Bacterial Cell Structure

Bacteria are prokaryotic microorganisms, meaning they lack a membrane-bound nucleus and organelles. They are structurally simple but highly efficient in function.

Key Components of Bacterial Cells:

a) Outer Structures:

1. Cell Wall:
   • Provides shape, protection, and structural integrity.
   • Composed of peptidoglycan (alternating N-acetylglucosamine and N-acetylmuramic acid).
   • Bacteria are classified based on cell wall structure into:
     • Gram-positive: Thick peptidoglycan layer (e.g., Staphylococcus).
     • Gram-negative: Thin peptidoglycan layer and outer membrane (e.g., E. coli).
2. Cell Membrane:
   • Composed of a phospholipid bilayer with proteins.
   • Functions:
     • Selective permeability.
     • Site for metabolic processes like respiration and photosynthesis.
3. Capsule/Slime Layer:
   • Polysaccharide layer outside the cell wall.
   • Functions:
     • Protects against desiccation.
     • Aids in attachment to surfaces (biofilm formation).
     • Prevents phagocytosis by host immune cells.
4. Flagella:
   • Long, whip-like structures used for movement (motility).
   • Composed of the protein flagellin.
   • Arrangement:
     • Monotrichous (single flagellum).
     • Lophotrichous (tuft at one end).
     • Amphitrichous (flagella at both ends).
     • Peritrichous (flagella all over).
5. Pili/Fimbriae:
   • Hair-like structures on the surface.
   • Functions:
     • Attachment to surfaces.
     • Exchange of genetic material during conjugation (sex pili).

b. Internal Structures:

1. Cytoplasm:
   • Gel-like matrix containing enzymes, nutrients, and ribosomes.
2. Nucleoid:
   • Region where the bacterial chromosome (circular, double-stranded DNA) is located.
   • Not surrounded by a nuclear membrane.
3. Plasmids:
   • Extra-chromosomal DNA, often carrying antibiotic resistance or virulence genes.
4. Ribosomes:
   • 70S ribosomes responsible for protein synthesis.
5. Inclusion Bodies:
   • Storage granules for nutrients (e.g., glycogen, sulfur, polyphosphate).
6. Endospores:
   • Highly resistant structures formed by some bacteria (e.g., Bacillus and Clostridium) under unfavorable conditions.

Chemoautotrophy

Chemoautotrophic bacteria derive energy from the oxidation of inorganic compounds and use this energy to fix carbon dioxide (CO₂) into organic compounds.

Key Features:

1. Energy Source: Inorganic compounds (e.g., ammonia, nitrites, sulfur, iron).
2. Carbon Source: Carbon dioxide.
3. Examples of Chemoautotrophic Bacteria:
   • Nitrosomonas: Oxidizes ammonia to nitrite.
   • Nitrobacter: Oxidizes nitrite to nitrate.
   • Thiobacillus: Oxidizes sulfur compounds.
   • Acidithiobacillus ferrooxidans: Oxidizes ferrous iron (Fe²⁺) to ferric iron (Fe³⁺).

Importance in Agriculture: Nitrogen Cycle: Nitrosomonas and Nitrobacter play a crucial role in soil nitrogen cycling by converting ammonia to nitrate, making nitrogen available to plants.

Photoautotrophy

Photoautotrophic bacteria derive energy from sunlight and use it to fix carbon dioxide into organic compounds.

Key Features:

1. Energy Source: Sunlight.
2. Carbon Source: Carbon dioxide.
3. Photosynthetic Pigments:
   • Bacteriochlorophyll: Found in photosynthetic bacteria (e.g., Rhodospirillum).
   • Chlorophyll-a: Found in cyanobacteria (e.g., Anabaena, Nostoc).

Types of Photoautotrophic Bacteria:

1. Cyanobacteria (Blue-Green Algae):
   • Perform oxygenic photosynthesis (release oxygen).
   • Found in aquatic and terrestrial habitats.
   • Fix atmospheric nitrogen (e.g., Anabaena, Nostoc).
2. Purple and Green Sulfur Bacteria:
   • Perform anoxygenic photosynthesis (do not release oxygen).
   • Use hydrogen sulfide (H₂S) as an electron donor instead of water.
   • Examples:
     • Purple sulfur bacteria (Chromatium).
     • Green sulfur bacteria (Chlorobium).

Importance in Agriculture:

• Soil Fertility: Cyanobacteria fix atmospheric nitrogen, enhancing soil nitrogen levels.
• Pond Fertilization: Cyanobacteria are used in aquaculture to increase productivity.

Bacterial Growth

Bacterial growth refers to an increase in the number of cells, not cell size. It occurs through binary fission, where one cell divides into two identical daughter cells.

Phases of Bacterial Growth Curve:

1. Lag Phase:
   • No increase in cell number; bacteria adapt to their environment.
   • Enzymes and metabolites are synthesized.
2. Log (Exponential) Phase:
   • Rapid cell division and exponential growth.
   • Best phase for studying bacterial physiology and for industrial applications.
3. Stationary Phase:
   • Growth rate slows due to nutrient depletion and waste accumulation.
   • Cell division equals cell death.
4. Death (Decline) Phase:
   • Cells die due to the exhaustion of resources and toxic waste accumulation.

Factors Affecting Bacterial Growth:

1. Nutrients:
   • Macronutrients (carbon, nitrogen, phosphorus, etc.).
   • Micronutrients (iron, magnesium, etc.).
2. Temperature:
   • Psychrophiles (cold-loving, e.g., Arthrobacter).
   • Mesophiles (moderate temperatures, e.g., E. coli).
   • Thermophiles (heat-loving, e.g., Thermus aquaticus).
3. pH:
   • Acidophiles (e.g., Acidithiobacillus).
   • Neutrophiles (e.g., E. coli).
   • Alkaliphiles (e.g., Bacillus alcalophilus).
4. Oxygen Requirement:
   • Obligate aerobes (require oxygen, e.g., Pseudomonas).
   • Obligate anaerobes (killed by oxygen, e.g., Clostridium).
   • Facultative anaerobes (grow with or without oxygen, e.g., E. coli).
   • Microaerophiles (require low oxygen levels, e.g., Helicobacter pylori).
5. Water:
   • Essential for metabolic activities.
6. Salt Concentration:
   • Halophiles thrive in high-salt environments (e.g., Halobacterium).