Unit 4: Microbial Technology

Classification of Microorganisms

1. Fundamental Concepts of Microbial Taxonomy

Microbial taxonomy is the science of organizing microorganisms into groups based on shared structural, physiological, genetic, and evolutionary relationships. It encompasses three distinct areas:

• Classification: Arranging organisms into taxonomic groups (taxa) based on similarities.
• Nomenclature: Assigning scientific names to taxonomic groups according to international rules.
• Identification: Determining that a particular isolate belongs to a recognized taxon.

Microbial Taxonomy is the systematic framework used to categorize microorganisms into distinct taxa based on morphological, biochemical, physiological, and genetic traits.

2. Major Systems of Classification

Over time, systems of classification have evolved as analytical technology advanced from simple light microscopy to molecular sequencing.

Whittaker Five-Kingdom System

Proposed by Robert Whittaker in 1969, this system categorizes living organisms into five kingdoms based on cellular structure and mode of nutrition:

1. Monera: Prokaryotic, unicellular organisms (e.g., Bacteria, Cyanobacteria).
2. Protista: Eukaryotic, mostly unicellular organisms (e.g., Protozoa, Microalgae).
3. Fungi: Eukaryotic, heterotrophic, chitin-containing organisms (e.g., Yeasts, Molds).
4. Plantae: Eukaryotic, multicellular, photosynthetic organisms.
5. Animalia: Eukaryotic, multicellular, ingestive heterotrophs.

Woese Three-Domain System

Proposed by Carl Woese in 1990, this modern evolutionary system relies on 16S rRNA gene sequencing. It divides all cellular life into three primary domains:

1. Archaea: Prokaryotic organisms lacking peptidoglycan in cell walls, often inhabiting extreme environments (extremophiles).
2. Bacteria: True prokaryotic organisms possessing peptidoglycan in cell walls.
3. Eukarya: Organisms with membrane-bound organelles and a true nucleus (Fungi, Protists, Plants, Animals).

3. Criteria Used in Microbial Classification

Microorganisms are categorized using both classical and molecular characteristics:

Morphological Characteristics

Includes cellular shape (cocci, bacilli, spirilla), flagellar arrangement (monotrichous, lophotrichous, peritrichous), endospore formation, and Gram-staining reaction.

Physiological and Biochemical Characteristics

Includes oxygen requirement (aerobic, anaerobic, microaerophilic, facultative anaerobic), optimal temperature range (psychrophiles, mesophiles, thermophiles), carbon and energy sources (autotrophs, heterotrophs, phototrophs, chemotrophs), and enzyme activity (catalase, oxidase, urease tests).

Molecular and Genotypic Characteristics

Includes Guanine-Cytosine (GC) ratio percentage, DNA-DNA hybridization, and 16S rRNA gene sequence analysis (for prokaryotes) or 18S rRNA gene sequence analysis (for eukaryotes).

4. Comparative Overview of Major Microbial Groups

5. Exam Notes and Common Mistakes

Exam Note: Gram-positive bacteria have a thick peptidoglycan layer that retains the crystal violet-iodine complex, appearing purple. Gram-negative bacteria have a thin peptidoglycan layer and an outer lipid membrane, which decolorizes with alcohol and takes the counterstain (safranin), appearing pink/red.

Common Mistake: Confusing Archaea with Bacteria. Remember that Archaea lack peptidoglycan in their cell walls and have ether-linked cell membrane lipids, making them genetically and biochemically distinct from Bacteria.

Microbial Culture Techniques

1. Composition and Types of Culture Media

Microbial culture media supply the essential nutrients required for microbial growth, including carbon, nitrogen, phosphorus, sulfur, trace elements, growth factors, and water.

Culture Medium: A balanced solid, liquid, or semi-solid nutrient formulation designed to support the growth, multiplication, and maintenance of microorganisms.

Classification Based on Physical State

• Liquid Media (Broth): Contains no gelling agent. Used for growing high cell density cultures and fermentation studies.
• Solid Media: Contains 1.5% to 2.0% agar as a solidifying agent. Used for isolating pure cultures and studying colony morphology.
• Semi-solid Media: Contains 0.2% to 0.5% agar. Used for testing bacterial motility and anaerobic growth.

Classification Based on Chemical Composition

• Synthetic (Defined) Media: Exact chemical composition is known. Used for quantitative nutritional and physiological studies.
• Complex (Undefined) Media: Exact chemical composition is unknown; contains crude complex ingredients like yeast extract, peptone, or beef extract. Used for routine culturing.

Classification Based on Functional Application

• Selective Media: Suppresses unwanted microbes and encourages desired organisms (e.g., MacConkey Agar contains bile salts and crystal violet to select for Gram-negative bacteria).
• Differential Media: Distinguishes colonies of different microbes based on biochemical reactions (e.g., Blood Agar tests for hemolysis; MacConkey Agar differentiates lactose fermenters from non-fermenters).
• Enriched Media: Contains complex nutrients like blood, serum, or egg yolk to support fastidious organisms (e.g., Chocolate Agar).
• Enrichment Media: Liquid media designed to favor the growth of a specific organism present in low numbers before isolation.

2. Sterilization and Aseptic Techniques

Aseptic techniques prevent contamination during media preparation, inoculation, and maintenance.

Sterilization Methods

1. Moist Heat Sterilization (Autoclaving): Uses steam under pressure (121°C at 15 psi for 15-20 minutes). Destroys all vegetative cells and endospores by denaturing proteins.
2. Dry Heat Sterilization: Uses hot air ovens (160°C–170°C for 2 hours). Destroys microbes by oxidation. Suitable for glassware and dry powders.
3. Filtration: Membrane filters with 0.22 μm pore size trap bacteria. Used for heat-labile liquids such as serum, antibiotic solutions, and vitamin mixes.
4. Radiation Sterilization: Uses ultraviolet light (for surface decontamination) or gamma rays (for heat-sensitive plasticware and medical supplies).

3. Inoculation and Isolation Techniques

Isolation methods separate individual microbial cells on solid agar so that each grows into a discrete pure colony (a clone from a single cell).

Streak Plate Method

A loopful of culture is streaked across four quadrants of an agar plate, diluting the sample progressively to yield isolated single colonies in the final quadrants.

Spread Plate Method

A diluted liquid sample (typically 0.1 mL) is pipetted onto the surface of an agar plate and spread evenly using a sterile glass spreader.

Pour Plate Method

A diluted sample is mixed directly with molten agar cooled to 45°C, poured into a sterile Petri dish, and allowed to solidify. Colonies grow both on top and within the medium.

4. Microbial Culture Preservation Methods

Measurement and Kinetics of Microbial Growth

1. Concept of Microbial Growth

Microbial growth refers to an increase in cell numbers (population growth) rather than an increase in individual cell size. Most bacteria reproduce by binary fission, where a single parent cell divides into two identical daughter cells.

2. Microbial Growth Curve in a Batch System

When inoculated into a fixed volume of liquid medium (closed batch system), microorganisms exhibit a characteristic growth curve with four distinct phases:

1. Lag Phase: Period of adaptation. Cell division is minimal or zero, but metabolic activity is intense (synthesis of enzymes, RNA, and ATP).
2. Log (Exponential) Phase: Rapid cell division occurs at a maximal constant rate. Population doubles at regular intervals. Cells are most sensitive to antibiotics during this phase.
3. Stationary Phase: Growth rate equals death rate. Population reaches maximum density due to nutrient depletion and accumulation of toxic waste products.
4. Death (Decline) Phase: Death rate exceeds growth rate. Population declines exponentially as nutrients are fully exhausted and toxic metabolites reach lethal levels.

3. Mathematical Kinetics of Microbial Growth

During the exponential growth phase, cell multiplication follows first-order kinetics.

Exponential Growth Formula: N = N0 * 2^n Where: N = Final cell number N0 = Initial cell number n = Number of generations (doublings)

Taking logarithm (base 10) on both sides:

log N = log N0 + n * log 2

Rearranging for number of generations (n):

n = (log N - log N0) / log 2 = (log N - log N0) / 0.3010

Specific Growth Rate Formula

The specific growth rate (μ) expresses the rate of population increase per unit cell mass or cell number per hour:

μ = (ln N - ln N0) / (t - t0)

Generation Time (Doubling Time) Formula

Generation time (td) is the time required for a population to double in size:

td = ln 2 / μ = 0.693 / μ

4. Monod Model for Substrate-Limited Growth

The relationship between specific growth rate (μ) and essential limiting substrate concentration (S) is described by the Monod equation:

Monod Equation: μ = (μmax * S) / (Ks + S) Where: μ = Specific growth rate (hour⁻¹) μmax = Maximum specific growth rate S = Substrate concentration (g/L) Ks = Half-saturation constant (substrate concentration at which μ = 0.5 * μmax)

5. Microbial Fermentation Modes

• Batch Culture: Closed system where media is provided initially and no fresh nutrients are added. Cells progress through all four growth phases.
• Continuous Culture: Open system where fresh medium is continually fed while culture broth containing cells and products is continuously removed at the same rate. Uses a Chemostat (controls nutrient feed rate) or a Turbidostat (controls culture turbidity).
• Fed-Batch Culture: Semi-open system where nutrients are added incrementally during fermentation without removing the culture broth until the end of the process. Ideal for high cell density production and avoiding substrate inhibition.

6. Methods for Measuring Microbial Growth

Colony Forming Unit Calculation Formula: CFU/mL = (Number of Colonies * Dilution Factor) / Volume Plated in mL

Strain Isolation and Isolation of Microbial Products

1. Isolation of High-Yield Microorganisms from Nature

The search for wild-type strains capable of producing useful biological materials involves systematic sampling and screening from habitats such as soil, marine sediment, and extreme environments.

Primary Screening

Separates valuable producers from non-producers in a large pool of isolates.

• Crowded Plate Technique: Isolates antibiotic producers by looking for zones of inhibition around crowded colonies on an agar plate.
• Auxiliary Streak Method: Streaks test organisms at right angles to candidate producer strains to detect growth inhibition.
• Enzymatic Screening: Uses specific substrates in agar (e.g., starch for amylase, casein for protease) to detect clear halo zones caused by substrate degradation.

Secondary Screening

Evaluates selected primary strains quantitatively to determine yield, fermentation parameters, side products, optimal physiological conditions, and potential toxicity.

2. Strain Improvement Strategies

Wild-type isolates typically yield low product quantities. Yields are enhanced using three primary approaches:

1. Random Mutation and Selection: Exposure to physical mutagens (UV radiation, X-rays) or chemical mutagens (NTG, EMS) followed by selection of overproducing mutants.
2. Protoplast Fusion: Removal of cell walls followed by fusion of somatic protoplasts using polyethylene glycol (PEG) to combine desirable traits from different strains.
3. Recombinant DNA Technology: Introducing specific cloned genes, high-copy-number plasmids, or metabolic engineering pathways to direct overproduction of targeted products.

3. Isolation and Recovery of Microbial Products (Downstream Processing)

Downstream processing encompasses all steps involved in recovering, purifying, and formulating biologically active products from fermentation broths.

Step 1: Cell Separation

Separates solid biomass from the liquid supernatant.

• Centrifugation: Uses high gravitational force to settle microbial cells (e.g., continuous disk-stack centrifuges).
• Filtration: Employs rotary vacuum drum filters or microfiltration membranes.

Step 2: Cell Disruption (For Intracellular Products)

Breaks open microbial cell walls to release intracellular proteins and products.

• Mechanical Methods: High-pressure homogenization, high-speed bead milling, ultrasound sonication.
• Non-Mechanical Methods: Enzymatic lysis (lysozyme), chemical treatment (detergents, alkali), osmotic shock.

Step 3: Product Concentration

Reduces water volume to concentrate the target molecule.

• Precipitation: Using ammonium sulfate, organic solvents (ethanol, acetone), or polymers (PEG).
• Ultrafiltration: Membrane separation based on molecular weight cut-offs (MWCO).
• Evaporation: Vacuum evaporation for heat-stable products.

Step 4: Product Purification

Achieves high purity by eliminating contaminating compounds.

• Ion Exchange Chromatography: Separates molecules based on net electrical charge.
• Affinity Chromatography: Highly specific separation based on biological affinity (e.g., enzyme-substrate, antigen-antibody).
• Size Exclusion (Gel Filtration) Chromatography: Separates components based on molecular size.

Step 5: Finishing and Formulation

Converts purified product into a stable, dry, marketable form using crystallization followed by spray drying or lyophilization (freeze-drying).

Application of Microbial Culture

1. Industrial Fermentation Products

Microorganisms are cultured on a large scale to produce primary and secondary metabolites.

Primary Metabolites: Compounds produced during the active growth phase (Log phase) that are directly essential for growth (e.g., Ethanol, Lactic acid, Amino acids). Secondary Metabolites: Compounds produced near the end of growth (Stationary phase) that are not essential for cellular growth (e.g., Antibiotics, Pigments, Toxins).

2. Major Applications Across Industries

3. Environmental Applications

• Wastewater Treatment: Uses mixed microbial consortia in activated sludge and trickling filter systems to oxidize organic waste and reduce Biological Oxygen Demand (BOD).
• Bioremediation: Utilization of specific microorganisms to break down toxic environmental pollutants such as heavy metals, pesticide residues, and crude oil spills into non-toxic end products.

4. Important Observations and Summary

• Microbial culture techniques bridge laboratory bench discovery and commercial manufacturing.
• Choosing between batch, continuous, and fed-batch fermentation depends on whether the product is a primary or secondary metabolite.
• Downstream processing accounts for a major portion (often 50% to 80%) of the total production cost in industrial bioprocess operations.