Factors That Influence Growth of Bacteria

Factors Influencing Bacterial Growth

Nutritional Requirements

• Bacteria obtain all necessary nutrients from their environment for growth.
• In the lab, bacteria are cultivated in culture media designed to provide all essential nutrients.
• Nutrients are categorized into:
  • Major elements
  • Trace elements
  • Growth factors

Major Elements

• Include C, H, O, N, S, P, K, Mg, Fe, Ca, Mn, and traces of Zn, Co, Cu, Mo.
• Found in water, inorganic ions, small molecules, and macromolecules.
• Serve functional and structural roles within the cell.
• Examples:
  • Carbon (C):
    • 50% of dry weight.
    • Sources: Organic compounds or CO_2.
    • Function: Main constituent of cellular material.
  • Oxygen (O):
    • 20% of dry weight.
    • Sources: H_2O, organic compounds.
    • Function: Constituent of cell material; O_2 is an electron acceptor in aerobic respiration.
  • Nitrogen (N):
    • 14% of dry weight.
    • Sources: NH3, NO3, organic compounds, N_2.
    • Function: Constituent of amino acids, nucleic acids, nucleotides, and coenzymes.
  • Hydrogen (H):
    • 8% of dry weight.
    • Sources: H2O, organic compounds, H2.
    • Function: Main constituent of organic compounds and cell water.
  • Phosphorus (P):
    • 3% of dry weight.
    • Sources: Inorganic phosphates (PO_4).
    • Function: Constituent of nucleic acids, nucleotides, phospholipids, LPS, teichoic acids.
  • Sulfur (S):
    • 1% of dry weight.
    • Sources: SO4, H2S, S, organic sulfur compounds.
    • Function: Constituent of cysteine, methionine, glutathione, and several coenzymes.
  • Potassium (K):
    • 1% of dry weight.
    • Sources: Potassium salts.
    • Function: Main cellular inorganic cation and cofactor for certain enzymatic reactions.
  • Magnesium (Mg):
    • 0.5% of dry weight.
    • Sources: Magnesium salts.
    • Function: Inorganic cellular cation, cofactor for certain enzymatic reactions.

Trace Elements

• Metal ions required in small amounts, often as contaminants.
• Function as cofactors for essential enzymatic reactions.
• Examples: Mn, Co, Zn, Cu.

Carbon and Energy Sources

• Bacteria require energy, carbon, and suitable physical conditions for growth (e.g., O_2 concentration, temperature, pH).
• Prokaryotes are categorized based on carbon and energy sources:
  • Photoautotrophs:
    • Energy source: Light.
    • Carbon source: CO_2.
    • Examples: Cyanobacteria.
  • Photoheterotrophs:
    • Energy source: Light.
    • Carbon source: Organic compounds.
    • Examples: Some purple bacteria.
  • Chemoautotrophs (Lithotrophs):
    • Energy source: Inorganic compounds (e.g., NH3, NO2, H_2S).
    • Carbon source: CO_2.
    • Examples: A few bacteria.
  • Chemoheterotrophs:
    • Energy source: Organic compounds.
    • Carbon source: Organic compounds.
    • Examples: Most bacteria, some Archaea, almost all eukaryotes (animals, protozoa, fungi).

Growth Factors

• Organic compounds required in small amounts that organisms cannot synthesize due to blocked or missing pathways.

• Fulfill specific roles in biosynthesis.

• Categories:

  • Purines and pyrimidines: for synthesis of nucleic acids (DNA, RNA).
  • Amino acids: required for synthesis of proteins.
  • Vitamins: needed as coenzymes and as functional groups of certain enzymes.

• Some bacteria (e.g., E. coli) do not require growth factors, while others (e.g., Lactobacillus) do.

• Auxotrophs: Mutant strains requiring growth factors not needed by the wild type (e.g., E. coli trp- requiring tryptophan).

• Examples of Vitamins:

  • P-Aminobenzoic acid:
    • Coenzyme form: Precursor for synthesis of folic acid
    • Function: Precursor for synthesis of folic acid
  • Folic acid:
    • Coenzyme form: Tetrahydrofolate
    • Function: Synthesis of thiamine, purine bases, serine, methionine, pentothenate.
  • Nicotinic acid:
    • Coenzyme form: NAD (Nicotinamide adinine dinucleotide and NADP)
    • Function: Electron carrier in dehydrogenation reactions

• Most pathogenic bacteria require complex media (e.g., blood serum, tissue extracts).

• Some pathogens do not grow on artificial media (e.g., Treponima pallidium, Mycobacterium leprae).

Selective Medium

• A medium with components added to prevent the growth of certain bacteria while promoting the growth of desired species.
• Physical conditions like temperature and pH can also be adjusted for selection.
• Example: MacConkey agar inhibits Gram-positive bacteria while allowing Gram-negative bacteria to grow.

Differential Medium

• A medium containing chemicals or reagents that produce a specific growth pattern or change, allowing differentiation between bacterial species.
• Example: MacConkey agar with neutral red indicator differentiates between lactose fermenters (red colonies) and non-lactose fermenters (colorless colonies).

Selective Differential Medium

• Contains both selective and differential agents.
• Example: MacConkey agar

Enrichment Medium

• Contains blood or tissue extracts to support the growth of fastidious organisms (e.g., pathogenic microorganisms).

Basal (Complex) Medium

• Used for organisms with unknown nutritional requirements.
• Exact composition is unknown.
• Examples: Nutrient agar, malt extract agar, potato dextrose agar.

Defined Medium

• Chemical composition is known.
• Used for precise biochemical studies.
• Example: Mineral salts agar.

Nutrient Agar Composition

• 0. 5% peptone: Provides organic nitrogen.
• 0. 3% beef extract/yeast extract: Provides vitamins, carbohydrates, and nitrogen.
• 1. 5% agar: Solidifying agent.
• 0. 5% sodium chloride: Maintains salt concentration similar to cytoplasm.

Effect of Oxygen

• Bacteria exhibit a wide range of responses to molecular oxygen (O_2).
• Obligate aerobes: Require oxygen for growth; use O_2 as a final electron acceptor in aerobic respiration (e.g., Bacillus, Pseudomonas).
• Obligate anaerobes: Do not need or use oxygen; oxygen is toxic (e.g., Clostridium botulinum).
• Facultative anaerobes (Facultative aerobes): Switch between aerobic and anaerobic metabolism (e.g., Staphylococcus aureus).
• Aerotolerant: Live by fermentation alone, regardless of oxygen presence (e.g., Lactobacillus).
• Microaerophiles: Grow well in low oxygen and high carbon dioxide concentrations (e.g., Campylobacter).
• The response of organisms to O_2 depends on the presence of enzymes that react with oxygen radicals.

Oxygen Radicals and Enzymes

• Addition of a single electron to O2 forms superoxide (O2^-) radical.
• Superoxide radicals can damage cells and produce hydrogen peroxide (H2O2) and hydroxyl radicals (OH•).
• Hydroxyl radicals are highly reactive and can damage DNA.
• Aerobic, facultative, aerotolerant, and microaerophilic organisms have protective mechanisms against toxic oxygen forms.
• Superoxide dismutase: Converts superoxide radicals to hydrogen peroxide.
• Catalase: Converts hydrogen peroxide to molecular oxygen and water.
• Peroxidase: Converts hydrogen peroxide to water (in organisms lacking catalase).
• Obligate anaerobes lack superoxide dismutase, catalase, and/or peroxidase, leading to lethal oxidations.
• Carotenoid pigments protect photosynthetic organisms by reacting with singlet oxygen radicals.

Oxygen Requirements Summary

Methods for Cultivation of Anaerobes

• Candle jar: Combustion reduces oxygen concentration.
• Anaerobic jar: Gas pack generates CO2 and H2. Palladium catalyst removes residual oxygen.
• Anaerobic chamber: Mixture of hydrogen, nitrogen, and carbon dioxide with palladium catalyst to remove residual oxygen.

Effect of Temperature

• Microorganisms grow in a range of temperatures defined by cardinal points:
  • Minimum temperature: Below which growth stops.
  • Maximum temperature: Above which growth stops.
  • Optimum temperature: Range within which growth is most rapid.

Temperature Ranges

• Psychrophiles: Cold-loving; grow at 0°C, optimum 10-15°C.
• Psychrotrophs: Grow at 0°C, optimum in mesophilic range (near room temperature).
• Mesophiles: Moderate temperature-loving microorganisms.
• Thermophiles: Heat-loving microorganisms.
• Extreme thermophiles (Hyperthermophiles): Grow at very high temperatures

Temperature Ranges for Prokaryotic Microorganisms (°C)

Temperature example for growth summary (°C)

Specific Examples of Temperature Ranges

Adaptations to Temperature

• Psychrophiles:
  • Unsaturated fatty acids in the cytoplasmic membrane remain liquid at low temperatures.
  • Enzymes function at low temperatures.
• Thermophiles:
  • Saturated fatty acids in the cytoplasmic membrane are stable at high temperatures.
  • Membranes of hyperthermophiles contain phytane, a branched, saturated isoprenoid.
  • Structural proteins, ribosomal proteins, transport proteins (permeases), and enzymes are very stable at high temperatures.
  • Proteins are dehydration and have slight changes in their primary structure, which accounts for their thermal stability.

Effect of pH

• The range of pH for growth is defined by:
  • Minimum pH: Below which growth stops.
  • Maximum pH: Above which growth stops.
  • Optimum pH: pH at which growth is best.

pH preference

• Acidophiles: Grow well below neutral pH.
• Neutrophiles: Grow best at neutral pH.
• Alkaliphiles: Grow best under alkaline conditions.

Optimum pH Examples

• Obligate acidophiles: Require a low pH for growth (e.g., Thiobacillus sp.).
• Many fungi are acidophiles.
• Culture media for human pathogens typically have a pH near 7.

Effects of pH on Bacterial Cells

• Extreme pH affects the structure of macromolecules.
• High pH can disrupt hydrogen bonds in DNA.
• Moderate pH changes can alter the ionization of amino acid functional groups, leading to protein denaturation.

Effect of Water Activity

• Water is the solvent for biochemical activities.
• Water availability is indicated by relative humidity (RH) or water activity (A_w).
• Pure water has an A_w of 1.00.
• Water activity is affected by solute concentration; higher solute concentration means lower water activity. A_w of human blood is 0.99, seawater 0.98, maple syrup 0.90, Great Salt Lake 0.75, agricultural soils 0.9-1.00, fresh vegetables 0.98- 0.99.
• Microorganisms can only use water in its free form.
• The most common solute is salt (NaCl).

Classification based salt requirement:

• Halophiles: Require salt for growth.
  • Mild halophiles: 1-6% salt.
  • Moderate halophiles: 6-15% salt.
  • Extreme halophiles: 15-30% salt.
• Halotolerant: Able to grow at moderate salt concentrations, but prefer no NaCl.
• Osmophiles: Live in high sugar environments.
• Xerophiles: Live in dry environments.

Osmotic Pressure

• The force with which water moves across the cytoplasmic membrane.
• In equilibrium, there should not be large differences in solute concentrations inside and outside the cell.

Tonicity

• Isotonic solution: No net flow of water.
• Hypertonic solution: Higher external solute concentration causes water to move out, leading to dehydration and inhibited growth.
• Hypotonic solution: Higher internal solute concentration causes water to move in, leading to cell burst or rupture.
• Adding solutes like salt and sugar preserves foods by osmotically withdrawing water from microbial cells.