Microbial Metabolism, Growth, and Molecular Biology: Study Guide

Chapter 5: Microbial Metabolism

Cellular Respiration

Cellular respiration is a metabolic process by which cells convert biochemical energy from nutrients into adenosine triphosphate (ATP), releasing waste products. The process involves glycolysis, the Krebs cycle, and the electron transport chain.

• Location in Prokaryotes: Glycolysis and the Krebs cycle occur in the cytoplasm; the electron transport chain is located in the plasma membrane.

• Location in Eukaryotes: Glycolysis occurs in the cytoplasm; the Krebs cycle and electron transport chain occur in the mitochondria.

• Key Steps:

  1. Glycolysis: Glucose is broken down into pyruvate, producing ATP and NADH.

  2. Krebs Cycle: Pyruvate is further oxidized, generating NADH, FADH2, and CO2.

  3. Electron Transport Chain: Electrons from NADH and FADH2 are transferred through membrane proteins, driving ATP synthesis.

• Overall Equation:

  $\mathrm{C_6H_{12}O_6} + 6\mathrm{O_2} \rightarrow 6\mathrm{CO_2} + 6\mathrm{H_2O} + \text{ATP}$

Example: Escherichia coli uses aerobic respiration when oxygen is available, maximizing ATP yield.

Fermentation and Its Role in Energy Production

Fermentation is an anaerobic process that allows cells to generate ATP without an electron transport chain. It regenerates NAD+ by transferring electrons to organic molecules.

• Key Features:

  • Occurs in the cytoplasm of both prokaryotes and eukaryotes.

  • Yields less ATP than respiration (typically 2 ATP per glucose).

  • End products include lactic acid, ethanol, and CO2.

• Importance: Enables survival in anaerobic environments and is used in food production (e.g., yogurt, bread, alcohol).

Example: Lactobacillus species ferment sugars to produce lactic acid in yogurt production.

Metabolic Diversity Among Organisms

Microorganisms exhibit diverse metabolic strategies based on their sources of carbon and energy.

• Carbon Source Categories:

  • Autotrophs: Use CO2 as their carbon source.

  • Heterotrophs: Require organic compounds for carbon.

• Energy Source Categories:

  • Phototrophs: Obtain energy from light.

  • Chemotrophs: Obtain energy from chemical compounds.

• Combined Categories:

  • Photoautotrophs: Light for energy, CO2 for carbon (e.g., cyanobacteria).

  • Chemoheterotrophs: Organic compounds for both energy and carbon (e.g., most bacteria, fungi).

Example: Rhodospirillum is a photoheterotroph, using light for energy and organic compounds for carbon.

Chapter 6: Microbial Growth and Its Control

Chemical and Physical Requirements for Growth

Microbial growth depends on various chemical and physical factors, including temperature, pH, and oxygen availability.

• Temperature Categories:

  • Psychrophiles: Grow best at 0–15°C.

  • Mesophiles: Grow best at 20–45°C (most human pathogens).

  • Thermophiles: Grow best at 55–80°C.

• pH Categories:

  • Acidophiles: Thrive at pH < 5.5.

  • Neutrophiles: Thrive at pH 5.5–7.9.

  • Alkaliphiles: Thrive at pH > 8.

• Oxygen Requirement Categories:

  • Obligate aerobes: Require oxygen.

  • Obligate anaerobes: Oxygen is toxic.

  • Facultative anaerobes: Can grow with or without oxygen.

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

  • Microaerophiles: Require low levels of oxygen.

Example: Clostridium species are obligate anaerobes, while Staphylococcus species are facultative anaerobes.

Biofilms

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

• Formation: Involves initial attachment, microcolony formation, maturation, and dispersal.

• Significance: Biofilms protect microbes from environmental stress and antibiotics, and are important in medical and industrial contexts.

Example: Dental plaque is a biofilm formed by oral bacteria.

Growth Curve Phases

The microbial growth curve describes population changes over time in a closed system (batch culture).

• Lag Phase: Cells adapt to new environment; little to no division.

• Log (Exponential) Phase: Rapid cell division and population growth.

• Stationary Phase: Growth rate slows; nutrient depletion and waste accumulation.

• Death Phase: Cells die at an exponential rate.

Example: In laboratory broth cultures, E. coli exhibits all four phases over 24–48 hours.

Methods for Estimating Microbial Growth

Several methods are used to quantify microbial populations.

• Direct Methods:

  • Plate counts (colony-forming units, CFUs)

  • Microscopic counts

• Indirect Methods:

  • Turbidity measurement (spectrophotometry)

  • Metabolic activity assays

Example: Plate counts are used to estimate viable bacteria in water samples.

Chapter 7: Molecular Information Flow and Protein Processing

DNA Replication

DNA replication is the process by which a cell duplicates its DNA before cell division.

• Location in Prokaryotes: Cytoplasm

• Location in Eukaryotes: Nucleus

• Key Steps:

  1. Initiation at the origin of replication

  2. Elongation by DNA polymerase

  3. Termination and separation of daughter DNA molecules

• Equation for DNA Synthesis:

  $\text{(dNMP)}_n + dNTP \rightarrow \text{(dNMP)}_{n+1} + PP_i$

Example: E. coli replicates its circular chromosome in about 40 minutes.

Prokaryotic vs. Eukaryotic DNA Structure and Gene Expression

There are fundamental differences in DNA organization and gene expression between prokaryotes and eukaryotes.

Feature	Prokaryotes	Eukaryotes
DNA Structure	Circular, single chromosome, no histones	Linear, multiple chromosomes, histones present
Location	Cytoplasm (nucleoid)	Nucleus
Gene Expression	Transcription and translation are coupled	Transcription in nucleus, translation in cytoplasm

Example: In bacteria, ribosomes begin translating mRNA before transcription is complete.

Protein Synthesis Steps

Protein synthesis involves two main processes: transcription and translation.

• Transcription: DNA is transcribed into messenger RNA (mRNA) by RNA polymerase.

• Translation: Ribosomes read mRNA and assemble amino acids into a polypeptide chain.

• Key Steps:

  1. Initiation

  2. Elongation

  3. Termination

Example: The lac operon in E. coli is transcribed and translated to produce enzymes for lactose metabolism.

Types of Genetic Transfers

Genetic transfer in microbes increases genetic diversity and can occur by several mechanisms.

• Transformation: Uptake of free DNA from the environment.

• Transduction: Transfer of DNA by bacteriophages (viruses that infect bacteria).

• Conjugation: Direct transfer of DNA between cells via a pilus.

Example: Antibiotic resistance genes can spread among bacteria by conjugation.

Additional info: Academic context and examples have been added to ensure completeness and clarity for exam preparation.