Bacteria and Archaea

Bacterial Structure and Growth

Bacteria exhibit a variety of shapes and arrangements, which are important for identification and classification. Their growth is influenced by environmental and nutritional factors, and their population dynamics follow characteristic phases. Understanding these aspects is fundamental to microbiology.

• Bacterial Shapes:

  • Cocci: Spherical or nearly spherical.

  • Bacilli: Rod-shaped; some form endospores.

  • Pleomorphic: Variable or morphologically indistinct; e.g., coccobacilli (elongated and spherical).

  • Spirals:

    • Vibrios: Curved or comma-shaped rods.

    • Spirillum: Thick, rigid, spiral organisms.

    • Spirochetes: Thin, flexible spirals.

• Bacterial Arrangements:

  • Cocci:

    • Diplococci: Pairs (e.g., Streptococcus pneumoniae).

    • Streptococci: Chains (e.g., Streptococcus pyogenes).

    • Tetrads: Groups of four.

    • Sarcinae: Cubes of eight.

    • Staphylococci: Grapelike clusters.

  • Bacilli:

    • Diplobacilli: Paired rods.

    • Streptobacilli: Chains.

    • Palisade: Bacilli attached along the long axis.

• Binary Fission: The primary method of bacterial reproduction, resulting in two identical daughter cells. Requires sufficient metabolites and regulatory events to initiate DNA replication. Once started, DNA synthesis must complete. When nutrients are depleted, division ceases.

Bacterial Population Growth Curve

Bacterial populations grow in predictable phases when cultured in a closed system.

• Lag Phase: Adaptation to medium; cells are metabolically active but not dividing.

• Log (Exponential) Phase: Rapid cell division; population doubles at each generation.

• Stationary Phase: Growth slows as nutrients deplete and waste accumulates; cell division equals cell death.

• Death Phase: Cell death exceeds cell division; population declines.

Methods for Measuring Microbial Growth

Microbial growth can be measured by assessing cell mass or cell number.

• Cell Mass:

  • Dry Weight: Cells are washed, dried, and weighed; accurate but time-consuming.

  • Turbidity: Measured with a spectrophotometer; optical density correlates with cell mass.

• Cell Number:

  • Direct Cell Count: Microscopically using counting chambers (e.g., Petroff-Hauser chamber).

  • Viable Counts: Only living cells are counted; methods include pour plating, spread plating, and most probable number (MPN).

Factors Influencing Bacterial Growth

Bacterial growth is affected by nutritional, physical, and chemical factors.

• Nutritional Requirements:

  • Phototrophs: Use light for energy.

  • Chemotrophs: Use chemical compounds for energy.

  • Autotrophs: Use CO2 as carbon source.

  • Heterotrophs: Use organic compounds as carbon source.

  • Photoautotrophs: Sunlight for energy, CO2 for carbon (e.g., photosynthetic bacteria, algae).

  • Chemoautotrophs: Chemical compounds for energy, CO2 for carbon.

  • Photoheterotrophs: Sunlight for energy, organic compounds for carbon.

  • Chemoheterotrophs: Organic compounds for both energy and carbon.

  • Nitrogen: Some bacteria use inorganic compounds (e.g., nitrates).

  • Sulfur and Phosphorus: Obtained from organic compounds or inorganic salts.

  • Minerals: Required in trace amounts.

• Temperature:

  • Mesophiles: Optimal growth at 25–40°C; most bacteria, including pathogens.

  • Thermophiles: Grow best at ≥45°C; found in hot environments.

  • Psychrophiles: Grow at ≤0°C; optimal at ~15°C; found in cold regions.

  • Psychrotrophs: Grow slowly at 0°C; optimal at 25–30°C; cause food spoilage.

• Osmotic Pressure:

  • Osmotolerant: Can withstand some osmotic pressure.

  • Osmophiles: Require high solute concentration.

  • Halophiles: Require high salt concentration (e.g., Halobacterium).

• Hydrostatic Pressure:

  • Barotolerant: Survive increased pressure.

  • Barophiles: Grow best under high pressure; found in deep oceans.

• Atmospheric Conditions:

  • Obligate Aerobes: Require oxygen.

  • Obligate Anaerobes: Cannot tolerate oxygen.

  • Aerotolerant Anaerobes: Tolerate oxygen but do not use it; fermenters.

  • Facultative Anaerobes: Can grow with or without oxygen.

  • Microaerophiles: Require low oxygen concentration (2–10% less than atmospheric).

  • Capneic Bacteria: Require elevated CO2 levels.

• pH:

  • Neutrophiles: Optimal pH 5–8.

  • Acidophiles: Optimal pH <5.5.

  • Alkaliphiles: Optimal pH >8.5.

Replication and Expression of Bacterial DNA

Bacterial DNA replication is tightly regulated and essential for cell division. Expression of genes allows adaptation to environmental changes.

• Binary Fission: DNA replication precedes cell division; regulatory events ensure completion.

• Gene Expression: Allows bacteria to respond to environmental stimuli.

Classification of Bacteria and Archaea

Criteria for Classification

Bacteria and archaea are classified based on morphological, biochemical, and genetic characteristics. Bergey’s Manual of Systematic Bacteriology is the standard reference for bacterial taxonomy.

• Prokaryotes: Divided into bacteria and archaea.

• Bacterial Species: Group of bacteria with similar characteristics.

• Phenotypic Classification: Based on microscopic/macroscopic morphology.

• Biochemical Markers: Presence/absence of enzymes, fermentation abilities.

Binomial System of Nomenclature

Bacteria are named using a two-part system: genus and species (e.g., Escherichia coli).

• Genus: Capitalized.

• Species: Lowercase.

Medically Important Bacteria (According to Bergey’s Manual)

Bacteria are grouped based on Gram reaction, oxygen requirements, morphology, and pathogenicity. Below is a summary of major groups and examples.

Archaea

Archaea are prokaryotes with unique features distinct from bacteria. Methanogens are a notable group, producing methane in anaerobic environments and rapidly killed by oxygen.

• Methanogens: Found in swamps, marshes, marine sediments, hydrothermal vents.

• Metabolic Byproduct: Methane.

• Oxygen Sensitivity: Rapidly killed by exposure to oxygen.

Summary Table: Bacterial Growth Factors

Example: Bacterial Growth Equation

The exponential growth of bacteria can be described mathematically:

Where: Nt = number of cells at time t N0 = initial number of cells n = number of generations

Additional info: This equation assumes binary fission and ideal conditions during the log phase.