Microbiology Fundamentals: Growth, Metabolism, and Control

Posted on Jul 3, 2025 in Biology

Microbiology Essentials: Core Concepts

Key Microbial Definitions

Binary Fission
A process where a cell elongates, chromosomes replicate, nuclear material divides, the membrane invaginates to the center, and the wall thickens and grows inward, resulting in two daughter cells.
CHONPS
Essential elements for life:
  • Carbon (synthesis of organic molecules)
  • Hydrogen (source of electrons)
  • Oxygen (electron acceptor, component of organic molecules)
  • Nitrogen (amino acids)
  • Phosphorus (phospholipids, nucleic acids)
  • Sulfur (some amino acids)
Catalase
An enzyme that catalyzes the breakdown of H₂O₂ into O₂ and H₂O.
Hydrogen Peroxide
A compound where the peroxide ion is O₂²⁻.
Peroxidase
An enzyme that catalyzes the breakdown of H₂O₂.
Superoxide Radical
O₂⁻, an anion with one unpaired electron.
Superoxide Dismutase
An enzyme that converts superoxide to O₂ and H₂O₂.

Microbial Growth and Environmental Influences

Measuring Microbial Growth

Methods of measuring microbial growth include:

  • Direct Methods: Microorganisms are seen and counted. Examples include microscopic counts, plate counts (pour plate, spread plate), filtration, and most probable number (MPN).
  • Indirect Methods: Growth is inferred. Examples include turbidity measurements, metabolic activity assessments, and dry weight determination.

Freezing preserves foods because it significantly slows the growth of bacteria. Mesophilic bacteria remain dormant, thus preventing rapid food spoilage.

The total number of cells can be calculated using the formula: Number of initial cells × 2n generations = Total number of cells.

Nitrogen and Phosphorus encourage microbial growth after an oil spill. These elements are essential for synthesizing proteins, phospholipids, nucleic acids, and ATP.

Microbial Temperature Preferences

  • Psychrophile: An organism that thrives in cold temperatures, often an obligate halophile and obligate aerobe, characterized by cold, salty, and aerobic conditions.
  • Thermophiles: Organisms that thrive in hot temperatures (e.g., 45°C), but not at room temperature.
  • A mesophile would typically be killed at temperatures around 60°C.
  • When incubated at 0°C, a psychrotroph would show slow and steady growth, represented by a line that increases over time.

Oxygen Requirements for Microbes

  • Facultatively Anaerobic: Can grow with or without oxygen. (Would show similar colony numbers in both aerobic and anaerobic conditions).
  • An organism with peroxidase and superoxide dismutase but lacking catalase is most likely an aerotolerant anaerobe.
  • Regarding Clostridium and Streptococcus: Both are catalase-negative. Clostridium is killed by oxygen because it lacks the necessary enzymes to detoxify reactive oxygen species, while Streptococcus possesses enzymes that allow it to tolerate oxygen.

Iron deficiency is not a characteristic of biofilms.

Trace elements are small mineral requirements for microbial growth.

Microbial Culture Media Types

  • Chemically Defined Medium: The exact chemical composition is known.
  • Complex Medium: The exact chemical composition is not known (e.g., contains yeast/protein extracts or unknown placeholders).
  • A medium showing color based on species and inhibiting the growth of other species is both selective and differential.
  • Minimal Media: Contains only the essential nutrients for bacteria, used to define species as heterotrophs, and provides core sources of carbon and nitrogen.
  • Reducing media are not used to culture aerobes.
  • Media with a pH indicator and high salt concentration can likely inhibit the growth of most bacteria.

Yeast Growth Experiment Analysis

Consider two flasks with yeast cells:

  • Flask A: Yeast cells in glucose-minimal salt broth with aeration.
  • Flask B: Yeast cells in an anaerobic jar.

Observations:

  • Flask A makes more ATP.
  • Flask B produces more alcohol.
  • Flask B has a shorter generation time.
  • Flask A produces more cell mass.
  • Flask A has higher absorbance (indicating more cells).

Bacterial Enumeration and Calculations

  • Number of bacteria on soap: 1.68 × 10⁸.
  • Bacteria in saliva sample: Not all bacteria will grow because microbes grown on nutrient agar are non-fastidious (can grow without special conditions). Fastidious bacteria require very particular food sources.
  • If 100 colonies are on a 1:1000 dilution plate, the number of bacteria per mL in the original sample is calculated as: 100 colonies / (10⁻³ dilution factor × 1 mL plated) = 100,000 bacteria/mL.
  • A 1 mL ocean water sample is diluted 1:10 in a series. 0.1 mL of the second dilution tube (which is a 1:100 dilution) is plated. After incubation, the plate has 191 colonies, indicating that the original sample contained 191,000 bacteria/mL. (Calculation: 191 colonies / (0.1 mL × 10⁻² dilution) = 191,000 bacteria/mL).

Microbial Control Methods and Agents

  • Pasteurization: Reduces microbes without significantly altering taste.
  • Resistance to Chemical Biocides: Gram-positive bacteria, fungi, endospores, and prions show varying levels of resistance.
  • Limitation of Autoclave: Cannot be used with heat-labile materials.
  • Disinfectants:
    • Which disinfectant disrupts the plasma membrane? Bisphenols.
    • Halogens do not act by decreasing the surface tension of microbes.
    • Glutaraldehyde is a highly effective control agent due to its broad-spectrum activity and safety profile.
    • Mismatched pair: Chlorhexidine is not typically used for disinfection of stethoscopes (often alcohol or quaternary ammonium compounds are used).
  • Antimicrobial Agents: Heat, radiation, and some chemicals result in lethal damage to nucleic acids.
  • Microbial controls usually do not target the mitochondria.
  • Cleaning an arm with Betadine solution is called antisepsis.
  • An agent reducing the number of bacteria on a toilet seat is a disinfectant.
  • Mismatched pair: UV radiation is not primarily associated with desiccation (drying); it causes DNA damage.
  • Physical Methods: Boiling or autoclaving denature proteins as their mechanism of action.

Potential Exam Questions

  • Given a “recipe” for a medium, identify if it’s a complex or defined medium. (Complex media have ingredients like “yeast/protein extracts” or unknown placeholders. Chemically defined media will have exact chemical compositions).
  • Catalase catalyzes H₂O₂ + 2H⁺ → 2H₂O.
  • Stationary Phase: Cells grow and die in similar numbers, resulting in a plateau in population size.
  • Direct measurement methods include: plate count (pour plate, spread plate), filtration, most probable number (MPN), and direct microscopic count. Indirect forms include: measuring turbidity, metabolic activity, and dry weight.