Welcome to Topic 6: Bacteria, Viruses, and the Great Evolutionary Race!

In this chapter, we are going to dive into the world of pathogens—the microorganisms that cause disease. Specifically, we will look at Bacteria and Viruses. We’ll use two major global health challenges, Tuberculosis (TB) and HIV, as our primary examples to see how these tiny invaders infect us, survive our defenses, and even "outsmart" our immune systems. Don't worry if it seems like a lot of detail at first; we will break it down step-by-step!

1. Bacteria vs. Viruses: Knowing the Enemy

Before we look at specific diseases, we need to understand the fundamental differences between the two main types of pathogens. Think of bacteria as independent organisms that can often survive on their own, while viruses are like biological "hijackers" that must take over a host cell to function.

Bacterial Structure

Bacteria are prokaryotic cells. You might remember some of these structures from Topic 3:

  • Circular DNA: Not contained in a nucleus.
  • Plasmids: Small loops of extra DNA.
  • Cell Wall: Made of peptidoglycan (prevents the cell from bursting).
  • Capsule: A slimy outer layer that protects the bacterium from the host's immune system.
  • Ribosomes: Smaller than eukaryotic ones (known as \(70S\) ribosomes).
  • Pili: Hair-like structures that help bacteria stick to surfaces or each other.
  • Flagellum: A tail-like structure for movement.

Viral Structure

Viruses are much smaller than bacteria and are non-living because they cannot reproduce without a host cell. Their structure is simple:

  • Nucleic Acid: A core of either DNA or RNA (the genetic "blueprint").
  • Capsid: A protective protein coat surrounding the nucleic acid.
  • Envelope: Some viruses (like HIV) have an extra outer layer made of lipids stolen from a host cell’s membrane.
  • Attachment Proteins: These stick out from the surface and allow the virus to "lock onto" specific host cells.

Quick Review: Bacteria are complete cells; viruses are just genetic material in a protein box. Bacteria can be killed by antibiotics, but viruses cannot!

2. Mycobacterium tuberculosis (TB)

Mycobacterium tuberculosis is the bacterium that causes TB. It primarily attacks the lungs but can spread elsewhere. It is a master of disguise and survival.

How TB Infects the Body

1. Inhalation: TB is spread via droplet infection (coughing or sneezing).
2. Phagocytosis: When the bacteria reach the lungs, white blood cells called macrophages engulf them.
3. Survival: Usually, a macrophage would destroy a bacterium. However, TB bacteria have a thick, waxy cell wall that allows them to survive inside the macrophage.
4. Tubercles: The immune system seals off the infected macrophages in tiny, hard lumps called tubercles. In this "latent" phase, the person has no symptoms and isn't contagious.

Active TB and Symptoms

If the person’s immune system weakens (due to age, malnutrition, or another infection like HIV), the bacteria break out of the tubercles and start destroying lung tissue. Symptoms include:

  • Persistent, bad cough (often coughing up blood).
  • Fever and night sweats.
  • Extreme fatigue and weight loss (this is why TB was historically called "Consumption").
If left untreated, the damage to the lungs reduces the surface area for gas exchange, which can lead to death.

Did you know? About one-third of the world's population is estimated to have "latent" TB, meaning they carry the bacteria but aren't currently sick!

3. Human Immunodeficiency Virus (HIV)

HIV is a virus that specifically targets the very cells meant to protect us: the T helper cells. Without enough T helper cells, the entire immune system collapses.

The HIV Infection Process

HIV is a retrovirus, meaning it uses RNA to make DNA. Here is how it takes over:

  1. Attachment: A protein on the HIV envelope called \(gp120\) binds to a receptor called \(CD4\) on the surface of T helper cells.
  2. Entry: The viral envelope fuses with the cell membrane, and the viral RNA enters the cell.
  3. Reverse Transcription: The virus uses an enzyme called reverse transcriptase to turn its viral RNA into DNA.
  4. Integration: Another enzyme, integrase, inserts this viral DNA into the host cell's own DNA.
  5. Replication: The host cell is now "tricked" into using its own machinery to create new viral proteins and RNA.
  6. Budding: New viruses burst out of the cell, often killing the T helper cell in the process.

From HIV to AIDS

As the number of T helper cells drops, the patient develops AIDS (Acquired Immune Deficiency Syndrome). Crucial Point: People do not die of HIV itself. They die from opportunistic infections (like pneumonia or TB) that their body can no longer fight off because the immune system is too weak.

Common Mistake to Avoid: Don't confuse Reverse Transcriptase with DNA Polymerase. Reverse transcriptase is the specific enzyme HIV uses to turn RNA back into DNA.

4. The "Evolutionary Race" and Evasion Mechanisms

Pathogens and their hosts are in a constant "arms race." As our immune systems evolve better ways to find and kill pathogens, the pathogens evolve new ways to hide or resist. This is often called evasion mechanisms.

How TB Evades the Immune System

  • Hiding inside cells: By living inside macrophages, TB stays "hidden" from antibodies in the blood.
  • Suppressing lysosomes: TB bacteria can produce proteins that prevent the macrophage's digestive enzymes (lysosomes) from fusing with the bacteria, so they don't get digested.

How HIV Evades the Immune System

  • Antigenic Variation: HIV mutates incredibly fast. This changes the shape of the antigens on its surface. By the time the immune system makes antibodies for one "look" of the virus, the virus has already changed its "disguise."
  • Killing Immune Cells: By specifically destroying T helper cells, the virus destroys the "command center" of the immune response.
  • Latency: HIV can stay "hidden" inside the host's DNA for years without making new viruses, making it invisible to the immune system.

Key Takeaway: The "evolutionary race" explains why it is so hard to create a vaccine for HIV—it simply changes too fast for our immune memory to keep up!

Summary Checklist

  • Can you describe 3 structural differences between bacteria and viruses?
  • Do you know the roles of reverse transcriptase and integrase in HIV?
  • Can you explain why T helper cells are so important in an HIV infection?
  • Do you understand how Mycobacterium tuberculosis can stay dormant in the body?
  • Can you define opportunistic infection in the context of AIDS?

Note: For more details on how the body fights back using B and T cells, or how we use antibiotics to treat these infections, check out the next chapters in this section!