Microbiology Study Notes: Prokaryotic and Eukaryotic Cell Structure & Light Microscopy

Chapter 4 – Functional Anatomy of Prokaryotic and Eukaryotic Cells

Overview of Cell Structure

This topic introduces the basic structural differences between prokaryotic and eukaryotic cells, focusing on their primary functions and components.

• Prokaryotic Cells: Cells lacking a true nucleus and membrane-bound organelles. Examples include Bacteria and Archaea.

• Eukaryotic Cells: Cells with a true nucleus and membrane-bound organelles. Examples include Fungi, Protozoa, Algae, and multicellular organisms.

• Cell Shapes: Common bacterial shapes include bacillus (rod-shaped), coccus (spherical), spirillum (spiral), spirochete, and arrangements such as pairs (diplococci), chains (streptococci), and clusters (staphylococci).

• Cell Arrangement: Bacteria are classified by their shape and arrangement, e.g., "streptococci" are chains of cocci.

Cell Structures and Functions

Cells possess specialized structures that contribute to their survival and function.

• Capsule: A gelatinous outer layer found in some bacteria, providing protection and aiding in adherence.

• Biofilm: A community of microorganisms attached to a surface, embedded in a self-produced matrix.

• Flagella: Long, whip-like appendages used for motility. The arrangement and number of flagella can vary among species.

• Fimbriae and Pili: Short, hair-like structures involved in attachment and genetic exchange.

• Cell Wall: Provides structural support and shape. In bacteria, the cell wall contains peptidoglycan.

• Gram-Positive vs. Gram-Negative Cell Walls: Gram-positive bacteria have thick peptidoglycan layers; Gram-negative bacteria have thin peptidoglycan and an outer membrane.

• Endospore: A dormant, tough, and non-reproductive structure produced by certain bacteria for survival in harsh conditions.

• Plasma Membrane: A selectively permeable barrier controlling the movement of substances in and out of the cell.

• Organelles in Eukaryotes: Includes nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, and others.

Cell Wall Composition and Gram Staining

The Gram stain is a differential staining technique used to classify bacteria based on cell wall structure.

• Gram-Positive Bacteria: Retain the crystal violet stain and appear purple due to thick peptidoglycan.

• Gram-Negative Bacteria: Do not retain the crystal violet stain and appear pink/red due to thin peptidoglycan and an outer membrane.

• Endotoxin: Lipopolysaccharide component of Gram-negative cell walls, can cause fever and shock.

Specialized Bacterial Structures

Certain bacteria possess unique structures that enhance their survival and pathogenicity.

• Mycoplasma: Bacteria lacking a cell wall, making them resistant to antibiotics targeting cell wall synthesis.

• Archaea: Prokaryotes with distinct cell wall compositions, often found in extreme environments.

• Endospores: Highly resistant structures formed by genera such as Bacillus and Clostridium.

Antibiotic Targets and Resistance

Antibiotics target specific bacterial structures, but resistance can develop through various mechanisms.

• Cell Wall Synthesis Inhibitors: e.g., penicillins and cephalosporins.

• Protein Synthesis Inhibitors: e.g., tetracyclines and aminoglycosides.

• Antibiotic Resistance: Can arise from genetic mutations or acquisition of resistance genes.

Human Microbiome

The human microbiome consists of all microorganisms living in and on the human body, influencing health and disease.

• Functions: Digestion, immune system modulation, protection against pathogens.

• Dysbiosis: Imbalance in the microbiome can contribute to disease.

Table: Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Membrane-bound Organelles	Absent	Present
Cell Wall Composition	Peptidoglycan (Bacteria), varied (Archaea)	Cellulose (Plants), chitin (Fungi), absent in animals
Size	Generally smaller (0.2–2.0 μm)	Generally larger (10–100 μm)
Examples	Bacteria, Archaea	Fungi, Protozoa, Algae, animals, plants

Lab Week 2 – Introduction to Light Microscopy

Microscope Parts and Functions

Light microscopy is a fundamental technique in microbiology for observing microorganisms and cell structures.

• Ocular Lens (Eyepiece): The lens you look through, typically 10x magnification.

• Objective Lenses: Lenses closest to the specimen, usually ranging from 4x to 100x magnification.

• Stage: Platform where the slide is placed.

• Condenser: Focuses light onto the specimen.

• Diaphragm: Adjusts the amount of light passing through the specimen.

• Coarse and Fine Focus: Used to bring the specimen into clear view.

Use of Oil Immersion

Oil immersion is used with the 100x objective lens to increase resolution by reducing light refraction.

• Application: Place a drop of immersion oil on the slide before using the 100x objective.

• Purpose: Oil has a similar refractive index to glass, allowing more light to enter the objective lens.

Calculating Total Magnification

Total magnification is determined by multiplying the magnification of the ocular lens by that of the objective lens.

• Formula:

$\text{Total Magnification} = \text{Ocular Magnification} \times \text{Objective Magnification}$

• Example: If the ocular lens is 10x and the objective lens is 40x, total magnification is $10 \times 40 = 400\times$.

Summary Table: Microscope Parts and Functions

Part	Function
Ocular Lens	Magnifies image, usually 10x
Objective Lens	Magnifies specimen, typically 4x, 10x, 40x, 100x
Stage	Holds the slide
Condenser	Focuses light on specimen
Diaphragm	Controls light intensity
Coarse/Fine Focus	Brings specimen into focus

Additional info: These notes expand on the syllabus outline by providing definitions, examples, and context for key microbiology concepts and laboratory techniques.