Pathogens and Infection

Welcome to Pathogens and Infection!

In this chapter, we are going to explore the world of biological "invaders." Think of your body as a high-security fortress. Every day, it is under constant threat from microscopic organisms trying to get inside. We will learn about the different types of attackers (bacteria and viruses), how they try to break in, and the "general" defense systems your body uses to kick them out before they cause too much trouble.

This is a foundational part of Unit 4 (Microbiology and Immunity). While this chapter focuses on the invaders and the first line of defense, later chapters will look at the "special forces" (your immune system) and how we use medicine to help.

1. Bacteria vs. Viruses: Knowing the Enemy

Not all pathogens are the same. The two biggest players you need to know for your exam are bacteria and viruses. They are built differently and behave differently.

Bacteria

Bacteria are prokaryotic cells. They are living organisms that can often survive on their own. As a quick refresher from Unit 2, bacteria have a cell wall, plasmids (circular DNA), and 70S ribosomes. They reproduce by binary fission.

Viruses

Viruses are much smaller than bacteria and are non-living. They cannot reproduce on their own; they must hijack a host cell to make copies of themselves. A virus consists of:

• Nucleic Acid: Either DNA or RNA (the genetic "blueprint").
• Capsid: A protective protein coat that surrounds the nucleic acid.
• Envelope: Some (but not all) viruses have an extra outer layer made of lipids taken from the host cell's membrane.

The Four "Must-Know" Viruses

The Edexcel syllabus requires you to know these specific examples. Use this list to keep them straight:

1. Ebola Virus: Contains RNA. It is an enveloped virus that causes severe hemorrhagic fever.

2. Tobacco Mosaic Virus (TMV): Contains RNA. It lacks an envelope and infects plants, causing a "mosaic" pattern of discoloration on leaves.

3. Human Immunodeficiency Virus (HIV): Contains RNA. It is an enveloped retrovirus that targets the human immune system.

4. Lambda Phage (\(\lambda\) phage): Contains DNA. It is a virus that specifically infects bacteria (a bacteriophage). It has a complex "head and tail" structure and no envelope.

Quick Review: Remember that viruses are "acellular"—they don't have a cytoplasm, cell membrane (unless they steal one), or ribosomes!

2. Viral Life Cycles: Lytic and Latency

When a virus infects a cell, it usually follows one of two paths:

A. The Lytic Cycle: This is the "fast" path. The virus inserts its genetic material, forces the host cell to manufacture viral proteins and nucleic acids, assembles new viruses, and then causes the cell to burst (lysis), releasing the new viruses to infect other cells.

B. Latency (The Lysogenic Cycle): This is the "hidden" path. The viral DNA is integrated into the host's own DNA. Every time the host cell divides, the viral DNA is copied along with it. The virus stays "dormant" (latent) until a trigger (like stress or UV light) causes it to enter the lytic cycle.

3. Keeping Pathogens Out: The Barriers

Before a pathogen can cause an infection, it has to get into your body. We have three main physical and chemical barriers:

• Skin: The primary physical barrier. It consists of a tough layer of keratin that is very hard for pathogens to penetrate unless there is a cut.
• Stomach Acid: We swallow many pathogens in our food. The hydrochloric acid (\(HCl\)) in our stomach has a very low \(pH\) (around \(2.0\)), which denatures the enzymes of most bacteria, killing them.
• Gut and Skin Flora: Your body is covered in "friendly" bacteria. These compete with pathogens for space and nutrients, making it much harder for harmful microbes to settle and grow.

4. Non-Specific Immune Responses

If a pathogen makes it past the barriers, your body launches non-specific responses. These are "general" defenses—they attack any invader the same way, regardless of what it is.

Inflammation

When tissue is damaged, chemicals like histamine are released. This causes vasodilation (widening of blood vessels) and makes capillary walls "leaky." This allows more blood and white blood cells to reach the site of infection. Common sign: Swelling, redness, and heat.

Lysozyme

This is an enzyme found in secretions like tears, saliva, and mucus. Lysozyme acts as an antibacterial agent by breaking down the cell walls of certain bacteria.

Interferon

When a cell is infected by a virus, it produces a protein called interferon. This protein signals neighboring cells to produce antiviral proteins, stopping the virus from spreading further.

Phagocytosis

This is the process where specialized white blood cells called phagocytes (like macrophages) engulf and digest pathogens.
Step-by-step:
1. The phagocyte recognizes the foreign antigens on the pathogen.
2. The phagocyte membrane wraps around the pathogen, trapping it in a vacuole called a phagosome.
3. Lysosomes (organelles containing digestive enzymes) fuse with the phagosome.
4. The enzymes break down the pathogen, destroying it.

Top Tip: Phagocytosis is "non-specific" because the phagocyte will eat anything that it identifies as "non-self."

5. Two Major Infections: TB and HIV

The syllabus highlights two specific diseases you need to understand in the context of infection.

Mycobacterium tuberculosis (TB)

TB is caused by a bacterium. It usually enters the body through the respiratory route (breathing in droplets).
- Once in the lungs, phagocytes engulf the bacteria.
- In healthy people, the bacteria are "walled off" in small, hard lumps called tubercles. The bacteria stay alive but dormant.
- If the immune system weakens, the bacteria can break out, multiply, and destroy lung tissue. This is "active TB."

Human Immunodeficiency Virus (HIV)

HIV is a virus that destroys the very system meant to protect us. It enters the body through blood or bodily fluids.
- It specifically targets T-helper cells (a type of white blood cell).
- HIV uses an enzyme called reverse transcriptase to turn its RNA into DNA, which then integrates into the host cell's DNA.
- As the number of T-helper cells drops, the person's immune system becomes so weak they can no longer fight off simple infections. This stage is known as AIDS (Acquired Immune Deficiency Syndrome).

Don't worry if this seems like a lot! Just remember: TB is a bacterial lung invader that hides in tubercles, while HIV is a viral "hijacker" that destroys immune cells.

Summary: Key Takeaways

• Bacteria are living cells; viruses are non-living genetic packages.
• Know your four viruses: Ebola, TMV, HIV, and Lambda Phage.
• Lytic is the active/bursting cycle; Latency is the hidden/dormant cycle.
• Your first lines of defense are skin, stomach acid, and flora.
• If they get in, inflammation, lysozyme, interferon, and phagocytosis take over.
• TB affects the lungs; HIV destroys T-helper cells.

For the next steps in your revision, check out the chapter on Immunity and Antibiotics to see how the body uses B and T cells to create long-term protection!