Microbial Cell Structure and Function

Genus and Species Naming Conventions

Microbiologists use a standardized system for naming organisms, known as binomial nomenclature.

• Genus name is capitalized; species name is lowercase.

• Both names are italicized when typed; when handwritten, underlining is acceptable if italics are not possible.

• Example: Escherichia coli

Cell Wall Structure: Gram-Positive vs. Gram-Negative Bacteria

The Gram stain differentiates bacteria based on cell wall architecture.

• Gram-positive bacteria have a thick peptidoglycan layer and stain purple with crystal violet.

• Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane containing lipopolysaccharide (LPS); they stain pink with safranin.

• Acid-fast bacteria (e.g., Mycobacterium tuberculosis) have waxy, lipid-rich cell walls containing mycolic acids and require acid-fast staining.

Example: Staphylococcus aureus is Gram-positive; Escherichia coli is Gram-negative.

Key Bacterial Structures

• Pili: Enable attachment to surfaces, conjugation (exchange of 5genetic material), and very slow movement via extension and contraction.

• Flagella: Responsible for chemotaxis (movement toward or away from chemical stimuli).

• Endospores: Formed by some bacteria (e.g., Bacillus, Clostridium) via asymmetric cell division and differentiation; provide survival under harsh conditions, not sexual reproduction.

Microbial Metabolism and Growth

Heterocysts and Nitrogen Fixation

Heterocysts are specialized cells in cyanobacteria chains that fix nitrogen for the colony.

• Contain nitrogenase enzyme, which is inactivated by oxygen.

• Heterocysts seal themselves off from oxygen and stop photosynthesis to protect nitrogenase.

• Normal cells fix carbon and share it with heterocysts; heterocysts fix nitrogen and share it with the colony.

• Heterocysts do not divide again, demonstrating cellular differentiation and evolutionary altruism.

Biofilm Formation

Biofilms are structured microbial communities attached to surfaces and embedded in a self-produced matrix.

• Exopolysaccharides (EPS) are required to form the biofilm matrix.

• Steps: Attachment → Growth → Quorum sensing → EPS secretion → Specialization (sporulation, swarmer cell release).

Scientific Notation and Microbial Growth Calculations

Microbial growth is often exponential, and scientific notation is used to express large cell numbers.

• Generation time: Time required for a population to double.

• Formula: where is initial cell number, is number of generations.

• Example: Starting with cells, after 4 hours (2 doublings), .

Growth Curves

Bacterial growth rate varies with temperature and over time.

• Temperature Growth Curve: Paraboloid shape, peak at optimum temperature (e.g., 30°C), zero growth at minimum and maximum temperatures.

• At low temperatures: Membranes gel, enzymes slow.

• At high temperatures: Membranes become too fluid, enzymes denature.

• Time Growth Curve: Lag phase → Log (exponential) phase → Stationary phase → Death phase → Gasp phase (long-term survival).

Microscopy, Staining, and Classification

Staining Methods

• Gram Stain: Differentiates based on peptidoglycan thickness.

• Acid-Fast Stain: Used for bacteria with waxy, lipid-rich cell walls (e.g., Mycobacterium).

• Counterstain (safranin) is used to visualize Gram-negative cells after crystal violet is washed out.

Domain Classification

• Bacteria and Archaea are prokaryotic and do not belong to the Eukarya domain.

• Archaea are more closely related to Eukarya than Bacteria, but all lack a true nucleus.

Microbial Genetics

DNA Replication

• Replication in bacteria is bidirectional from a single origin, as in eukaryotes.

• Replication forks move in both directions from the origin.

Microbial Nutrition and Membrane Transport

Membrane Structure and Fluidity

• Fluid Mosaic Model: Plasma membrane is fluid, not rigid.

• Saturated phospholipids: Straight tails, pack tightly, less fluid.

• Unsaturated phospholipids (cis): Kinked tails, pack loosely, more fluid; important for maintaining membrane fluidity at low temperatures.

Transport Mechanisms

• Passive diffusion: Movement down concentration gradient, no energy required.

• Facilitated diffusion: Uses channels, still passive.

• Active coupled transport: Uses energy from one solute moving down its gradient to move another against its gradient (e.g., symporters, antiporters).

• PTS (Phosphotransferase System) sugar transport: Sugar enters cell down gradient, then is phosphorylated (using PEP energy), so it no longer counts as sugar, maintaining the gradient.

Microbial Diseases and Koch's Postulates

Koch's Postulates (Germ Theory of Disease)

Four steps to prove a microorganism causes disease:

1. Pathogen must be found in all diseased organisms, not in healthy ones.

2. Pathogen must be isolated and grown in pure culture.

3. Pure culture must cause disease when introduced into a healthy organism.

4. Pathogen must be re-isolated from the newly diseased organism and shown to be the same.

Limitations: Some pathogens (e.g., Staphylococcus aureus) can be present in healthy individuals.

Comparison Tables

Gram-Positive vs. Gram-Negative Cell Wall Structure

Passive vs. Active Transport

Saturated vs. Unsaturated Phospholipids

Additional info:

• FTSZ is a protein that forms a contractile ring at the site of bacterial cell division.

• Gasp phase is a long-term survival phase after death phase in batch culture.

• Conjugation is the process by which bacteria exchange genetic material via pili.

• LPS (lipopolysaccharide) released from lysed Gram-negative bacteria can cause toxic effects (endotoxin shock).