Industrial microbiology

Microbiology (B.Sc) · Applied microbiology · free study notes for exams, placements and interviews

In short

Industrial microbiology is the use of microorganisms - bacteria, actinomycetes, yeasts and moulds - and their enzymes to make commercially valuable products such as antibiotics, enzymes, organic acids, amino acids, vitamins, vaccines, probiotics and biofuels at tonne scale. It covers the whole chain: isolating and improving the producing strain, designing cheap media, growing it under controlled conditions, then recovering and purifying the product to pharmacopoeial quality. It carries heavy weight in B.Sc/M.Sc Applied Microbiology papers, CSIR-UGC NET and GAT-B/BET entrance tests, and is the core technical round for jobs at Biocon, Serum Institute of India, Bharat Biotech, Aurobindo, Advanced Enzymes, Novozymes South Asia, Praj Industries, Hester Biosciences and Amul/NDDB culture units. India is the world's largest vaccine producer by doses and a major supplier of fermentation-derived statins, penicillins and enzymes, so the employable skills are strain handling, aseptic technique, assay and documentation.

Explained

Scope and industrially important microorganisms

Definition: the applied science of exploiting microbial metabolism on an industrial scale; the product may be the cell itself (baker's yeast, SCP, biofertiliser, vaccine), a metabolite (ethanol, citric acid, penicillin), an enzyme, or a transformation of a substrate (steroid hydroxylation, vinegar).
Qualities of an ideal industrial strain: genetically stable; high and reproducible yield; grows fast on a cheap, non-sterile-tolerant medium; non-pathogenic and non-toxigenic; produces a single major product with few by-products; easy to separate from broth; amenable to genetic manipulation; not readily attacked by phage.
Bacteria: *Bacillus subtilis/licheniformis/amyloliquefaciens* (α-amylase, protease, Bacitracin), *Corynebacterium glutamicum* (glutamate, lysine), *Lactobacillus/Streptococcus thermophilus* (lactic acid, dairy starters), *Clostridium acetobutylicum* (acetone-butanol), *Acetobacter aceti* (vinegar), *Xanthomonas campestris* (xanthan gum), *E. coli* K-12 hosts (recombinant insulin, hGH).
Actinomycetes (the antibiotic factory): *Streptomyces griseus* (streptomycin), *S. venezuelae* (chloramphenicol), *S. aureofaciens* (tetracyclines), *S. erythreus/Saccharopolyspora erythraea* (erythromycin), *Amycolatopsis orientalis* (vancomycin).
Fungi: *Penicillium chrysogenum* (penicillin), *Aspergillus niger* (citric acid, gluconic acid, amylase, pectinase), *A. oryzae* (koji enzymes), *Aspergillus terreus* (lovastatin), *Trichoderma reesei* (cellulase), *Rhizopus* (fumaric acid, steroid transformation), *Blakeslea trispora* (β-carotene).
Yeasts and algae: *Saccharomyces cerevisiae* (ethanol, baker's yeast, heterologous proteins), *Pichia pastoris / Komagataella* (recombinant proteins), *Yarrowia lipolytica* (lipase, SCO), *Spirulina (Arthrospira)* and *Chlorella* (food supplements, large Indian production in Tamil Nadu).
Culture collections: MTCC (IMTECH Chandigarh), NCIM (NCL Pune), NCDC (NDRI Karnal), ATCC/NCIB abroad; patent deposits under the Budapest Treaty at MTCC/NAIMCC - strains are deposited, not described, when patenting.

Screening, isolation and strain improvement

Primary screening = detect the activity qualitatively from many isolates. Methods: crowded plate technique (inhibition zones around a colony), giant colony/agar plug and streak technique, auxanography, indicator dyes (CaCO₃ clearing for acid producers, starch-iodine for amylase, skim-milk clearing for protease, tributyrin for lipase).
Secondary screening: shake-flask fermentation of selected isolates to measure actual titre, spectrum, stability and by-products; then bench-scale fermenter (5-10 L) confirmation. Only a few isolates out of thousands survive.
Enrichment culture: selective pressure (sole carbon source, temperature, pH, antibiotic) applied to soil/water samples before plating.
Strain improvement routes:
1. Selection of natural variants - repeated single-colony isolation of the best performers.
2. Mutation (mutagenesis) and screening - UV, X-rays, nitrosoguanidine (NTG), EMS, nitrous acid; survival kept around 0.1-10% so mutants are enriched. The classic Wisconsin series of *P. chrysogenum* (NRRL 1951 → Wis Q-176 → modern strains) raised penicillin titres from ~ mg/L to ~50 g/L.
3. Rational selection - auxotrophs and analogue-resistant (regulatory) mutants to break feedback control; e.g. a homoserine auxotroph of *C. glutamicum* overproduces lysine, AEC (S-2-aminoethyl-cysteine)-resistant mutants are feedback-insensitive.
4. Recombination - protoplast fusion, transduction/conjugation in *Streptomyces*, hybridisation in yeast.
5. Recombinant DNA and metabolic engineering - gene dosage, stronger promoters, blocking competing pathways, exporting product; used for insulin, hepatitis B surface antigen, rituximab biosimilars, phytase.
Preservation of improved strains: lyophilisation, liquid N₂ (−196 °C) with glycerol/DMSO, soil/silica stocks, mineral-oil overlay, repeated subculture only as a last resort (risk of strain degeneration).

Raw materials, media and inoculum development

Industrial media are formulated on cost per kg of product, not per litre of medium - the carbon source is usually the largest single consumable cost.
Carbon sources: cane molasses (Indian mainstay, 45-50% total sugars), corn/maize starch and hydrolysates, sucrose, sulphite liquor, whey lactose, vegetable oils, hydrocarbons/alkanes (SCP); for lignocellulosic ethanol - rice straw and bagasse.
Nitrogen sources: corn steep liquor (also supplies growth factors and phenylacetic acid precursors), soybean/groundnut meal, yeast extract, urea, ammonium sulphate/phosphate, ammonia (also used for pH control).
Others: phosphate (limited deliberately in secondary-metabolite fermentations), Mg, S, trace metals; precursors (phenylacetic acid for penicillin G, cobalt for vitamin B₁₂, chloride for chlortetracycline); inducers (starch for amylase, lactose/IPTG for recombinant proteins); inhibitors (methylene blue/ferrocyanide to trap Fe and Mn in citric acid); antifoam (silicone oils, vegetable oils).
Media types: synthetic/defined (reproducible, costly, used for assays and recombinant proteins) versus complex/crude (cheap, variable batch-to-batch, harder downstream).
Inoculum development (train): working stock (lyophilised/cryovial) → slant/agar plate → shake flask (seed) → seed fermenter → production fermenter; each stage typically 5-10% v/v of the next, transferred in late exponential phase to avoid a long lag.
Why inoculum quality matters: adequate concentration, active morphology (pellet vs filamentous form in *A. niger*, spore count for actinomycetes), freedom from contamination and phage; a bad inoculum cannot be corrected later in the batch.

Product classes: primary and secondary metabolites

Primary metabolites - formed during growth (trophophase), essential to the cell: ethanol, lactic/acetic/citric acid, amino acids, nucleotides, vitamins. Production is growth-associated; yields are limited by the stoichiometry of the pathway.
Secondary metabolites - formed mainly after growth slows (idiophase), not needed for growth: antibiotics, alkaloids, gibberellins, statins, pigments. Strongly regulated by catabolite repression (glucose) and phosphate; hence slow-feeding of glucose and low phosphate.
Antibiotics: β-lactams (penicillin G/V; 6-APA by penicillin acylase → semisynthetic amoxicillin, ampicillin; cephalosporin C → 7-ACA), aminoglycosides (streptomycin, gentamicin), tetracyclines, macrolides (erythromycin), glycopeptides (vancomycin), polyenes (amphotericin B, nystatin).
Enzymes: amylases and glucoamylase (starch, baking), proteases (detergents, leather, dairy), lipases, cellulases and xylanases (textile, paper, feed, ethanol), pectinases (fruit juice clarification), glucose isomerase (HFCS), phytase (poultry feed), penicillin acylase, lactase, asparaginase (ALL therapy).
Organic acids: citric (*A. niger*, surface/submerged/solid-state), lactic (*Lactobacillus*, feedstock for PLA), gluconic, itaconic, acetic (quick vinegar/Frings generator).
Amino acids and nucleotides: L-glutamate (MSG, biotin-limited *C. glutamicum*), lysine, threonine, tryptophan; IMP/GMP flavour enhancers.
Vitamins and others: B₁₂ (*Pseudomonas denitrificans*, *Propionibacterium*), riboflavin (*Ashbya gossypii*), β-carotene, ergosterol; polysaccharides xanthan, dextran, pullulan; biopolymers PHB/PHA; biosurfactants rhamnolipid, sophorolipid.
Whole-cell products: baker's and feed yeast, SCP (*Candida*, *Methylophilus*, Spirulina), probiotics (*Lactobacillus rhamnosus*, *Bacillus clausii* - the Indian OTC market), biofertilisers (*Rhizobium*, *Azotobacter*, *Azospirillum*, PSB, mycorrhiza), biopesticides (*Bacillus thuringiensis*, *Trichoderma*, *Beauveria*), vaccines and recombinant therapeutics.
Microbial transformations (biotransformation): single-step, highly stereospecific conversions - 11-α hydroxylation of progesterone by *Rhizopus*, sorbitol → sorbose by *Gluconobacter* in vitamin C manufacture, steroid and statin side-chain modification.

Product recovery and downstream processing

General sequence: broth conditioning → solid-liquid separation → cell disruption (only for intracellular products) → concentration/primary isolation → purification → polishing and formulation.
Separation: rotary vacuum/filter press (filamentous moulds, with filter aid), continuous disc-stack or decanter centrifuges (bacteria, yeast), membrane microfiltration/ultrafiltration (TFF), flotation for yeast.
Cell disruption: high-pressure homogeniser and bead mill (mechanical, large scale); sonication, osmotic shock, enzymatic lysis (lysozyme), alkali/detergent (non-mechanical, lab scale).
Primary isolation: solvent extraction (penicillin into amyl acetate/butyl acetate at low pH, back-extracted into buffer), precipitation (ammonium sulphate salting-out, isoelectric precipitation, calcium citrate for citric acid), adsorption on activated carbon or resin, ion exchange (streptomycin, amino acids).
Purification: chromatography - ion exchange, gel filtration, hydrophobic, and affinity (Protein A for monoclonal antibodies); crystallisation (penicillin K/Na salts, amino acids).
Finishing: sterile filtration (0.22 µm), lyophilisation or spray-drying, aseptic filling; for parenterals, endotoxin and sterility release testing.
Rules of thumb: downstream processing is typically 50-70% of total manufacturing cost for high-purity biologicals; each step costs yield, so overall yield = product of step yields; the first step should give the biggest volume reduction.
Effluent and by-products: spent broth and mycelium have high BOD/COD; mycelial cake is sold as feed/manure, spent wash in distilleries is concentrated and incinerated for zero liquid discharge as required by CPCB norms for molasses distilleries.

Quality, regulation and biosafety in India

Drugs: manufacture of antibiotics, vaccines and biologicals is licensed under the Drugs and Cosmetics Act, 1940 and Rules, 1945; GMP requirements are in Schedule M, substantially revised by notification in December 2023 (risk-based quality management, product quality review, computerised-system validation), with phased compliance timelines for small manufacturers that have been extended more than once - verify the current deadline notification before quoting it. Premises and facilities for biologicals fall under Schedule M Part I plus specific parts; CDSCO and the state drug authorities inspect, and the Indian Pharmacopoeia (IPC, Ghaziabad) sets monographs and assay methods.
Clinical/market approval of biologicals follows the New Drugs and Clinical Trials Rules, 2019 and the CDSCO-DBT Guidelines on Similar Biologics (2016).
Biosafety for GM microorganisms: Rules, 1989 (Manufacture, Use, Import, Export and Storage of Hazardous Micro-organisms/GMOs) under the Environment (Protection) Act, 1986; the approval chain is IBSC (institute) → RCGM (DBT) → GEAC (MoEFCC), with SBCC/DLC for state-level monitoring; recombinant pharma products follow the DBT-CDSCO recombinant guidelines. Risk Groups 1-4 and containment levels BSL-1 to BSL-4 govern laboratory practice.
Other applicable law and standards: the four Labour Codes in force from 21 November 2025 (which replaced the Factories Act, Minimum Wages Act, Industrial Disputes Act and BOCW Act) now govern plant safety, hazardous-process duties, working hours and welfare; Water Act 1974 and Air Act 1981 with CPCB/SPCB consents for effluent and emissions; Food Safety and Standards Act 2006 (FSSAI) for enzymes, cultures and nutraceuticals; Insecticides Act 1968 with CIB&RC registration for biopesticides and the FCO 1985 schedule for biofertiliser specifications; BIS standards exist for biofertilisers, industrial alcohol and microbiological test methods - quote an IS number only after checking its current edition, since many are revised or withdrawn.
In-plant quality control: sterility and bioburden checks, environmental monitoring (settle plates, air samplers), media fill validation, potency/bioassay (cup-plate zone of inhibition against a standard), HPLC assay, LAL endotoxin test, batch manufacturing records and deviation handling. GST on most pharmaceutical and enzyme products falls in the 5% or 18% slab under the 5/18/40 structure effective 22 September 2025 - the exact rate is product-specific.

Economics and scale-up considerations

Cost drivers: raw material (carbon/nitrogen), utilities (sterilisation steam, chilled water, and aeration/agitation power - the dominant utility in aerobic fermentation), downstream chemicals and solvents, labour, QC/validation, effluent treatment, plant depreciation.
Scale-up basis: geometric similarity is kept where possible and one parameter is held constant - usually P/V (power per unit volume), k_La, or impeller tip speed; oxygen transfer, not nutrient, is the usual limit in large aerobic tanks.
Batch vs fed-batch vs continuous: batch is simple and the regulatory norm for pharma; fed-batch (slow feeding of glucose) avoids catabolite repression and the Crabtree effect and is standard for penicillin, baker's yeast and recombinant proteins; continuous culture gives high productivity (SCP, effluent treatment) but risks contamination and strain drift, so is rare in pharma.
Productivity levers: titre (g/L), batch time including turnaround, and downstream recovery - the "titre × rate × yield" triad.

Key points

Industrial microbiology - large-scale use of microbes/their enzymes to make saleable products; spans strain, medium, process and recovery.
Ideal industrial strain - genetically stable, high-yielding, fast-growing on cheap medium, non-pathogenic, easily separated.
Primary screening - qualitative detection of activity (crowded plate, agar plug, indicator dyes); secondary screening quantifies titre in shake flasks.
Mutation and screening - mutagen dose set for ~0.1-10% survival; most mutants are worse, so throughput of screening decides success.
Auxotrophic and analogue-resistant mutants - break feedback inhibition and repression; basis of lysine and glutamate overproduction.
Primary metabolite - growth-associated, essential (ethanol, citric acid, amino acids); secondary metabolite - idiophase, non-essential (antibiotics, statins).
Catabolite repression - excess glucose shuts off secondary metabolism; countered by fed-batch feeding or slowly used carbon sources (lactose, molasses).
Precursor - directs product structure (phenylacetic acid → penicillin G); inducer - switches on enzyme synthesis; inhibitor - diverts flux (ferrocyanide in citric acid).
Corn steep liquor and molasses - cheap complex nitrogen/carbon sources that also supply growth factors; variable in composition.
Inoculum train - stock → slant → shake flask → seed vessel → production vessel, each ~5-10% v/v, transferred in late log phase.
Yield coefficient Y_x/s, Y_p/s - grams of biomass or product per gram of substrate consumed; compared against theoretical pathway yield.
Volumetric productivity - g/L/h; the industrial figure of merit, since it includes batch time and turnaround.
Downstream processing - separation → disruption → isolation → purification → formulation; often 50-70% of manufacturing cost.
Overall recovery - product of individual step yields; shortening the train is the cheapest way to raise it.
Scale-up - usually at constant P/V or k_La; oxygen transfer limits large aerobic tanks.
Regulation - Drugs and Cosmetics Act 1940 with revised Schedule M GMP (2023 notification, phased), CDSCO licensing, IP monographs.
GM microorganisms - Rules, 1989 under the Environment (Protection) Act, 1986; IBSC → RCGM → GEAC approval chain.
Strain preservation - lyophilisation and liquid nitrogen are the methods of choice against degeneration (silica/soil stocks and mineral-oil overlay are lesser alternatives); repeated subculture is not.

Formulas and examples

Specific growth rate: µ = (1/X)(dX/dt); integrated: ln(X₂/X₁) = µ(t₂ − t₁)
X = biomass concentration (g dry cell weight per litre), t = time (h), µ in h⁻¹

Doubling time: t_d = ln 2 / µ = 0.693 / µ (h); generations n = (log₁₀X₂ − log₁₀X₁)/0.301

Monod equation: µ = µ_max · S /(K_s + S); S = limiting substrate (g/L), K_s = saturation constant (g/L)

Chemostat at steady state: D = F/V = µ; residual substrate S = K_s·D /(µ_max − D)
F = feed flow (L/h), V = working volume (L), D = dilution rate (h⁻¹)

Yield coefficients: Y_x/s = −ΔX/ΔS (g cells/g substrate); Y_p/s = ΔP/ΔS (g product/g substrate)
Volumetric productivity Q_p = P_final /t_total (g L⁻¹ h⁻¹); specific productivity q_p = Q_p/X (g g⁻¹ h⁻¹)

Overall recovery = Π (step yields); Product per batch = titre (g/L) × working volume (L) × recovery

Oxygen transfer rate: OTR = k_La (C* − C_L) = X · q_O₂ at steady state
k_La in h⁻¹, C* = saturation dissolved O₂ (mg/L), C_L = actual DO (mg/L), q_O₂ = specific O₂ uptake (mg g⁻¹ h⁻¹)

Penicillin potency: 1 IU = 0.6 µg penicillin G sodium, so 1 mg penicillin G sodium = 1,667 IU

Example 1 - growth rate and doubling time. A *Bacillus licheniformis* seed culture grows from 0.50 to 4.00 g DCW/L in 5 h.

ln(4.00/0.50) = ln 8 = 2.0794
µ = 2.0794/5 = 0.416 h⁻¹
t_d = 0.693/0.416 = 1.67 h (100 min); generations = log₁₀8/0.301 = 0.903/0.301 = 3 doublings ✓

Example 2 - citric acid yield and efficiency. *A. niger* converts 100 g/L sucrose to 70 g/L citric acid.

Stoichiometry: C₁₂H₂₂O₁₁ (342 g) + 3 O₂ → 2 C₆H₈O₇ (2 × 192 = 384 g) + 3 H₂O
Theoretical Y_p/s = 384/342 = 1.123 g/g
Actual Y_p/s = 70/100 = 0.70 g/g → conversion efficiency = 0.70/1.123 = 62.3% of theoretical

Example 3 - batch output and recovery (penicillin). A 200 m³ fermenter reaches 50 g/L penicillin G in a 120 h cycle; downstream recovery is 85%.

Working volume 200 m³ = 200,000 L → broth penicillin = 200,000 × 50 = 10,000,000 g = 10.0 tonnes
Recovered = 10.0 × 0.85 = 8.5 tonnes of penicillin G sodium per batch
Volumetric productivity = 50/120 = 0.417 g L⁻¹ h⁻¹
Potency of the recovered salt = 8.5 × 10⁹ mg × 1,667 IU/mg = 1.42 × 10¹³ IU (≈ 14.2 million mega-units)

Example 4 - Monod and chemostat. For a strain with µ_max = 0.50 h⁻¹ and K_s = 0.20 g/L, in a 10 L chemostat fed glucose at S₀ = 10 g/L.

At S = 1.80 g/L: µ = 0.50 × 1.80/(0.20 + 1.80) = 0.90/2.00 = 0.45 h⁻¹
Set D = µ = 0.45 h⁻¹ → F = D × V = 0.45 × 10 = 4.5 L/h
Check residual S = K_s·D/(µ_max − D) = 0.20 × 0.45/(0.50 − 0.45) = 0.09/0.05 = 1.8 g/L ✓
With Y_x/s = 0.45 g/g: X = Y_x/s(S₀ − S) = 0.45 × 8.2 = 3.69 g/L; biomass productivity = D·X = 0.45 × 3.69 = 1.66 g L⁻¹ h⁻¹

Example 5 - raw-material cost per kg of product. A citric acid unit must ship 100 tonnes/month; fermentation Y_p/s = 0.70 kg/kg sugar, downstream recovery 90%, molasses at 50% total sugars costs ₹8,000/tonne.

Citric acid needed in broth = 100/0.90 = 111.1 t
Sugar required = 111.1/0.70 = 158.7 t → molasses = 158.7/0.50 = 317.5 t
Molasses cost = 317.5 × 8,000 = ₹25.4 lakh/month → ₹25.4 per kg of product from the carbon source alone (indicative; molasses price varies sharply with the sugar season)

	Primary metabolite	Secondary metabolite
Phase of formation	Trophophase (growth)	Idiophase (after growth slows)
Need for cell	Essential	Not essential for growth
Examples	Ethanol, citric acid, lysine, vitamins	Penicillin, streptomycin, lovastatin
Typical titre	High (tens to >100 g/L)	Lower (mg/L to tens of g/L)
Key control	Pathway feedback, stoichiometry	Catabolite and phosphate repression
Process mode	Batch or continuous	Fed-batch, two-stage

Scale	Volume	Purpose
Shake flask	100 mL - 2 L	Screening, medium optimisation
Bench/laboratory fermenter	2 - 20 L	Kinetics, k_La, DO/pH control studies
Pilot plant	100 - 1,000 L	Scale-up data, engineering batches
Production	10 - 300 m³	Commercial manufacture

Where it is used

Pharmaceutical fermentation: penicillins and 6-APA, cephalosporins, statins and recombinant insulin - Aurobindo, Alembic, Cipla, Biocon (Bengaluru), erstwhile Hindustan Antibiotics (Pimpri) as the classic PSU example; roles as fermentation officer, QC microbiologist, QA/validation executive.
Vaccines and biologicals: Serum Institute of India (Pune), Bharat Biotech (Hyderabad), Biological E, Panacea Biotec - upstream cell/bacterial culture, sterility and potency testing, GMP documentation.
Industrial enzymes: Advanced Enzyme Technologies (Thane), Novozymes South Asia (Bengaluru), Camson/Zytex - enzymes for detergents, textiles, leather (Chennai cluster), baking, feed and juice processing.
Alcohol and bioenergy: molasses and grain distilleries, 2G ethanol refineries (Praj Industries technology, IOC Panipat), compressed biogas plants under the SATAT programme - fermentation operators and effluent/ZLD microbiologists.
Agri-inputs: biofertiliser and biopesticide units licensed under FCO 1985 and the Insecticides Act 1968, with *Rhizobium*, *Trichoderma* and *Bt* production; state biofertiliser labs and IARI/ICAR units.
Food cultures and probiotics: dairy starter cultures at NDRI/Amul, baker's yeast, probiotic formulations; also contract research and CDMO roles (Syngene, Sai Life) in strain development and process scale-up.

Interview questions

Q: What makes a microorganism suitable for industrial use? A: Genetic stability with a high, reproducible yield of a single major product, fast growth on a cheap medium, non-pathogenicity, easy separation from broth and amenability to genetic improvement.
Q: Difference between primary and secondary screening? A: Primary screening qualitatively detects whether an isolate has the desired activity (zones, dye changes) in large numbers; secondary screening quantifies titre, stability and by-products in shake-flask fermentations of the shortlisted isolates.
Q: Why are auxotrophic mutants used for amino acid production? A: Blocking a branch of the pathway starves the cell of the end-product that normally feedback-inhibits the first enzyme, so the intermediate or the desired amino acid accumulates and is excreted.
Q: Why is a fed-batch process preferred for penicillin? A: Penicillin is a secondary metabolite repressed by excess glucose; slow glucose feeding keeps the concentration low enough to avoid catabolite repression while sustaining a long production phase, and also limits oxygen and heat loads.
Q: What is the role of corn steep liquor in penicillin fermentation? A: It is a cheap complex nitrogen source supplying amino acids, vitamins and trace elements, and historically phenylacetic acid-type precursors that favour penicillin G.
Q: Why is downstream processing often more expensive than fermentation? A: The product is dilute and in a complex broth; multiple separation and purification steps are needed to reach pharmacopoeial purity, each with capital cost, consumables and yield loss.
Q: How would you calculate overall recovery across five steps of 90% each? A: Multiply them - 0.9⁵ = 0.59, so about 59%; this is why reducing the number of steps raises yield more than improving any single step.
Q: Which parameter is normally kept constant during scale-up and why? A: Usually power per unit volume or k_La, because oxygen transfer rather than nutrient supply limits large aerobic fermenters; impeller tip speed is used when shear sensitivity matters.
Q: Which approvals are needed in India to produce a recombinant therapeutic protein? A: IBSC clearance at the institute, RCGM (DBT) approval for the recombinant work and RCGM/GEAC clearance under the Rules, 1989, plus CDSCO manufacturing licence and GMP compliance under the Drugs and Cosmetics Act with revised Schedule M.
Q: How is an antibiotic's potency determined in a plant QC lab? A: By a microbiological cup-plate (agar diffusion) bioassay comparing the zone of inhibition of the sample against a reference standard on a seeded plate, supported by HPLC chemical assay.
Q: Why is lyophilisation better than repeated subculture for strain maintenance? A: Freeze-drying arrests metabolism, so the genotype is frozen in time; repeated subculture allows spontaneous mutants and revertants to take over and the strain degenerates in yield.
Q: A fermenter batch shows normal growth but no product. What would you check? A: Loss of the production trait in the inoculum (strain degeneration or plasmid loss), missing precursor or inducer, excess glucose or phosphate causing repression, and wrong pH, temperature or DO during the production phase.

Common mistakes

Treating industrial microbiology as only fermentation - the strain, medium, inoculum train, recovery and regulatory compliance are equally examinable.
Confusing primary/secondary screening (stages of selecting a strain) with primary/secondary metabolites (classes of product).
Writing penicillin as a growth-associated product; it is a secondary metabolite made mainly after growth slows.
Using a mutagen dose that kills nearly everything, or screening too few survivors, then concluding mutagenesis failed.
Reporting yield without stating the basis - g product per g substrate consumed, per g substrate supplied, or as a percentage of theoretical.
Forgetting that overall recovery multiplies, and quoting the best single-step yield as the plant yield.
Maintaining production strains by routine subculture, and ignoring batch-to-batch variability of molasses and corn steep liquor when a yield suddenly drops.
Quoting old statutes or withdrawn IS numbers - the Factories Act has been replaced by the Labour Codes in force from 21 November 2025, and Schedule M was revised in 2023.