Comprehensive Study Notes: Microbial Growth, Metabolism, and Laboratory Techniques

Microbial Growth and Reproduction

Binary Fission, Budding, and Spore Formation

Microorganisms reproduce by several mechanisms, with binary fission being the most common in bacteria. In binary fission, a cell divides into two genetically identical daughter cells. Budding involves the formation of a new organism from a protrusion on the parent cell, common in yeast. Spore formation is a reproductive strategy in some bacteria and fungi, where spores are produced to withstand harsh conditions.

• Binary Fission: Symmetrical division; rapid population increase.

• Budding: Asymmetrical division; new cell grows from a specific site.

• Spore Formation: Produces resistant spores; survival under adverse conditions.

Generation Time

Generation time is the time required for a bacterial population to double. It varies among species and environmental conditions.

• Shorter generation time = faster growth.

• Calculated using growth curves in batch cultures.

Four Stages of Bacterial Growth in Batch Culture

Bacterial growth in a closed system follows four distinct phases:

• Lag Phase: Adaptation, no increase in cell number.

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

• Stationary Phase: Nutrient depletion, growth rate equals death rate.

• Death Phase: Decline in viable cells due to exhaustion of resources.

Optimal, Minimum, and Maximum Temperatures and pH

Microbes have specific ranges for growth:

• Minimum: Lowest temperature/pH for growth.

• Optimal: Temperature/pH at which growth is fastest.

• Maximum: Highest temperature/pH for growth.

Temperature Classifications of Microbes

Microbes are classified by their preferred temperature ranges:

• Psychrophiles: Grow best at 0–15°C.

• Psychrotrophs: Grow at 20–30°C, can tolerate cold.

• Mesophiles: Grow at 25–40°C; most pathogens.

• Thermophiles: Grow at 50–60°C.

• Hyperthermophiles: Grow above 80°C.

Most pathogens are mesophiles.

Microbial pH Preferences

Microbes are also classified by their pH tolerance:

• Acidophiles: Thrive in acidic environments (pH < 5.5).

• Alkaliphiles: Thrive in basic environments (pH > 8).

• Neutrophiles: Prefer neutral pH (6.5–7.5).

Osmotic Stress and Halophiles

Halophiles are microbes that thrive in high salt concentrations. They combat osmotic stress by accumulating compatible solutes.

Classification by Carbon and Energy Source

Microbes are classified by how they obtain carbon and energy:

• Autotrophs: Use CO2 as carbon source.

• Heterotrophs: Use organic compounds as carbon source.

• Phototrophs: Use light as energy source.

• Chemotrophs: Use chemicals as energy source.

Laboratory Techniques and Media

Types of Media

Media are used to grow and identify microbes:

• Complex Media: Contains nutrients from extracts; composition varies.

• Defined Media: Exact chemical composition known.

• Selectives Media: Inhibits some microbes, allows others to grow.

• Differential Media: Distinguishes microbes by biochemical reactions.

Streak Plate Technique

The streak plate technique is used to isolate pure colonies by spreading bacteria over an agar surface.

Culture of Anaerobic Microbes

Anaerobic microbes require environments without oxygen. Methods include:

• Use of anaerobic jars or chambers.

• Reducing agents in media (e.g., thioglycollate).

Cell Enumeration Methods

Counting microbial cells can be done by:

• Direct Methods: Microscopy, plate counts.

• Indirect Methods: Turbidity, metabolic activity.

Decontamination, Sterilization, Disinfection, Microbiostatic, Microbicidal, Antiseptic

Term	Definition
Decontamination	Removal of microbes to safe levels
Sterilization	Destruction of all microbial life
Disinfection	Destruction of most pathogens (not spores)
Microbiostatic	Inhibits microbial growth
Microbicidal	Kills microbes
Antiseptic	Used on living tissue to reduce microbes

Germicides and Their Classes

Germicides are chemicals that kill microbes. Classes include:

• High-level: Kill all microbes, including spores.

• Intermediate-level: Kill most pathogens, not spores.

• Low-level: Kill some microbes, not spores or mycobacteria.

Factors in Selecting Germicides

• Microbial target

• Surface compatibility

• Toxicity

• Cost

Microbial Metabolism

Metabolism, Anabolism, and Catabolism

Metabolism is the sum of all chemical reactions in a cell. Anabolism builds complex molecules; catabolism breaks them down.

• Anabolism: Requires energy; biosynthesis.

• Catabolism: Releases energy; breakdown of molecules.

Adenosine Triphosphate (ATP)

ATP is the cell's energy currency. It is ideal because it stores and releases energy efficiently and is regenerated by ADP/ATP cycling.

• Energy released by hydrolysis: $ATP ightarrow ADP + P_i$

Enzymes and Coenzymes

Enzymes are biological catalysts that speed up reactions. Coenzymes are organic molecules that assist enzymes.

• Lower activation energy for reactions.

• Specific for substrates.

• Examples: NAD+, FAD.

Enzyme Inhibition

Enzyme activity can be inhibited by:

• Competitive Inhibition: Inhibitor competes with substrate.

• Noncompetitive Inhibition: Inhibitor binds elsewhere, changing enzyme shape.

Oxidation-Reduction Reactions

These reactions transfer electrons between molecules, crucial for energy production.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

Phosphorylation Mechanisms

ATP is produced by:

• Substrate-level phosphorylation

• Oxidative phosphorylation

• Photophosphorylation

Cellular Respiration

Process by which cells harvest energy from organic molecules.

• Glycolysis: Glucose → pyruvate; produces ATP and NADH.

• Krebs Cycle: Pyruvate → CO2; produces ATP, NADH, FADH2.

• Electron Transport Chain: NADH/FADH2 → ATP via oxidative phosphorylation.

Fermentation

Fermentation is an anaerobic process that generates ATP by substrate-level phosphorylation.

• Lactic Acid Fermentation: Pyruvate → lactic acid.

• Alcoholic Fermentation: Pyruvate → ethanol and CO2.

Comparison of Aerobic and Anaerobic Respiration

Feature	Aerobic Respiration	Anaerobic Respiration
Final Electron Acceptor	O2	Non-oxygen (e.g., nitrate, sulfate)
ATP Yield	High	Lower
End Products	CO2, H2O	Varies (e.g., lactic acid, ethanol)

Biochemical Tests and Identification

Biochemical Tests

Biochemical tests identify microbes based on metabolic properties.

• Catalase Test: Detects enzyme catalase.

• Oxidase Test: Detects cytochrome oxidase.

• Fermentation Tests: Detects acid/gas production from sugars.

Other Identification Techniques

Additional tools include microscopy, molecular methods, and immunological assays.

Examples and Applications

• Mycobacteria: Acid-fast bacteria; cause tuberculosis.

• Endospores: Resistant structures formed by Bacillus and Clostridium.

• Protozoa: Unicellular eukaryotes; some are pathogens.

• Yeasts: Fungi; reproduce by budding.

Additional info:

• Some terms and details were inferred from context and standard microbiology curriculum.

• Tables were reconstructed to clarify comparisons and classifications.