Bacterial Cell Structure, Components, and Growth Kinetics

Posted on Nov 4, 2025 in Biology

Bacterial Cell Structure and Components

Cell Envelope

Plasma Membrane

The fundamental barrier defining the cell boundary.

Cell Wall

  • Gram-Positive Wall (Gram +)
  • Gram-Negative Wall (Gram -)

Organs of Locomotion and Adhesion

  • Flagella: Used for motility.
  • Pili or Fimbriae: Used for adhesion and conjugation.

Internal Components (Cytoplasm)

The cytoplasm is a gelatinous substance containing essential structures:

  • Nucleoid
  • Ribosomes
  • Inclusion Bodies

External Components

  • Capsule: A dense, organized layer.
  • Glycocalyx: General term for external polysaccharide layers.
  • Mucus Layer: A diffuse, unorganized layer.

Cytoplasm Details

The bacterial cytoplasm is gelatinous and contains the chromosome.

  • Nucleoid: Contains the genetic material (chromosomes and plasmids, which are DNA).
  • Ribosomes
  • Inclusion Bodies

Inclusion Bodies (Storage Granules)

These bodies serve as reserves for nutrients or provide specific functions:

Organic Inclusion Bodies

  • Poly-beta-hydroxybutyrate (PHB) and Glycogen: Serve as carbon and energy stocks.
  • Cyanophycin: Accumulates excess nitrogen.
  • Carboxysomes: Contain enzymes (e.g., RuBisCO) necessary for carbon fixation.
  • Gas Vacuoles: Provide buoyancy, allowing the cell to adjust its position in the water column.

Inorganic Inclusion Bodies

  • Polyphosphate Granules or Volutin: Serve as a reserve of phosphate.
  • Sulfur Granules: Temporarily accumulate elemental sulfur.

Key Molecular Components

Ribosomes

Bacterial ribosomes are smaller than eukaryotic ribosomes. Their primary function is protein synthesis.

Molecular Chaperones

These are protein partners that facilitate the correct folding of newly synthesized polypeptides.

Plasmids

Plasmids are double-stranded, circular molecules of DNA that can exist and replicate independently of the main bacterial chromosome.

External Layers: Glycocalyx and Mucus

Glycocalyx

The glycocalyx (sometimes referred to as microcapsules) aids in adhesion, particularly to solid objects in aquatic environments, often initiating biofilm formation.

Mucus Layer

A diffuse, unorganized polysaccharide matrix that often comprises one or more types of bacteria, contributing to biofilms.

Role of Outer Layers (Capsule/Glycocalyx)

These external structures provide crucial survival advantages:

  • Protection from phagocytes (immune evasion).
  • Nutrient reserve.
  • Formation of Biofilms, offering protection against antimicrobial agents and environmental stress.

Endospores: Bacterial Survival Mechanisms

The endospore is an inactive, specialized structure produced within the cytoplasm of certain bacteria, conferring extreme resistance to stressful environmental situations:

  • Heat
  • Ultraviolet Radiation
  • Chemical Disinfectants
  • Desiccation (Drying)

Endospore Properties

  • Dormancy: Cells in latency show no signs of life, principally due to the lack of free water within the spore structure.
  • High Resistance: Exhibits high resistance to heat, acids, bases, staining, radiation, disinfectants, and antibiotics.
  • Survival Mechanism: Endospore formation is not a mechanism of reproduction, but rather a crucial survival mechanism in harsh environments.

General Characteristics of Bacteria

Bacteria are single-celled organisms that multiply primarily by cell division (binary fission).

They are ubiquitous, found in air, soil, water, plants, and animals, including humans.

Extreme Environments

Thermophilic Bacteria: These organisms live in extreme thermal environments (temperatures typically exceeding 100°C).

Bacterial Morphology

Common bacterial shapes include:

  1. Cocci (Spherical)
  2. Bacilli (Rod-shaped)
  3. Vibrios (Comma-shaped)
  4. Spirillum (Spiral/Wavy)

Bacterial Colonization and Transmission

Humans are naturally colonized by diverse bacterial populations (microbiota).

Methods of Transmission

Bacteria can be transmitted through various routes:

  • Saliva and aerosols (droplets)
  • Blood contact
  • Direct skin contact
  • Genital secretions
  • Fecal-oral route
  • Vectors (e.g., mosquitoes)

Sites of Colonization and Contamination

Bacteria inhabit or contaminate numerous body sites:

  • Eyes
  • Nose and Throat (Upper Respiratory Tract)
  • Mouth (Oral Cavity)
  • Lower Respiratory Tract
  • Skin
  • Stomach and Intestines (Gastrointestinal Tract)
  • Urinary Tract
  • Genital Tract

Microbial Growth and Kinetics

Defining Growth

  • Microbial Growth: Usually refers to an increase in the number of cells in a population.
  • Individual Growth: The increase in the size and weight of a single cell, typically a prelude to cell division.
  • Population Growth: The increase in the total number of cells resulting from individual cell growth and division.

Growth Rate Parameters

Growth Rate: The change in cell number or mass per unit time.

  1. Generation: The process resulting in the formation of two daughter cells from one parent cell.
  2. Generation Time (Doubling Time): The time required for a microbial population to double in number.

Factors Affecting Microbial Growth

External Factors (Environmental/Cultural Conditions)

  • Physicochemical Conditions: pH, Temperature (T°), Water Activity (Aw), Oxygen (O₂), Carbon Dioxide (CO₂).
  • Nutritional Conditions: Availability and ratio of essential nutrients (e.g., Carbon/Nitrogen ratio, typically 10:1 to 12:1).

Internal Factors

  • Metabolic capacity and genetic makeup of the organism.

Nutrient Concentration Effects

The concentration of available nutrients significantly affects both the microbial growth rate and the final biomass yield of a microorganism.

Microbial Culture Methods

Batch Culture

Batch culture involves the growth of microorganisms in a fixed volume of medium. The nutrient composition is continuously altered and eventually exhausted by growth, leading to distinct growth phases (lag, exponential, stationary, death).

Continuous Cultivation

Continuous cultivation maintains a constant culture volume by continuously adding fresh medium while simultaneously removing an equal volume of spent medium and cells. This allows for prolonged exponential growth.

Chemostat

The chemostat is the most commonly used device for continuous culture. Its design allows for the precise control and management of both the population density and the culture growth rate by regulating the dilution rate and nutrient concentration.