Microbiology Study Guide: Cell Structure, Classification, and Microbial Growth Control

Prokaryotic and Eukaryotic Cell Structure

Prokaryotes vs. Eukaryotes

Understanding the differences between prokaryotic and eukaryotic cells is fundamental in microbiology. Prokaryotes include bacteria and archaea, while eukaryotes encompass protists, fungi, plants, and animals.

• Prokaryotes: One circular chromosome, not in a membrane; lack histones and organelles; cell walls contain peptidoglycan (bacteria) or pseudomurein (archaea); divide by binary fission.

• Eukaryotes: Paired chromosomes in a nuclear membrane; contain histones and organelles; cell walls (when present) are polysaccharide-based; divide by mitosis.

Feature	Prokaryote	Eukaryote
Chromosome	Circular, no membrane	Paired, in nuclear membrane
Histones	Absent	Present
Organelles	Absent	Present
Cell Wall	Peptidoglycan (bacteria), pseudomurein (archaea)	Polysaccharide (when present)
Division	Binary fission	Mitosis

Structure of a Prokaryotic Cell

Prokaryotic cells typically lack membrane-enclosed organelles. All bacteria contain cytoplasm, ribosomes, a plasma membrane, and a nucleoid. Most bacteria have cell walls, which play roles in virulence, identification, and as targets for antimicrobial agents.

• Capsule: Contributes to virulence by preventing phagocytosis.

• Cell Wall: Provides structural support and protection; composition varies between Gram-positive and Gram-negative bacteria.

• Flagella: Filamentous appendages for motility; composed of filament, hook, and basal body.

• Plasmids: Extrachromosomal DNA elements encoding antibiotic resistance or toxin production.

Gram Staining and Cell Wall Differences

Gram-Positive vs. Gram-Negative Bacteria

Gram staining is a differential technique used to classify bacteria based on cell wall structure. Gram-positive bacteria have thick peptidoglycan walls, while Gram-negative bacteria have thin peptidoglycan and an outer membrane containing lipopolysaccharides.

Characteristic	Gram-Positive	Gram-Negative
Peptidoglycan Thickness	Thick	Thin
Outer Membrane	Absent	Present
Teichoic Acids	Present	Absent
LPS (Lipopolysaccharide)	Absent	Present
Susceptibility to Penicillin	High	Low
Flagella Basal Body	2 rings	4 rings

Classification and Identification of Microorganisms

Methods of Classification

Microorganisms are classified based on cellular organization, morphology, staining characteristics, and biochemical tests. Carl Woese established the three-domain system: Bacteria, Archaea, and Eukarya.

• Morphological Characteristics: Useful for identifying eukaryotes.

• Differential Staining: Gram and acid-fast stains.

• Biochemical Tests: Detect presence of specific enzymes.

• Dichotomous Keys: Identification keys based on successive questions.

Control of Microbial Growth

Principles and Methods

Controlling microbial growth is essential in medical and laboratory settings. Methods include heat, radiation, and filtration. Heat is the most widely used method for sterilization.

• Decontamination: Treatment to make objects safe to handle.

• Disinfection: Targets pathogens but does not eliminate all microorganisms.

• Sterilization: Removal or destruction of all living microorganisms.

• Use-Dilution Test: Evaluates effectiveness of disinfectants.

• Disk-Diffusion Method: Tests antimicrobial activity.

Biochemical Tests and Metabolic Pathways

Fermentation and Respiration

Microorganisms utilize various metabolic pathways for energy production, including glycolysis, the Krebs cycle, and fermentation. Biochemical tests help identify bacteria based on their metabolic capabilities.

• Fermentation: Anaerobic process producing acids, gases, or alcohols.

• Respiration: Complete oxidation of pyruvic acid to CO2; aerobic or anaerobic.

• Biochemical Tests: Detect enzymes such as urease, catalase, and oxidase.

Test	Purpose
Fermentation Test	Detects acid/gas production from carbohydrate metabolism
Oxidase Test	Identifies cytochrome oxidase presence
Urease Test	Detects urea hydrolysis

Summary Table: Microbial Classification and Control

Domain	Cell Type	Cell Wall	Example
Bacteria	Prokaryote	Peptidoglycan	Escherichia coli
Archaea	Prokaryote	Pseudomurein	Methanogens
Eukarya	Eukaryote	Varies	Fungi, Protozoa