Biotechnology: Class 12 — Chapter 10: Bioprocessing and Biomanufacturing

• Primary Metabolites: Compounds directly produced during primary metabolic pathways essential for basic cellular functions, growth, and development (e.g., ethanol, amino acids).
• Secondary Metabolites: Intermediates or indirect products synthesized via specialized secondary metabolic pathways. They are not essential for basic survival but serve ecological functions such as defense against pathogens/herbivores, abiotic stress tolerance, or acting as pollinator attractants (e.g., antibiotics, pigments, alkaloids).

In 1928, Alexander Fleming at St. Mary's Hospital inadvertently discovered penicillin when he observed that a contaminating green mold on a Petri dish of Staphylococcus aureus inhibited bacterial growth.

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Development & Scale-up: The active antibacterial agent was identified as a metabolite secreted by the mold Penicillium notatum. Its therapeutic potential as an antibiotic was established by Ernest Chain and Howard Florey, enabling mass production to treat wounded soldiers in World War II. Fleming, Chain, and Florey were jointly awarded the Nobel Prize in 1945.

1. Maintenance of a strictly sterile environment to prevent foreign microbial contamination.
2. Adequate and continuous supply of sterile air/oxygen for cellular respiration.
3. Uniform mixing of nutrients, cells, and dissolved gases without inducing lethal shear stress.
4. Precise control and maintenance of optimum temperature.
5. Real-time monitoring and adjustment of environmental parameters such as pH and dissolved oxygen (DO).

• Agitator Shaft & Impeller: Driven by a motor to mix the vessel contents, ensuring a homogeneous suspension of cells and nutrients.
• Sparger: A system (often a ring with micro-holes) located at the bottom of the vessel to introduce pressurized, sterile air or oxygen bubbles into the liquid medium.
• Baffles: Metal strips attached to the inner wall of the vessel to break vortex formation, improving mixing efficiency and reducing power consumption.
• Jacket: An outer annular space surrounding the vessel through which cold or hot water is circulated to regulate internal temperature.
• Sensitivity Probes: Sensors inserted directly into the broth to continuously measure temperature and pH.
• Digital Controller: An automated system that processes probe signals and dynamically adjusts parameters (e.g., triggering pumps to add $1\text{ M } \text{NaOH}$ or $1\text{ N } \text{HCl}$ to maintain pH, or adjusting water bath circulation).

• Stirred Tank Bioreactor: The most conventional design; utilizes a mechanical agitator shaft and impellers to facilitate mixing and gas dispersion.
• Air-lift Bioreactor: Lacks mechanical moving parts; utilizes a draft tube to create air currents that circulate the fluid broth up and down in an inside-out loop.
• Bubble Column Bioreactor: Mixing is achieved solely by the upward movement of air bubbles introduced through a high-pressure sparger jet, providing a low-shear environment ideal for shear-sensitive cells.

Upstream processing encompasses all steps prior to the harvest of the bioreactor, focusing on media formulation, sterilization, and inoculation.

• Nutrient Media Formulation: Formulated based on stoichiometric requirements for cell growth and product synthesis. The general growth equation is: $$\text{Carbon and energy source} + \text{Nitrogen source} + \text{other requirements} \rightarrow \text{Cell biomass} + \text{products} + \text{CO}_2 + \text{H}_2\text{O} + \text{heat}$$
• Cellular Yield Coefficient ($Y$): Quantifies the efficiency of substrate conversion into biomass: $$Y = \frac{\text{Quantity of cell dry weight produced by the culture}}{\text{Quantity of carbon substrate utilised}}$$
• Inoculum Development Criteria: The starter culture must be:
  1. Healthy and in the active log/exponential growth phase.
  2. Capable of minimizing the lag phase upon transfer.
  3. Available in sufficient volume (typically 5-10% of the production volume).
  4. In a suitable morphological state and completely free of contamination.
  5. Stable in retaining its high-yielding product-forming genetic traits.
• Sterilization Protocols: Essential to eliminate contaminating microbes. Includes thermal sterilization of nutrient media, in-situ steam sterilization of the bioreactor vessel, and microfiltration of incoming air.

• Batch Mode: A closed culture system where a fixed initial charge of nutrient medium is inoculated. Growth proceeds through distinct phases until nutrients are exhausted. Substrate concentration $[S]$ decreases, while biomass $[X]$ increases until stationary phase.
• Fed-batch Mode: An semi-open system where growth-limiting substrates are fed intermittently or continuously to the bioreactor without removing any liquid culture. This maintains low residual substrate levels, avoiding substrate toxicity and extending the production phase.
• Continuous Mode: An open system where fresh sterile medium is continuously pumped into the bioreactor at the same rate that spent culture broth containing cells and products is withdrawn, establishing a steady-state dynamic equilibrium.

Downstream processing involves the physical and chemical unit operations required to isolate, concentrate, and purify the target product from the complex fermentation broth.

1. Solid-Liquid Separation

• Filtration: Used for separating large particles and biomass. Conventional filtration handles particles with pore diameter $d_p > 10\text{ }\mu\text{m}$ using canvas or glass fiber. Rotary vacuum filters are widely used in industry.
• Centrifugation: Separates cells and particles in the size range of $0.1\text{ }\mu\text{m}$ to $100\text{ }\mu\text{m}$ from the liquid phase using centrifugal force.

2. Cell Disruption (For Intracellular Products)

• Physical Methods: Mechanical milling in high-speed bead mills, high-pressure homogenization, or ultrasonic vibration (ultrasonication).
• Non-Mechanical Methods: Chemical lysis using surfactants, alkalis, organic solvents, osmotic shock, or enzymatic degradation of the cell wall (e.g., lysozyme).

3. Product Recovery & Extraction

• Liquid-Liquid Extraction: Separation of a target solute from a liquid mixture by contacting it with an immiscible solvent in which the product is preferentially soluble. The solute-rich phase is the *extract*, and the residual depleted liquid is the *raffinate*.

4. Purification Techniques

• Precipitation: Inducing target proteins or antibiotics to precipitate out of solution by adding neutral salts (e.g., ammonium sulfate), organic solvents, or polymers.
• Membrane Separation: Pressure-driven processes categorized by pore size cut-offs:

• Dialysis: A membrane separation process used to remove low-molecular-weight solutes ($10\text{--}500\text{ Da}$) like organic acids and inorganic salts from a solution across a selectively permeable membrane driven by concentration gradients.