Bacteria – Cell Structure, Chemoautotrophy, Photoautotrophy, and Growth

Introduction

Bacteria are among the earliest and most primitive organisms, appearing around 3.5 billion years ago. They are prokaryotes, meaning they lack a true nucleus and membrane-bound organelles. Despite their structural simplicity, bacteria show vast diversity in physiology, nutrition, and ecological roles. Found in soil, water, air, and extreme habitats, bacteria are also present as symbionts in plants, animals, and humans.

Their ability to utilize diverse energy sources makes them vital in biogeochemical cycles and in industries such as medicine, agriculture, and biotechnology. This chapter discusses bacterial cell structure, their nutritional modes like chemoautotrophy and photoautotrophy, and their growth patterns.

1. Bacterial Cell Structure

A. General Characteristics

• Unicellular, but may form colonies.
• Size ranges: 0.2–2 µm in diameter, 1–10 µm in length.
• Simple structure but highly efficient for survival and rapid reproduction.

B. Cell Envelope

1. Plasma Membrane: Phospholipid bilayer with proteins; functions in transport, respiration, and biosynthesis. Major site of energy metabolism as bacteria lack mitochondria.
2. Cell Wall: Provides rigidity; made of peptidoglycan.
   • Gram-positive: thick peptidoglycan, teichoic acids, stain purple.
   • Gram-negative: thin peptidoglycan, outer membrane with LPS, stain pink.
   • Acid-fast bacteria: contain mycolic acids (e.g., Mycobacterium).
3. Capsule/Slime Layer: Protective polysaccharide or protein covering; prevents desiccation, helps adhesion, and resists phagocytosis.

C. Cytoplasmic Components

• Nucleoid: Irregular region containing circular double-stranded DNA, attached to plasma membrane.
• Plasmids: Extra-chromosomal DNA; carry resistance or metabolic genes; used in genetic engineering.
• Ribosomes: 70S (30S + 50S subunits); site of protein synthesis.
• Inclusion Bodies: Storage granules (glycogen, sulfur, polyphosphate, gas vacuoles).

D. Surface Appendages

• Flagella: Motility structures made of flagellin; arrangement can be monotrichous, lophotrichous, amphitrichous, or peritrichous.
• Fimbriae: Short, hair-like; aid in attachment.
• Pili: Longer; sex pili help in genetic exchange (conjugation).

2. Chemoautotrophy in Bacteria

Chemoautotrophic bacteria derive energy by oxidizing inorganic compounds and use CO₂ as their carbon source. The released electrons generate ATP via oxidative phosphorylation, while CO₂ is fixed into organic molecules through the Calvin cycle.

Examples

• Nitrifying bacteria: Nitrosomonas (NH₃ → NO₂⁻), Nitrobacter (NO₂⁻ → NO₃⁻).
• Sulfur bacteria: Thiobacillus, oxidize H₂S → SO₄²⁻.
• Iron bacteria: Gallionella, oxidize Fe²⁺ → Fe³⁺.
• Hydrogen bacteria: Hydrogenomonas, oxidize H₂.

Importance

• Vital in nitrogen, sulfur, and iron cycles.
• Increase soil fertility by converting compounds into plant-usable forms.
• Used in wastewater treatment and bioremediation.

3. Photoautotrophy in Bacteria

Photoautotrophic bacteria utilize light as an energy source and CO₂ as carbon source. They may perform oxygenic or anoxygenic photosynthesis.

Types

• Cyanobacteria: Oxygenic photosynthesis; use water as electron donor, release O₂. Pigments: chlorophyll a, phycobiliproteins. Examples: Anabaena, Nostoc.
• Purple Sulfur Bacteria: Anoxygenic photosynthesis; use H₂S as electron donor, no O₂ released. Pigment: bacteriochlorophyll. Example: Chromatium.
• Green Sulfur Bacteria: Anoxygenic photosynthesis; electron donor is H₂S. Pigments: bacteriochlorophylls. Example: Chlorobium.

Significance

• Cyanobacteria contribute significantly to global oxygen production.
• Some cyanobacteria fix atmospheric nitrogen, enriching soils (e.g., in rice paddies).
• Sulfur bacteria play roles in sulfur cycling and thrive in anaerobic habitats.

4. Bacterial Growth

Bacterial growth refers to an increase in the number of cells, usually by binary fission.

A. Binary Fission

DNA replicates, attaches to plasma membrane, the cell elongates, and divides into two identical daughter cells.

B. Bacterial Growth Curve

1. Lag Phase: Cells active but not dividing; enzymes synthesized.
2. Log Phase: Rapid, exponential cell division; cells most metabolically active and susceptible to antibiotics.
3. Stationary Phase: Nutrient limitation and waste accumulation balance growth and death rates.
4. Death Phase: Cell death exceeds reproduction; loss of viability.

C. Factors Affecting Growth

• Nutrients availability (carbon, nitrogen, trace elements).
• pH: Most bacteria grow near neutral pH.
• Temperature ranges:
  • Psychrophiles: 0–20°C
  • Mesophiles: 20–45°C
  • Thermophiles: 45–80°C
  • Hyperthermophiles: >80°C
• Oxygen requirements: obligate aerobes, obligate anaerobes, facultative anaerobes, microaerophiles.