostatic: Inhibits the growth or reproduction of bacteria, allowing the host's immune system to clear the infection.

2. The Mode of Action: How Penicillin Attacks Bacteria

The syllabus specifically requires you to know how Penicillin works. Penicillin is one of the oldest and most widely used antibiotics, and its action is highly specific to bacterial structure.

The Target: The Bacterial Cell Wall

The key difference between bacterial (prokaryotic) cells and our (eukaryotic) cells is the presence of a strong, rigid cell wall in bacteria. This wall is made primarily of a polymer called peptidoglycan.

Did you know? The peptidoglycan wall gives the bacteria its shape and, crucially, protects it from osmotic lysis (bursting) when water moves in via osmosis.

Step-by-Step Action of Penicillin

Penicillin is a bactericidal antibiotic that acts by interfering with the synthesis of the cell wall.

1. Targeting Cell Wall Synthesis: Penicillin works primarily when bacteria are actively growing and dividing, as this is when they are building new cell walls.
2. Inhibition of Cross-Linking: Penicillin binds to and inhibits specific bacterial enzymes (often called transpeptidases) that are responsible for forming the cross-links between the long peptidoglycan chains.
3. Structural Weakness: Without these cross-links, the new cell wall layers are incomplete and structurally weak.
4. Osmotic Lysis: Since the inside of the bacterial cell has a much lower water potential (it is more concentrated) than the surrounding tissue fluid, water rushes into the cell via osmosis. The weak wall cannot withstand the high internal pressure, and the bacterium bursts (lysis).

Analogy Aid: Imagine building a house out of Lego bricks (the peptidoglycan strands). If Penicillin is added, it stops the "glue" (the cross-linking enzymes) from securing the bricks together. When pressure (osmosis) is applied, the weak structure collapses.

3. Why Antibiotics Do Not Affect Viruses

This is a critical distinction and often tested. If you have a cold (caused by a virus), an antibiotic will do absolutely nothing!

Fundamental Biological Differences

Antibiotics target structures or processes found in bacteria but not in human cells or viruses.

The mechanism of penicillin confirms this:

• No Peptidoglycan: Viruses are non-cellular structures. They do not have a cell wall made of peptidoglycan. They are simply a nucleic acid core (DNA or RNA) surrounded by a protein capsid and sometimes a lipid envelope (Syllabus 1.2.7).
• No Independent Metabolism: Viruses cannot reproduce, grow, or carry out metabolic reactions outside a host cell. They are obligate intracellular parasites. They hijack the host's machinery (our ribosomes, enzymes, and ATP) to make new viral particles.
• Lack of Target Sites: Penicillin targets bacterial enzyme systems involved in cell wall synthesis. Since viruses lack these enzyme systems and structures, the antibiotic has nothing to attack.

Key Takeaway: Antibiotics are highly specific. Viruses simply don't have the cellular machinery or walls that antibiotics are designed to disrupt.

4. The Consequences of Antibiotic Resistance

Antibiotics were once considered a "magic bullet," but their overuse and misuse have led to one of the biggest threats to global health: antibiotic resistance.

What is Resistance?

Antibiotic resistance occurs when bacteria evolve and develop mechanisms that allow them to survive exposure to an antibiotic that would normally kill them or stop their growth. This is a powerful example of natural selection (Syllabus 17.2.4).

The Consequences for Health and Society

The emergence of antibiotic-resistant strains (often called "superbugs," such as MRSA) has severe consequences:

Harder and Longer Treatments

• Ineffectiveness: Common antibiotics fail to cure infections. Patients remain sick for longer.
• Secondary Treatments: Doctors must resort to using "second-line" or "third-line" antibiotics, which are often more expensive, have more severe side effects, and may need to be administered intravenously (through a drip) in a hospital setting.

Increased Mortality and Morbidity

• Untreatable Infections: If a bacterium is resistant to all available drugs, the infection becomes untreatable, leading directly to patient death.
• Risk in Medicine: Simple surgical procedures, organ transplants, chemotherapy, and routine childbirth become incredibly risky, as the risk of a deadly, untreatable post-operative infection increases dramatically.

Economic Impact

• Higher Healthcare Costs: Longer hospital stays, expensive specialist drugs, and increased testing place an enormous financial strain on healthcare systems globally.
• Loss of Productivity: People are sick for longer, reducing the economic output of communities and nations.

Encouragement: Understanding these consequences shows you why this topic is so relevant to society! You are learning about a real-world crisis.

5. Steps Taken to Reduce the Impact of Antibiotic Resistance

Resistance is inevitable because bacteria reproduce so quickly and natural selection is constantly acting. However, we can take deliberate steps to slow the rate of evolution and reduce the impact of resistance.

A. Controlling Use in Humans and Animals

• Restrict Prescription: Doctors should prescribe antibiotics only when absolutely necessary (e.g., confirming a bacterial infection, not for viral colds).
• Patient Education: Patients must be educated to complete the full course of antibiotics prescribed, even if they feel better early. Stopping early leaves the hardiest bacteria alive, favouring resistant strains.
• Reducing Agricultural Use: Limiting the use of antibiotics in farm animals (often used as growth promoters or preventative measures) to prevent resistance from moving into the human food chain.

B. Hygiene and Infection Control

The best way to fight resistance is to stop infections from spreading in the first place.

• Improved Sanitation and Hygiene: Simple measures like frequent handwashing are crucial, especially in healthcare settings (hospitals and clinics).
• Isolation: Isolating patients infected with resistant strains (like MRSA) in hospitals to prevent transmission to other vulnerable patients.
• Vaccination: Using vaccines to prevent bacterial diseases (like some forms of pneumonia) reduces the overall need for antibiotics.

C. Research and Development

• New Drugs: Investment into research to develop new antibiotics, as the current rate of drug development is too slow to keep up with resistance.
• Alternative Treatments: Researching alternative therapies, such as bacteriophages (viruses that specifically kill bacteria) or other non-antibiotic drugs.