Microbiology: Infectious Agents, Nutrition, and Transport

Memory Trick for Exam Day:

• Prion = Protein only (no DNA/RNA).
• Viroid = RNA only (no protein coat).
• Satellite = Needs a helper virus.

The Nature of Viruses

Viruses are classified as obligate intracellular parasites. This means they cannot reproduce or carry out metabolic processes independently; they must hijack the machinery of a living host cell.

A virus is composed of a genome and a protein coat called a capsid. A critical rule for exams is that a viral genome consists of either DNA or RNA, but never both. While all viruses possess a capsid, only some viruses are "enveloped," meaning they possess an additional lipid bilayer surrounding the capsid.

Viral Life Cycles and Replication

Understanding how a virus infects a cell requires a step-by-step look at its replication cycle. Regardless of the type of virus, the process generally follows this specific sequence:

$\text{adsorption} \rightarrow \text{penetration} \rightarrow \text{uncoating} \rightarrow \text{synthesis} \rightarrow \text{assembly} \rightarrow \text{release}$

1. Adsorption: The virus attaches to specific receptors on the host cell surface.
2. Penetration: The virus enters the cell. While bacteriophages (viruses that infect bacteria) typically inject their genome into the host, animal viruses often enter via fusion with the cell membrane or through endocytosis.
3. Uncoating: The viral capsid is removed, releasing the nucleic acid into the host cell.
4. Synthesis: The host's machinery is hijacked to replicate the viral genome and produce viral proteins.
5. Assembly: New viral particles are constructed from the newly synthesized components.
6. Release: The new viruses exit the cell to infect others.

Lytic vs. Lysogenic Cycles

When discussing bacteriophages, it is essential to distinguish between two different pathways of infection:

• Lytic Cycle: The virus takes over the cell, replicates rapidly, and ultimately causes the host cell to undergo lysis (bursting), releasing the new virions.
• Lysogenic Cycle: Instead of killing the host immediately, the viral DNA integrates into the host's genome. In this state, the viral DNA is referred to as a prophage and can persist within the host for many generations.

Microbial Nutrition and Metabolism

Microbes are masters of adaptation, utilizing a wide variety of substances for growth. To understand microbial ecology, we must categorize nutrients by their chemical makeup and how organisms derive energy from them.

Nutrient Classification

Nutrients are broadly divided into two groups:

• Organic Nutrients: These contain both carbon and hydrogen. Common examples include carbohydrates, lipids, proteins, and nucleic acids.
• Inorganic Nutrients: These do not contain carbon-hydrogen bonds and include water, various gases, and minerals or salts.

Metabolic Strategies: Carbon and Energy

Microbes are classified based on two fundamental questions: "Where do they get their carbon?" and "Where do they get their energy?".

1. Carbon Sources

• Autotrophs: Use inorganic carbon, specifically $\text{CO}_2$, as their primary carbon source.
• Heterotrophs: Must consume organic molecules to obtain carbon.

2. Energy Sources

• Phototrophs: Derive their energy from light.
• Chemotrophs: Derive their energy from the oxidation of chemical compounds.

By combining these two classifications, we can identify four distinct nutritional types:

High-Yield Note: Most human pathogens are chemoheterotrophs, meaning they break down organic molecules to obtain both their carbon and their energy.

Nutrient Transport Mechanisms

For a microbe to grow, it must move nutrients from the external environment across the cell membrane and into the cytoplasm. This movement occurs via two primary mechanisms: passive and active transport.

Passive Transport

Passive transport is "free" in terms of energy; it does not require the cell to expend ATP. Substances move via passive transport down a concentration gradient, moving from an area of high concentration to an area of low concentration.

There are three main types of passive movement:

• Diffusion: The movement of solutes (dissolved particles) through a membrane.
• Osmosis: Specifically refers to the movement of water across a semi-permeable membrane.
• Facilitated Diffusion: This is still a passive process (no energy used), but it requires specific carrier or channel proteins to help move molecules that cannot pass through the lipid bilayer on their own.

Active Transport

Unlike passive transport, active transport requires the expenditure of energy (usually in the form of ATP) and the use of specific transport proteins. This mechanism is vital because it allows microbes to move nutrients against a concentration gradient—effectively pumping substances into the cell even when the internal concentration is already higher than the external environment. This is a critical survival mechanism in nutrient-poor environments.