Microbial Growth and Growth Phases

Binary Fission and Generation Time

Bacteria reproduce primarily by binary fission, a process in which a single cell divides into two genetically identical daughter cells. The generation time is the period required for a bacterial population to double in number.

• Binary fission: Asexual reproduction method in prokaryotes.

• Generation time: Varies by species and environmental conditions; can be as short as 20 minutes for some bacteria.

Phases of Bacterial Growth Curve

Bacterial populations in batch culture exhibit distinct growth phases:

• Lag phase: Cells are metabolically active, synthesizing enzymes and cellular components, but not dividing.

• Log (Exponential) phase: Rapid cell division; cells are most uniform and sensitive to antibiotics.

• Stationary phase: Rate of cell division equals rate of cell death; nutrient depletion and waste accumulation occur.

• Death (Decline) phase: Cell death exceeds cell division; population decreases.

Example: If a culture's cell count remains constant but cells are larger and more enzymatically active, it is in the lag phase.

Microbial Nutritional Types

Energy and Carbon Sources

Microbes are classified by their sources of energy, electrons, and carbon:

• Phototrophs: Obtain energy from sunlight.

• Chemotrophs: Obtain energy by oxidizing chemical compounds.

• Autotrophs: Use CO2 as their carbon source.

• Heterotrophs: Obtain carbon from organic compounds.

• Lithotrophs: Use inorganic molecules as electron donors.

• Organotrophs: Use organic molecules as electron donors.

Example: A chemolithoautotroph gets energy by oxidizing inorganic compounds (e.g., H2S) and uses CO2 as its carbon source.

Oxygen Requirements

Types of Oxygen Relationships

• Obligate aerobes: Require oxygen for growth.

• Obligate anaerobes: Cannot tolerate oxygen; oxygen is toxic.

• Facultative anaerobes: Can grow with or without oxygen, but grow better with oxygen.

• Aerotolerant anaerobes: Do not use oxygen but can tolerate its presence.

• Microaerophiles: Require low concentrations of oxygen.

• Capnophiles: Require elevated CO2 levels.

Example: Growth throughout a thioglycolate tube, with more growth at the top, indicates a facultative anaerobe.

Physical and Chemical Growth Factors

Temperature Preferences

• Psychrophiles: Grow best at low temperatures (0–20°C).

• Mesophiles: Prefer moderate temperatures (25–40°C); most human pathogens.

• Thermophiles: Grow at high temperatures (45–60°C or higher).

pH Preferences

• Neutrophiles: Prefer near-neutral pH (6.5–7.5).

• Acidophiles: Grow in acidic environments (pH < 6).

• Alkalinophiles: Grow in basic environments (pH > 8).

Osmotic Pressure and Salt Tolerance

• Obligate halophiles: Require high salt concentrations.

• Facultative halophiles: Tolerate but do not require high salt.

• Barophiles: Thrive under high pressure (e.g., deep ocean).

Example: A microbe that grows only in 15% NaCl is an obligate halophile.

Osmotic Effects on Cells

• Hypotonic solution: Water enters the cell; may cause swelling and lysis, especially in cells lacking a wall.

• Hypertonic solution: Water leaves the cell; causes shrinkage (plasmolysis).

Example: Lysozyme-treated bacteria in distilled water lyse due to loss of cell wall and osmotic influx.

Culture Media

Types of Media

• Defined (synthetic) medium: Exact chemical composition is known.

• Complex medium: Contains extracts/digests; composition not fully known.

• Selective medium: Inhibits unwanted microbes, allows target organisms to grow.

• Differential medium: Distinguishes organisms by visible changes (e.g., color change).

• Reducing medium: Removes/binds oxygen; used for anaerobes.

• Transport medium: Maintains viability and prevents contamination during transport.

Example: Mannitol salt agar is both selective (high salt) and differential (color change with mannitol fermentation).

Quorum Sensing and Surface Structures

Quorum Sensing

• Quorum sensing: Cell-to-cell communication via signaling molecules to detect population density and coordinate behavior (e.g., biofilm formation).

• Does not require direct contact; signals diffuse through the environment.

Surface Structures

• Fimbriae: Short, hair-like structures for attachment; important in biofilm formation.

• Glycocalyces: Polysaccharide layers aiding in attachment and protection.

• Sex pili: Rigid tubes for DNA transfer during conjugation (horizontal gene transfer).

• Flagella: Long, whip-like structures for motility.

Example: Conjugation involves DNA transfer via a sex pilus.

Prokaryotic Cell Structure

General Features

• Prokaryotes: Lack a true, membrane-bound nucleus; include Bacteria and Archaea.

• Eukaryotes: Have a true nucleus and membrane-bound organelles (e.g., protozoa, algae, fungi).

Cell Wall Composition

• Peptidoglycan: Polymer of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) cross-linked by peptides.

• Gram-positive: Thick peptidoglycan, teichoic acids.

• Gram-negative: Thin peptidoglycan, outer membrane with lipopolysaccharide (LPS).

Example: Gram-negative bacteria lose crystal violet stain during decolorization and appear pink/red after counterstaining.

Other Structures

• Bacterial inclusions: Storage granules for nutrients (e.g., glycogen, polyphosphate).

• Nucleoid: Region containing the bacterial chromosome (not membrane-bound).

• Ribosomes: Sites of protein synthesis; bacterial ribosomes are 70S (30S + 50S subunits).

• Cell membrane: Present in both prokaryotes and eukaryotes; eukaryotic membranes often contain cholesterol.

Example: Antibiotics targeting the 30S subunit inhibit bacterial protein synthesis without affecting human ribosomes.

Bacterial Shapes and Arrangements

• Coccobacillus: Short, oval rod intermediate between coccus and bacillus.

• Sarcina: Cubical packet of cocci.

• Vibrio: Comma-shaped bacterium.

• Spirillum: Rigid, spiral-shaped bacterium with external flagella.

• Spirochete: Flexible, spiral-shaped bacterium with axial filaments (endoflagella).

Eukaryotic Organelles

• Mitochondrion: Site of aerobic respiration; contains cristae and electron transport enzymes.

• Chloroplast: Site of photosynthesis; found in algae, not in protozoa or fungi.

• Golgi apparatus: Modifies, sorts, and packages proteins.

• Endoplasmic reticulum (ER): Transport network; rough ER has ribosomes.

• Lysosomes: Contain digestive enzymes.

Fungi, Protozoa, and Algae

Fungi

• Hypha: Single fungal filament.

• Mycelium: Mass of hyphae.

• Zygospores, ascospores, basidiospores: Sexual spores used in fungal classification.

• Conidia: Asexual spores.

Protozoa

• Trophozoite: Active, feeding stage.

• Cyst: Dormant, resistant stage.

Algae

• Algae: Simple eukaryotes; perform oxygenic photosynthesis; reproduce via alternation of generations.

• Distinguishing feature: Presence of chloroplasts (photosynthetic ability).

Example: A motile, unicellular eukaryote with mitochondria but no chloroplasts is a protozoan.

Archaea

• Archaea: Prokaryotes often found in extreme environments (high temperature, acidity, salinity, or pressure).

• Not considered pathogens due to their environmental requirements.

• Example: An organism living at pH 1 and 60°C is likely an archaeon.

Summary Table: Microbial Nutritional Types

Key Equations

• Generation time calculation:

Where: N = final cell number N0 = initial cell number n = number of generations

• Logarithmic growth rate:

Additional info:

• Some explanations and examples were expanded for clarity and completeness.

• Table entries for nutritional types were inferred and formatted for academic clarity.