Microbial Metabolism and Factors Affecting Microbial Growth

Microbial Metabolism

• Metabolism is the sum of all growth processes in living cells, involving 2000-3000 biosynthetic reactions governed by enzymes.
• Reactions are classified into:
  • Catabolism
  • Anabolism

Anabolism

• Anabolism (Biosynthesis): Enzyme-regulated, energy-requiring reactions.
• Involves building complex organic molecules from simpler ones.
• Associated with building cellular components and cell mass.
• Uses products of catabolism:
  • Building blocks are linked to form cellular components.
  • Requires energy from ATP or reducing powers.
• Reactions are anabolic or biosynthetic, generally dehydration synthesis reactions (release water) and endergonic (consume more energy than they produce).
• Examples:
  • Formation of proteins from amino acids.
  • Nucleic acids from nucleotides.
  • Polysaccharides from simple sugars.

Catabolism

• Catabolism: Breakdown; Enzyme-regulated chemical reactions that release energy.
• Complex organic compounds are broken down into simpler ones.
• Reactions are catabolic or degradative.
• Breakdown releases energy from breaking chemical bonds (Energy is neither created nor destroyed).
• Generally hydrolytic reactions (use water, break chemical bonds) and exergonic (produce more energy than they consume).
• Example: Cells break down sugars into $CO<em>2$ and $H</em>2O$.
• Objectives of Catabolism:
  • Energy: ATP
  • Reducing power: NADH, FADH, NADPH
  • Precursor metabolites (intermediates): Small building blocks for cell mass synthesis.
    • Examples: RCOOH for amino acid and protein synthesis, Oxaloacetate.
• Breakdown of nutrients releases energy, captured in Substrate level Phosphorylation and Oxidative level Phosphorylation.
• In microbes, this process occurs near inner cellular membranes.

Catabolism vs Anabolism

• Catabolism:
  • Complex to simple molecules.
  • Converging.
  • Exergonic (energy producing).
  • Involves oxidations.
  • Requires NAD+
  • Convergent process.
  • Yields ATP, NADH, and end products like $CO<em>2$, $H</em>2O$, and $NH_3$.
• Anabolism:
  • Simple to complex molecules.
  • Diverging.
  • Endergonic (energy consuming).
  • Involves reductions.
  • Requires NADPH.
  • Divergent process.
  • Uses ATP and precursor molecules like amino acids, sugars, fatty acids, and nitrogenous bases.

Types of Bacterial Metabolism

• Three common forms:
  • Aerobic pathway
  • Anaerobic pathway
  • Facultative pathway
• Differ in the final electron acceptor.

Aerobic Pathway

• Substrate or nutrients are broken down in the presence of oxygen as the final electron acceptor.
• Generates the most available energy for living cells.
• Example: E. coli yields 28 ATP molecules from 1 glucose molecule.

Anaerobic Pathway

• Also known as the fermentation pathway.
• Substrate (usually intermediate carbohydrate, e.g., glucose) is broken down in the absence of oxygen (toxic to strict anaerobes).
• Resulting intermediate carbohydrate acts as the final electron acceptor.
• End product is either alcohols or organic acids.
• Small amount of energy and reducing power is obtained.
• Example: 2 ATP per Glucose.
• Microbes require high nutrient concentration to survive by fermentation (Pasteur effect).

Facultative Pathway

• Some microorganisms can live in the presence and absence of oxygen.
• These are called facultative organisms.
• Preferentially live in aerobic environments.
• Majority of bacteria are facultative.

Anaerobic Respiration

• Present in some bacteria.
• Differs from aerobic and anaerobic pathways; the final electron acceptor is some other element than oxygen or an intermediary carbohydrate.
• Examples:
  • Nitrate reducing bacteria
  • Sulfate reducing bacteria

Overview of the Three Main Catabolic Pathways

• Aerobic Respiration:
  • Maximum net yield: 36-38 ATPs
  • Final electron acceptor: Oxygen
• Anaerobic Respiration:
  • Yields 2-36 ATPs
  • Final electron acceptor: Non-oxygen compound (e.g., $NO<em>3, CO</em>3$)
• Fermentation:
  • Yields 2 ATPs
  • Final electron acceptor: Organic compounds (alcohols, acids)

The Intrinsic and Extrinsic Parameters Affecting Microbial Growth

• Microorganisms greatly influence human life and activities (food, materials, health, medical, and environment).
• Understanding factors affecting microbial growth helps provide effective control measures.
• Parameters classified into two major classes:
  • Intrinsic
  • Extrinsic

The Intrinsic Parameters

• Describe factors associated with the nature of a substance (substrate) on which microorganisms can establish growth.
• Parameters:
  • pH
  • Moisture content
  • Oxidation-reduction potentials
  • Nutritional content
  • Antimicrobial constituents
  • Biological structures

1. pH

• Bacteria prefer pH between 6.5 - 7.5 (neutral to slightly acidic).
• Fungi are more suited to slightly alkaline pH of 7.5 - 8.5 (some yeasts and molds).
• Buffering (stabilizing the pH) using acid or alkaline can preserve materials.

2. Moisture Content

• Drying or desiccation is an old method of material preservation.
• Water or moisture content is a crucial criterion determining microbial growth.
• Technically known as Water Activity ($a_w$) of the substance.
• Describes pure water quantity available for biological growth.
• All living cells require water for growth.

3. Oxidation - Reduction Potential (ORP)

• ORP is how readily a substrate loses or gains electrons.
• Substrate losing electron: oxidized; gaining electron: reduced.
• OR potential measured using a potentiometer and expressed in millivolts (mv).
• Highly oxidized compound (readily accepts electron) is a good reducing agent with a positive (+) Eh (oxygen readily available), and vice versa.
• Redox potential of a solution is measured and reported as Eh.
• High -SH content substances, like meat, have negative Eh favoring anaerobic growth (e.g., solid meat Eh -200mv, minced meat and fruits Eh +200mv).
• Fresh foods of plant and animal origin are in a reduced state due to reducing substances like sugars, ascorbic acids, and sulphydryl groups of proteins.
• Oxygen diffusion and food processing (heating) can alter Eh (reduced or oxidized).
• Food stored in air has a wider Eh range (+Mv) than under vacuum.
• Oxygen presence or absence is important for microbial growth; microorganisms grouped by oxygen requirement/intolerance.
• Aerobes: +500 to +300 mV
• Anaerobes: +100 to -250 mV
• Facultative anaerobes: +300 to -100 mV
• Eh and presence/absence of oxygen determine microbial group growth in foods (important in spoilage and fermented foods).
• Spoilage example: putrefaction of meat by Clostridium spp.
• Fermentation example: Penicillium spp. in blue cheese

4. Nutrient Content

• Microbial growth depends on nutritional content.
• Biological life needs water, energy source, nitrogen and elements, minerals, vitamins, and growth factors.
• Microorganism colonization and growth depend on the substance's chemical constituents and enzymatic capabilities.
• Simple sugar materials are highly vulnerable to microbial spoilage compared to complex ones.

5. Antimicrobial Content

• Natural biological and synthetic materials contain antimicrobial constituents protecting or delaying spoilage.
• Example: spices like cloves and plant-based compounds contain eugenol, phenol, etc., deterring microbial growth.

6. Biological Structures

• Protective shells (eggs), skin (human tissues), resin (wood) protect materials from microbial spoilage.

The Extrinsic Parameters Affecting Microbial Growth

• Parameters associated with the environment surrounding materials.
• Limited to:
  • Temperature surrounding the material
  • Relative Humidity of the surrounding
  • Presence and concentration of gases in the environment

1. Temperature

• Microorganisms grow over a wide temperature range; temperature greatly affects them.
• Storage temperature affects microorganism colonization and proliferation.
• Controlling temperature and relative humidity enhances durability.

2. Relative Humidity

• Water is essential for biological life; surrounding relative humidity affects microorganism growth.
• Materials last longer at low or high RH.
• Drying agents (salts, silica gels, talcum powders, refrigeration, etc.) help preserve materials.

3. Gaseous Content

• The gaseous content of the atmosphere (carbon dioxide, oxygen, and nitrogen) greatly affects microorganism growth.
• Increased carbon dioxide (controlled or modified atmosphere) enhances shelf life of microbial sensitive materials.
• 10% concentration, generated using dry ice or gaseous carbon dioxide, is generally used.