Growing MicroOrganisms A

Introduction

• Exclusive copyright of Professor Omri.

• This presentation and accompanying PowerPoint slides are strictly for students enrolled in Introduction to Microbiology (BIOL-2026 E) in Fall term 2025 at Laurentian University.

• Unauthorized or commercial use, such as uploading to non-Laurentian servers, is strictly prohibited.

Course Details

• Course Code: BIOL-2026 E

• Course Name: Introduction to Microbiology

• Term: Fall 2025

• Chapter: 7

• Part: A

• Topic: Growing (and killing) microorganisms

• Date Range: 5-10 November

Learning Objectives

1. Understand how nutritional and environmental factors affect microbial growth.

2. Learn how to cultivate different types of microorganisms in the laboratory.

3. Gain skills in measuring and monitoring microbial populations.

4. Explore methods to eliminate microbes or inhibit their growth.

Key Topics to Explore

• Methods for isolating and cultivating microorganisms.

• Techniques for studying microorganisms that cannot be grown in lab settings.

• Strategies for preventing microbial growth and eliminating undesirable microbes.

• An understanding of factors influencing microbial growth.

Importance of Studying Microbial Growth

• Understanding microbial growth requirements enhances knowledge of microorganisms.

• Key examples:

  • Methanobrevibacter smithii: Crucial for the digestive system.

  • Biogeochemical cycling microbes: Facilitate carbon and nitrogen transfer.

  • Streptococcus pyogenes: Causes a multitude of human diseases.

  • Laboratory research on these microorganisms necessitates comprehension of their growth needs.

Challenges in Growing Microbes

• Cultivating microorganisms is often unexpectedly difficult.

• Microbes can be compared to animals in captivity; while some thrive in laboratory conditions, others struggle away from their natural habitats.

• Success in culture requires a deep understanding of both nutritional and environmental factors.

Microbial Diversity

• The biological diversity of microorganisms has evolved over 4 billion years, classified into three main categories:

  1. Phylogenetic Diversity: Evolutionary relationships.

  2. Physiological Diversity: Functional mechanisms.

  3. Metabolic Diversity: Methods of nutrient acquisition and processing.

• Microorganisms adapt to exploit available niches, limited only by chemical and physical constraints.

Basic Cell Metabolism

• All cells exhibit two principal metabolic processes:

  • Catabolism: Process of breaking down molecules to release energy.

  • Anabolism: Process of utilizing energy to build new molecules.

Microbial Nutritional Requirements

• Essential Nutrients:

  • Macronutrients (Required in large quantities, >0.1% of cell mass):

  • Carbon (C)

  • Nitrogen (N)

  • Phosphorus (P)

  • Sulfur (S)

  • Oxygen (O)

  • Micronutrients (Required in smaller amounts):

  • Iron (Fe)

  • Copper (Cu)

  • Sodium (Na)

  • Magnesium (Mg)

  • Manganese (Mn)

Fundamentals of Nutrition

• Energy Sources:

  1. Light: Utilized by photosynthetic organisms.

  2. Chemical Compounds:

  • Organic (e.g., glucose, acetate)

  • Inorganic (e.g., $H<em>2$, $NH</em>3$, $S^0$, $NO_2^-$)

• Carbon Sources:

  1. Organic Carbon (Heterotrophs):

  • Utilizes pre-existing carbon compounds.

  • Example: Most bacteria and fungi.

  1. Inorganic Carbon (Autotrophs):

  • Uses carbon dioxide ($CO_2$) exclusively.

  • Example: Plants and certain bacteria.

Microbes and Their Versatile Energy Harvesting

• Microbes show a remarkable adaptability in energy harvesting:

  • Phototrophs: Utilize light.

  • Chemoorganotrophs: Use organic chemicals.

  • Chemolithotrophs: Use inorganic chemicals.

• These classifications help predict microbial habitats, extending from human guts to ocean floors.

Nutritional Classifications Based on Energy and Carbon Sources

1. Photoautotrophs:

   • Energy: Light

   • Carbon: $CO_2$

   • Example: Cyanobacteria

2. Chemoorganoheterotrophs:

   • Energy: Chemical compounds

   • Carbon: Organic compounds

   • Example: $Escherichia coli$

3. Chemolithotrophs:

   • Energy: Inorganic compounds

   • Carbon: Either $CO_2$ or organic

   • Example: Sulfur-oxidizing bacteria.

Energy and Carbon Requirements for Microbes

• Energy sources:

  • Chemorganotrophs: Obtain energy from oxidation of organic compounds.

  • Chemolithotrophs: Obtain energy from inorganic compound oxidation, only present in prokaryotes.

  • Phototrophs: Capture light energy using pigments, can be oxygenic or anoxygenic.

• Carbon Sources:

  • Carbon is a major requirement of all organisms for energy storage and structural purposes:

  • Autotrophs: Primary producers, fix $C$ directly from $CO_2$.

  • Heterotrophs: Use organic molecules produced by autotrophs.

Nitrogen Acquisition

• Microbes must incorporate nitrogen into a usable form, which is crucial for synthesizing numerous macromolecules such as amino acids and nucleic acids.

Nutrient Concentration and Growth Rate

• Growth rates are influenced by nutrient availability within the environment.

• A nutrient present in the lowest concentration serves as a limiting factor for microbial growth.

Oxygen Requirements for Microbial Growth

1. Obligate Aerobes:

   • Require oxygen for growth and cannot survive without it.

2. Microaerophiles:

   • Need oxygen but at levels lower than those found in the atmosphere; high oxygen levels can be detrimental.

3. Facultative Anaerobes:

   • Capable of growing with or without oxygen but prefer oxygen when available.

4. Aerotolerant Anaerobes:

   • Do not utilize oxygen but can grow in its presence.

5. Obligate Anaerobes:

   • Cannot tolerate oxygen and perish in its presence.

Effects of Oxygen on Growth

• Aerobic Growth:

  • Obligatory aerobes require oxygen.

  • Microaerophiles grow optimally at low oxygen concentrations.

• Anaerobic Growth:

  • Aerotolerant anaerobes are unaffected by oxygen but do not use it.

  • Obligate anaerobes cannot grow when oxygen is present.

  • Facultative anaerobes can thrive without oxygen but perform better when oxygen is available.

Reactive Oxygen Species (ROS) and Tolerance

• Reactions to oxygen depend on the available defenses against Reactive Oxygen Species (ROS), which can damage key biomolecules.

pH Effects on Microbial Growth

• pH influences macromolecule structure and transmembrane electrochemical gradients.

• Each microorganism possesses an optimal pH range for growth and maintains a relatively neutral intracellular pH.

• Extremophiles can tolerate internal pH ranging from 4.6 to 9.5.

Water Availability and Osmotic Pressure Effects

• Varying solute concentrations can cause influx or efflux of water, leading to cellular stress, swelling, or shrinkage.

• Availability of water is essential for biochemical reactions and is measured in terms of water activity (aw):

  • Pure water aw = 1.0; seawater aw = 0.98.

  • Most bacteria prefer an aw > 0.9.

  • Food spoilage prevention strategies include dehydration or the addition of sugar/salt to decrease aw.

Temperature Requirements for Microbial Growth

• Microorganisms can be categorized based on temperature preferences into:

  1. Psychrophiles (Cold-loving):

  • Minimum: Below 0°C

  • Optimum: Below 15°C

  • Maximum: Around 20°C

  • Adaptations: Higher proportions of unsaturated fatty acids in membranes.

  1. Psychrotolerant (Cold-tolerant):

  • Can grow at 0°C; optimal growth at moderate temperatures (20-40°C); common in soil and temperate environments.

  1. Mesophiles (Moderate temperature):

  • Optimal growth at room/body temperature; common human pathogens are mesophiles.

Impact of Temperature on Microbial Growth

• Temperature influences macromolecular structure, membrane fluidity, and enzyme functionality.

• Each microorganism exhibits a specific temperature growth range.

Methods for Growing Microorganisms in the Laboratory

• Microorganisms can be cultivated based on:

  1. Physical State:

  • Solid media (with agar)

  • Liquid media (broths)

  1. Composition:

  • Complex media (undefined ingredients)

  • Defined media (known chemical composition)

  1. Purpose:

  • Selective media (support specific organism growth)

  • Differential media (help identify organisms)

  • Enriched media (support fastidious organisms)

Solid vs. Liquid Media

• Solid media:

  • Agar plates

• Liquid media:

  • Broths.

  • Agar preparation involves adding powdered agar to liquid media (1.5 g/100 mL), heating above 85°C to melt, cooling to 45°C, and pouring into plates to solidify.

Culture Media Details

• Agar:

  • Derived from algae; gels at specific temperatures, including 37°C.

  • Generally not degraded by most microbes; serves as a solidifying agent in microbiological media.

Colony Morphology

• Variations in colony form, texture, elevation, and color can be observed:

  • Compact circular colonies

  • Filamentous colonies.

Complex vs. Defined Media for Microbial Growth

Specialized Media

• Selective Media: Allows isolation of microbes with specific properties (e.g., salt tolerance).

• Differential Media: Distinguishes certain microbes based on visual responses (e.g., lactose fermentation of E. coli on MacConkey agar).

• Enriched Media: Increases specific microbial population from mixed cell types (e.g., Sabouraud dextrose agar for fungi).

Techniques for Obtaining Pure Cultures

1. Streak Plate Method:

   • Used to isolate pure cultures by separating mixed populations into individual colonies.

2. Spread Plate Method:

   • Used for counting and isolation; samples spread on agar surface.

3. Pour Plate Method:

   • Designed for very dilute samples; sample mixed with molten agar.

Benefits of Solid Medium

• Solid media preserves cells in fixed locations, facilitating isolation into individual populations.

• Allows separation of mixed cells into pure cultures.

Unculturable Bacteria

• Many microbes exist in complex communities (consortia) and depend on interrelated survival strategies involving chemical signaling and nutrient sharing.

• Stress, starvation, or death can result when attempting to grow isolated microbes.

  • Example: Some gut bacteria require short-chain fatty acids produced by other bacterial species.

Cultivation-Independent Methods for Studying Unculturable Bacteria

• DNA from unculturable bacteria can be amplified and sequenced, leading to comparison of genetic content across areas.

• Metagenomic techniques provide insight into microbial communities; however, confirmation with cultured organisms is essential for accurately formulating growth media.