Microbial Metabolism, Growth, and Control: Study Notes for Exam II

Microbial Metabolism

Metabolism: Anabolism and Catabolism

Metabolism refers to all the chemical reactions that occur within a microorganism. These reactions are divided into two main categories:

• Anabolism: The synthesis of complex molecules from simpler ones, requiring energy input. Anabolic reactions are essential for cell growth and repair.

• Catabolism: The breakdown of complex molecules into simpler ones, releasing energy. Catabolic reactions provide the energy needed for cellular processes.

Example: The breakdown of glucose during glycolysis is a catabolic process, while the synthesis of proteins from amino acids is anabolic.

Role of ATP in Metabolism

Adenosine triphosphate (ATP) is the primary energy carrier in cells. It stores energy released from catabolic reactions and supplies it for anabolic reactions.

• ATP is generated mainly through cellular respiration and fermentation.

• Energy is released when ATP is hydrolyzed to ADP and inorganic phosphate:

$\mathrm{ATP} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ADP} + \mathrm{P}_i + \text{energy}$

Enzymes: Structure and Function

Enzymes are biological catalysts that speed up chemical reactions without being consumed. Key components include:

• Substrate: The specific molecule upon which an enzyme acts.

• Active Site: The region on the enzyme where the substrate binds.

• Enzyme: The protein molecule that facilitates the reaction.

Example: The enzyme sucrase catalyzes the breakdown of sucrose into glucose and fructose.

Factors Influencing Enzymatic Activity

• Temperature: Each enzyme has an optimal temperature; high temperatures can denature enzymes.

• pH: Extreme pH values can alter enzyme structure and function.

• Substrate Concentration: Increased substrate increases reaction rate up to a saturation point.

• Inhibitors: Competitive and noncompetitive inhibitors can decrease enzyme activity.

Oxidation and Reduction (Redox Reactions)

Redox reactions involve the transfer of electrons between molecules:

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

These reactions are essential for energy production in cells.

Electron Transport Chain (ETC) and Chemiosmosis

The ETC is a series of protein complexes in the cell membrane that transfer electrons, releasing energy used to pump protons and generate ATP via chemiosmosis.

• Proton gradient drives ATP synthesis through ATP synthase.

$\text{ADP} + \text{P}_i \xrightarrow{\text{ATP synthase}} \text{ATP}$

Glycolysis

Glycolysis is the process of breaking down glucose into pyruvate, producing ATP and NADH.

• Occurs in the cytoplasm.

• Net gain: 2 ATP and 2 NADH per glucose molecule.

Aerobic and Anaerobic Respiration

• Aerobic Respiration: Uses oxygen as the final electron acceptor; produces the most ATP.

• Anaerobic Respiration: Uses other inorganic molecules (e.g., nitrate, sulfate) as final electron acceptors; yields less ATP.

Fermentation

Fermentation is an anaerobic process that regenerates NAD+ by transferring electrons to organic molecules, producing products like lactic acid or ethanol.

• Yields only 2 ATP per glucose.

Krebs Cycle (Citric Acid Cycle)

The Krebs cycle completes the oxidation of glucose derivatives, generating NADH, FADH2, and ATP.

• Occurs in the cytoplasm (prokaryotes) or mitochondria (eukaryotes).

• Produces CO2 as a waste product.

Microbial Growth

Classification of Microbes by Temperature Preference

Microbes are classified based on their optimal temperature ranges:

Group	Optimal Temperature Range (°C)
Psychrophiles	−5 to 15
Mesophiles	20 to 45
Thermophiles	55 to 80

Example: Most human pathogens are mesophiles.

pH Control in Culture Media

The pH of culture media is controlled to maintain optimal growth conditions and prevent the accumulation of acidic or basic metabolic byproducts that could inhibit microbial growth.

• Buffers are often added to media to stabilize pH.

Biofilms: Formation and Infection Potential

Biofilms are structured communities of microbes attached to surfaces and embedded in a self-produced matrix.

• Biofilms protect microbes from environmental stress and antibiotics.

• They are significant in medical settings due to their role in persistent infections (e.g., on catheters).

Chemically Defined vs. Complex Media

• Chemically Defined Media: Exact chemical composition is known.

• Complex Media: Contains extracts (e.g., peptones, yeast extract); composition varies.

Colony Definition

A colony is a visible mass of microbial cells arising from a single cell or group of cells on solid media.

Binary Fission

Binary fission is the primary method of reproduction in prokaryotes, where one cell divides into two identical daughter cells.

Phases of Microbial Growth and Generation Time

Microbial growth in batch culture follows distinct phases:

Phase	Description
Lag	Cells adapt to environment; no division
Log (Exponential)	Rapid cell division; population doubles at constant rate
Stationary	Growth rate slows; nutrient depletion/waste accumulation
Death	Cells die faster than they divide

Generation time is the time required for a cell to divide (or for a population to double).

The Control of Microbial Growth

Definitions of Key Terms

• Sterilization: Removal or destruction of all microbial life.

• Disinfection: Destruction of vegetative pathogens on inanimate objects.

• Antisepsis: Destruction of vegetative pathogens on living tissue.

• Degerming: Mechanical removal of microbes from a limited area.

• Sanitization: Lowering microbial counts to safe public health levels.

• Biocide/Germicide: Agents that kill microbes.

• Bacteriostasis: Inhibiting, not killing, microbes.

• Asepsis: Absence of significant contamination.

Effects of Microbial Control Agents on Cellular Structures

• Damage to cell membranes (loss of selective permeability).

• Denaturation of proteins and enzymes.

• Damage to nucleic acids (DNA/RNA).

Comparison of Physical Methods: Autoclaving, Boiling, Pasteurization, Dry Heat

Method	Conditions	Effectiveness
Autoclaving	121°C, 15 psi, 15 min	Sterilizes; kills endospores
Boiling	100°C, 10 min	Kills most pathogens; not endospores
Pasteurization	63°C for 30 min or 72°C for 15 sec	Reduces spoilage organisms; not sterilization
Dry Heat	170°C, 2 hr	Sterilizes; slower than moist heat

Factors Affecting Disinfection Effectiveness

• Number of microbes present

• Microbial characteristics (e.g., endospore formation)

• Concentration and nature of disinfectant

• Contact time and temperature

• Presence of organic matter

Chemical Sterilizers

• Examples include ethylene oxide, glutaraldehyde, and peracetic acid.

• Used for heat-sensitive materials.

Microbial Type and Control Effectiveness

The type of microbe affects susceptibility to control methods. For example, endospores and mycobacteria are more resistant than vegetative bacteria.

Additional info: Where the original notes were brief, academic context and examples were added for clarity and completeness.