Microbial Metabolism, Growth, and Control: Study Guide for BIOL 2040 Exam 2 (Chapters 5-7)

Microbial Metabolism

Metabolism Overview

Metabolism encompasses all chemical reactions within a cell, divided into catabolic and anabolic pathways. Catabolism breaks down complex molecules to release energy, while anabolism uses energy to build complex molecules.

• Catabolism: Breakdown of complex molecules; exergonic (releases energy).

• Anabolism: Synthesis of complex molecules; endergonic (requires energy).

• ATP Generation: ATP is produced during catabolic reactions and consumed during anabolic reactions.

• Example: Glycolysis is a catabolic pathway that generates ATP.

Enzymes and Chemical Reactions

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy. Their activity is influenced by environmental factors and inhibitors.

• Enzyme-Substrate Complex: Substrate binds to the enzyme's active site.

• Factors Affecting Enzyme Activity:

  • Temperature and pH: Extreme values can denature enzymes.

  • Substrate Concentration: Higher concentration increases reaction rate until saturation.

  • Inhibitors:

    • Competitive: Block active site.

    • Noncompetitive: Bind allosterically, altering enzyme shape.

• Example: Enzyme activity graph shows maximum rate at saturation.

Types of ATP Generation

Cells generate ATP through three main mechanisms, each with distinct processes and energy sources.

• Substrate-Level Phosphorylation: Direct transfer of phosphate to ADP.

• Oxidative Phosphorylation: Uses electron transport chain (ETC) and chemiosmosis.

• Photophosphorylation: Utilizes light energy in photosynthetic organisms.

• Equation: $\text{ADP} + \text{P}_i \rightarrow \text{ATP}$

Carbohydrate Catabolism (Energy Production)

Carbohydrate catabolism is central to microbial energy production, involving glycolysis, the Krebs cycle, and the electron transport chain.

• Glycolysis: Glucose is converted to 2 pyruvic acid, 2 ATP, and 2 NADH.

• Krebs Cycle: Pyruvic acid is converted to Acetyl-CoA; produces NADH, FADH2, CO2, and ATP.

• Electron Transport Chain (ETC): Electrons pass through carriers, producing ATP via chemiosmosis.

• Aerobic Respiration: Final electron acceptor is oxygen.

• Anaerobic Respiration: Final electron acceptor is an inorganic molecule (not oxygen).

• Fermentation: Produces ATP without oxygen; end products include lactic acid, ethanol, and CO2.

Enzyme Activity and Substrate Saturation

Enzyme activity increases with substrate concentration until all active sites are occupied, reaching a maximum rate known as enzyme saturation.

• Key Concept: Enzymes cannot increase reaction rate beyond saturation.

Oxygen and Glucose Consumption

Aerobic metabolism is more efficient than fermentation, yielding more ATP per glucose molecule.

• Example: Aerobic respiration produces up to 38 ATP per glucose, while fermentation yields only 2 ATP.

Microbial Growth

Growth Requirements

Microbial growth depends on physical and chemical factors, including temperature, pH, and osmotic pressure.

• Temperature:

  • Psychrophiles: Cold-loving.

  • Mesophiles: Moderate-temperature-loving (human pathogens).

  • Thermophiles: Heat-loving.

• pH: Most bacteria prefer pH 6.5-7.5; acidophiles thrive in acidic environments.

• Osmotic Pressure: Plasmolysis occurs in hypertonic solutions.

Oxygen Requirements

Microbes vary in their oxygen requirements, which affects their growth patterns and ecological niches.

• Obligate Aerobes: Require oxygen.

• Obligate Anaerobes: Killed by oxygen.

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

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

Bacterial Reproduction

Bacteria primarily reproduce by binary fission, leading to exponential population growth under optimal conditions.

• Binary Fission: Cell divides into two identical daughter cells.

• Generation Time: Time required for a cell to divide.

Bacterial Growth Phases

Bacterial populations progress through distinct growth phases in batch culture.

• Lag Phase: Cells adjust to environment; no division.

• Log Phase: Exponential growth.

• Stationary Phase: Growth rate equals death rate.

• Death Phase: Cells die faster than they reproduce.

Measuring Microbial Growth

Microbial growth can be measured directly or indirectly to estimate population size and viability.

• Direct Methods:

  • Plate Count (Colony Forming Units, CFUs).

  • Filtration for small sample sizes.

• Indirect Methods:

  • Turbidity (spectrophotometer).

The Control of Microbial Growth

Definitions & Terms

Controlling microbial growth is essential in healthcare, food safety, and laboratory settings. Key terms describe the extent and method of microbial control.

• Sterilization: Destruction of all microbial life.

• Disinfection: Destruction of harmful microbes on non-living surfaces.

• Antisepsis: Destruction of harmful microbes on living tissues.

• Sanitization: Reducing microbial counts to safe levels.

Microbial Death & Control Agents

The effectiveness of microbial control agents depends on several factors, including microbial characteristics and exposure conditions.

• Factors Affecting Death Rate: Number of microbes, time of exposure, microbial characteristics (e.g., endospores are highly resistant).

• Mechanisms of Control: Disrupt membrane permeability, damage proteins (enzymes), damage nucleic acids (DNA/RNA).

Physical Methods of Microbial Control

Physical methods are widely used to control microbial growth, especially in sterilization and disinfection processes.

• Heat:

  • Moist Heat: Autoclaving, boiling, pasteurization.

  • Dry Heat: Incineration, flaming, hot-air sterilization.

• Filtration: Used for heat-sensitive materials.

• Radiation:

  • Ionizing (X-rays, gamma rays): Destroys DNA.

  • Non-ionizing (UV light): Causes thymine dimers.

Chemical Methods of Microbial Control

Chemical agents are used to disinfect, sterilize, and preserve materials and surfaces.

• Disinfectants & Antiseptics:

  • Phenols & Phenolics: Disrupt plasma membranes.

  • Halogens: Iodine, chlorine.

  • Alcohols: Ethanol, isopropanol.

  • Heavy Metals: Silver, copper, mercury.

• Sterilants: Aldehydes (formaldehyde, glutaraldehyde), ethylene oxide gas.

• Food Preservatives: Sorbic acid, benzoic acid, nitrates & nitrites (prevent endospore germination in meats).

Thermal Death Time

Thermal death time is the minimum time required to kill all microbes at a specific temperature.

• Example: Bacillus cereus endospores are killed at 121°C for 15 minutes by autoclaving.

Autoclaving Effect

Autoclaving is a highly effective sterilization method, killing all microbes and spores.

• Key Concept: Autoclaving at 121°C for 15 minutes ensures complete sterilization.

Disk-Diffusion Test for Disinfectants

The disk-diffusion test compares the effectiveness of disinfectants by measuring zones of inhibition.

• Key Concept: Larger zone of inhibition indicates a more effective disinfectant.

Bacterial Growth in Disinfectants

Disinfectants can be classified as bacteriostatic (inhibit growth) or bactericidal (kill bacteria) based on their effect after incubation.

• Bacteriostatic: Prevents bacterial growth.

• Bactericidal: Kills bacteria.

Key Study Tips

• Understand enzyme activity and ATP production (glycolysis, Krebs cycle, ETC).

• Know bacterial growth phases and oxygen requirements.

• Memorize sterilization, disinfection, and antisepsis definitions.

• Review physical and chemical microbial control methods.

• Practice interpreting growth curves and disinfection tests.

• Be familiar with aerobic vs. anaerobic respiration and fermentation.