Microbial Growth, Classification, and Metabolism: Study Guide

Microbial Growth and Classification

Oxygen Requirements and Bacterial Classification

Bacteria can be classified based on their oxygen requirements, which affects their growth and metabolism.

• Obligate Aerobes: Require oxygen for growth.

• Obligate Anaerobes: Cannot grow in the presence of oxygen.

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

• Microaerophiles: Require low levels of oxygen.

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

Example: A bacterium that grows slowly in the absence of oxygen and more quickly in its presence is likely a facultative anaerobe.

Genetic Engineering and Oxygen Classification

Genetically engineering an obligate anaerobe to express superoxide dismutase (an enzyme that detoxifies oxygen radicals) may allow it to survive in oxygenated environments. However, its classification as an obligate anaerobe would only change if it can now grow in the presence of oxygen.

Phases of Bacterial Growth

Bacterial growth in batch culture follows four main phases:

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

• Log (Exponential) Phase: Rapid cell division; population doubles at a constant rate.

• Stationary Phase: Growth rate slows as nutrients deplete and waste accumulates; cell division equals cell death.

• Death Phase: Cells die at an exponential rate.

Key Point: The log phase shows an increase in cell numbers, while the stationary phase shows no net increase.

Bacterial Generation Time and Cell Counting

Generation time is the time it takes for a bacterial population to double. It can be calculated using cell counts at different time points.

• Direct Cell Count: Using a hemocytometer to count cells directly.

• Viable Cell Count: Diluting and plating cells to count colony-forming units (CFU).

Example Calculation: If you start with 250 cells/mL and after 4 hours (with a 30-minute generation time) you expect 8 generations: $N = N_0 \times 2^n$ where $N_0$ is the initial number and $n$ is the number of generations.

Serial Dilution and Plate Counting

Serial dilution is used to estimate the number of viable bacteria in a sample.

• Each dilution reduces the concentration by a known factor (e.g., 1:10).

• Plates are inoculated with a measured volume from each dilution.

• CFU/mL is calculated as: $\text{CFU/mL} = \frac{\text{Number of colonies} \times \text{Dilution factor}}{\text{Volume plated (mL)}}$

Example: If plate I has 550 CFU, plate II has 70 CFU, and plate IV has 5 CFU, use the plate with 30-300 colonies for the most accurate count.

Disinfection, Antiseptics, and Sterilization

Antiseptics vs. Disinfectants

Antiseptics are chemicals used on living tissue to reduce the risk of infection, while disinfectants are used on inanimate objects to destroy microorganisms.

• Use Antiseptics: On skin or mucous membranes (e.g., before injections).

• Use Disinfectants: On surfaces, instruments, or equipment.

Methods of Microbial Control

• Mechanical: Filtration (removes microbes from liquids or air).

• Physical: Heat (autoclaving, pasteurization), radiation (UV, gamma rays).

• Chemical: Alcohols, phenolics, halogens, aldehydes.

Classification:

• Sterilant: Destroys all forms of microbial life, including spores (e.g., autoclave, ethylene oxide gas).

• Disinfectant: Destroys most microbes, but not necessarily spores (e.g., bleach).

• Antiseptic: Safe for use on living tissue (e.g., iodine, alcohol).

Sterilization vs. Disinfection

Sterilization is the complete destruction or removal of all forms of microbial life, including spores. Disinfection reduces the number of pathogenic organisms but may not eliminate spores.

Microbial Metabolism and Bioenergetics

Aerobic Respiration and ATP Yield

Aerobic respiration in chemotrophs generates up to 32 ATP per glucose molecule. This number is calculated by summing ATP from glycolysis, the Krebs cycle, and oxidative phosphorylation.

• Glycolysis: 2 ATP (net)

• Krebs Cycle: 2 ATP

• Oxidative Phosphorylation: ~28 ATP

Note: Bacteria may not always reach this maximum due to differences in electron transport chain efficiency and use of proton motive force for other processes.

Electron Transport Chain (ETC)

The ETC is a series of protein complexes that transfer electrons from donors (like NADH) to acceptors (like O2), generating a proton gradient used to produce ATP.

• Electrons move through complexes I-IV, pumping protons across the membrane.

• ATP synthase uses the proton gradient to synthesize ATP from ADP and Pi.

Types of Chemolithotrophs

Aerobic chemolithotrophs use oxygen as the terminal electron acceptor, while anaerobic chemolithotrophs use other molecules (e.g., nitrate, sulfate).

• Aerobic chemolithotrophs generally generate more ATP via oxidative phosphorylation due to the higher reduction potential of oxygen.

Phototrophs: Anoxygenic vs. Oxygenic

Anoxygenic phototrophs do not produce oxygen during photosynthesis (e.g., purple sulfur bacteria), while oxygenic phototrophs (e.g., cyanobacteria, plants) do.

Cyanobacteria and Photosynthesis

Cyanobacteria are important for their ability to fix both CO2 and N2. They contain both photosystem I and II and perform oxygenic photosynthesis.

Calvin Cycle and Photosystems

Organisms with both photosystem I and II and the Calvin cycle are classified as oxygenic phototrophs (e.g., cyanobacteria, plants).

ED Pathway vs. EM Pathway

The Entner-Doudoroff (ED) pathway and Embden-Meyerhof (EM) pathway are two glycolytic pathways. The EM pathway (classic glycolysis) generally produces ATP at a faster rate than the ED pathway.

Nitrogen Cycle

Stages and Microbial Involvement

The nitrogen cycle involves several key stages, each mediated by specific groups of microbes:

Stage	Microbes Involved	Description
Nitrogen Fixation	Rhizobium, Azotobacter, cyanobacteria	Conversion of N2 to NH3
Nitrification	Nitrosomonas, Nitrobacter	NH3 to NO2- to NO3-
Assimilation	Plants, microbes	Uptake of NO3- or NH3 into organic molecules
Ammonification	Decomposers	Organic N to NH3
Denitrification	Pseudomonas, Paracoccus	NO3- to N2

Diagram: The nitrogen cycle is a continuous process involving these stages and microbes.

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard microbiology curricula.