Microbiology basics

In short

Microbiology is the study of organisms too small to see with the unaided eye — bacteria, archaea, viruses, fungi, algae and protozoa — including their structure, growth, genetics, cultivation, control and uses. For a biotechnology engineer it is the foundation subject: almost every bioprocess (insulin, enzymes, antibiotics, vaccines, ethanol, probiotics, biofertilisers) runs on a microbial or cell-culture host, and every such process must be kept sterile and monitored. It is examined in GATE Biotechnology (BT) and GATE Life Sciences (XL) under "Microbiology", in CSIR-UGC NET Life Sciences Unit on Cellular Organisation and Microbiology, and in university papers as Microbiology I/II with a compulsory lab viva. Placement technical rounds at Biocon, Serum Institute of India, Bharat Biotech, Panacea Biotec, Dr. Reddy's, Syngene, Thermo Fisher and FSSAI/NABL-accredited testing labs lean heavily on staining, sterilisation, media, plate counts and contamination control. Regulatory practice in India — Schedule M (revised 2023, Good Manufacturing Practices), IP (Indian Pharmacopoeia) microbial limit tests, DBT/RCGM biosafety rules — is built on exactly these basics.

Explained

Scope, history and classification of microorganisms

• Branches: bacteriology, virology, mycology (fungi), phycology (algae), protozoology, immunology; applied branches — industrial, food, medical, agricultural, environmental microbiology.
• Landmark names examined: Leeuwenhoek (first observation), Pasteur (disproved spontaneous generation with swan-neck flasks; pasteurisation; fermentation is microbial), Koch (pure-culture technique, solid media, Koch's postulates, TB bacillus), Lister (antiseptic surgery), Jenner (smallpox vaccination), Fleming (penicillin, 1928), Winogradsky and Beijerinck (soil/nitrogen-fixing microbes), Ivanovsky and Beijerinck (tobacco mosaic virus), Woese (three domains from 16S rRNA).
• Koch's postulates: (1) organism present in all diseased hosts, (2) isolated in pure culture, (3) reproduces disease on inoculation into a healthy host, (4) re-isolated from that host. Limitations: non-culturable organisms, asymptomatic carriers, viruses needing living cells, polymicrobial disease.
• Three domains (Woese): Bacteria, Archaea, Eukarya. Archaea have ether-linked branched lipids, no peptidoglycan (pseudomurein or S-layers), and include methanogens, halophiles and thermophiles; they share transcription/translation features with eukaryotes.
• Whittaker's five kingdoms (Monera, Protista, Fungi, Plantae, Animalia) is the classical scheme still asked; Bergey's Manual of Systematic Bacteriology is the standard bacterial taxonomy reference.
• Nomenclature: binomial, italicised, genus capitalised — *Escherichia coli*; abbreviate as *E. coli* after first use. Strain designations follow (e.g. *E. coli* K-12 MG1655).
• Modern identification: 16S rRNA gene sequencing (universal, ~1,540 bp, with hypervariable V3–V4 regions), MALDI-TOF MS in clinical labs, DNA–DNA hybridisation and now average nucleotide identity (ANI ≥ 95–96% = same species) for species delineation.
• Indian culture collections: MTCC (Microbial Type Culture Collection, IMTECH Chandigarh), NCIM (NCL Pune), NCCS Pune (cell lines); both MTCC and NCIM are International Depositary Authorities for patent deposits under the Budapest Treaty.

Microbial cell structure and the prokaryote–eukaryote divide

• Prokaryote: no nuclear membrane, no membrane-bound organelles, 70S ribosomes, single circular chromosome (nucleoid), typical size 0.5–5 µm, binary fission, 1–5 Mb genome.
• Eukaryote (fungi, algae, protozoa): true nucleus, mitochondria/ER/Golgi, 80S cytoplasmic ribosomes, linear chromosomes with histones, mitosis and meiosis, 10–100 µm.
• Shapes: coccus, rod (bacillus), spiral (spirillum rigid, spirochaete flexible), comma (vibrio), pleomorphic; arrangements — diplo-, strepto- (chains), staphylo- (clusters), tetrad, sarcina.
• Cell wall: peptidoglycan (murein) — alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked β-1,4, with a tetrapeptide side chain on NAM cross-linked by transpeptidase. Lysozyme cuts the NAG–NAM bond; penicillin inhibits transpeptidase (hence acts only on growing cells).
• Gram-positive wall: thick peptidoglycan (20–80 nm), many layers, teichoic and lipoteichoic acids, no outer membrane; retains crystal violet → purple.
• Gram-negative wall: thin peptidoglycan (2–7 nm) plus an outer membrane with lipopolysaccharide (LPS) — lipid A (the endotoxin), core polysaccharide, O-antigen; periplasmic space; porins. Decolourised by alcohol, counterstained pink/red by safranin.
• Acid-fast wall (*Mycobacterium*, *Nocardia*): mycolic acids make it waxy; needs Ziehl–Neelsen staining (carbol fuchsin with heat, acid-alcohol decolouriser, methylene blue counterstain) — the basis of India's sputum smear TB diagnosis under the National TB Elimination Programme.
• Other structures: capsule/glycocalyx (virulence, biofilm, anti-phagocytic), slime layer, flagellum (motility; monotrichous, lophotrichous, amphitrichous, peritrichous; rotates by proton-motive force; H antigen), fimbriae/pili (adhesion), sex pilus (conjugation), plasmids (R, F, Col, degradative — the backbone of recombinant DNA work), inclusion bodies (PHB granules, polyphosphate, gas vacuoles), mesosomes, 70S ribosomes (30S + 50S).
• Endospore (*Bacillus*, *Clostridium*): dormant, dehydrated, calcium-dipicolinate-rich core with cortex, spore coat and exosporium; survives boiling, desiccation and radiation — the reason moist heat at 121 °C is the sterilisation benchmark. Stained by Schaeffer–Fulton (malachite green/safranin). Sporulation is a differentiation response to nutrient starvation.

Nutrition, growth and growth kinetics

• Nutritional classes: by carbon — autotroph (CO₂) vs heterotroph (organic C); by energy — phototroph vs chemotroph; by electron donor — lithotroph (inorganic) vs organotroph. So *E. coli* is a chemoorganoheterotroph, nitrifying bacteria are chemolithoautotrophs.
• Oxygen relations: obligate aerobe, obligate anaerobe, facultative anaerobe, microaerophile, aerotolerant anaerobe. Anaerobes lack superoxide dismutase/catalase. Anaerobic culture uses thioglycollate broth, anaerobic jars with GasPak, Robertson's cooked meat medium.
• Physical requirements: temperature — psychrophile (<20 °C), mesophile (20–45 °C, includes all human pathogens at 37 °C), thermophile (45–80 °C, *Thermus aquaticus* → Taq polymerase), hyperthermophile (>80 °C, *Pyrococcus furiosus* → Pfu polymerase); pH — acidophile, neutrophile, alkaliphile; water activity and osmotic tolerance — halophiles, osmophiles; CO₂ — capnophiles.
• Culture media types: by consistency (liquid broth, semi-solid, solid with 1.5–2% agar — melts at ~95 °C, sets at ~40 °C, not digested by most bacteria); by composition (synthetic/defined vs complex — nutrient broth, peptone water); by function — enriched (blood, chocolate agar), enrichment (selenite F broth for *Salmonella*), selective (MacConkey with bile salts and crystal violet; EMB; SS agar; TCBS for *Vibrio*; Sabouraud with low pH for fungi), differential (MacConkey — lactose fermenters pink; blood agar — haemolysis patterns; EMB — green metallic sheen for *E. coli*), transport (Cary–Blair, VR medium), assay media.
• Pure culture methods: streak plate (quadrant streaking), pour plate, spread plate, serial dilution, micromanipulation, enrichment culture; maintained by subculture, agar slants, glycerol stocks at −80 °C, lyophilisation (freeze-drying) and liquid-nitrogen cryopreservation.
• Batch growth curve (four phases):
  1. Lag — no division; adaptation, enzyme induction, increase in cell size; length depends on inoculum age/size and medium shift.
  2. Log/exponential — balanced growth at constant maximum specific growth rate µ; cells are physiologically uniform and most sensitive to antibiotics; this is the phase used for competent-cell preparation and most kinetic measurements.
  3. Stationary — growth rate = death rate; limiting nutrient exhausted, toxic products accumulate; secondary metabolites (most antibiotics), sporulation, stringent response.
  4. Decline/death — exponential loss of viability; may be followed by prolonged survival of a sub-population.
• Diauxic growth: two log phases on glucose + lactose, separated by a second lag — glucose repression (cAMP–CRP and inducer exclusion in the lac system).
• Continuous culture: chemostat (dilution rate D = F/V controls µ; at steady state µ = D), turbidostat; advantages — constant physiology, high productivity; limitations — contamination risk, wall growth, strain mutation over long runs. Fed-batch is the industrial norm for high-cell-density recombinant protein production.
• Growth measurement: turbidity (OD₆₀₀, total cells), viable plate count (CFU/mL — the standard, misses VBNC cells), direct microscopic count with haemocytometer/Petroff–Hausser, dry cell weight, most probable number (MPN, for coliforms in water), membrane filtration, flow cytometry, ATP bioluminescence, protein/DNA assay.

Control of microorganisms: sterilisation, disinfection and antimicrobials

• Definitions: sterilisation — destruction of *all* life, including spores; disinfection — killing pathogens on inanimate objects; antisepsis — on living tissue; sanitisation — reducing counts to a safe level; -cidal kills, -static inhibits; asepsis — preventing entry of microbes.
• Moist heat: autoclaving at 121 °C, 15 psi (≈ 103 kPa gauge, ~1.0 bar above atmosphere), 15–20 minutes — the reference cycle; also 134 °C for 3–4 min for instruments. Kills by protein denaturation; must displace all air (steam traps/vacuum) because trapped air lowers the real temperature. Validation: chemical indicators (autoclave tape, Browne's tube), biological indicator — spores of *Geobacillus stearothermophilus* (formerly *Bacillus stearothermophilus*).
• Other moist methods: boiling (100 °C, disinfection only — spores survive), tyndallisation (100 °C, 30 min on 3 successive days), pasteurisation — LTLT 63 °C/30 min, HTST 72 °C/15 s, UHT 135–140 °C/few seconds. BIS specifies pasteurised milk requirements (phosphatase-negative) under the Food Safety and Standards (FSS) Act, 2006 regulations administered by FSSAI.
• Dry heat: hot-air oven 160–170 °C for 1–2 hours (glassware, powders, oils); incineration; red/flaming for loops. Dry heat needs higher temperature and longer time than moist heat because there is no latent heat transfer and no hydrolysis of proteins.
• Filtration: membrane filters of 0.22 µm for sterilising heat-labile liquids (media additives, antibiotics, vitamins, serum); 0.45 µm for water-quality counts; HEPA filters (99.97% at 0.3 µm) in laminar flow hoods and cleanrooms; depth filters for large volumes. Note filtration removes but does not kill, and most viruses and mycoplasma pass 0.22 µm (use 0.1 µm for mycoplasma).
• Radiation: ionising — gamma (⁶⁰Co), electron beam, used for plastics, syringes, sutures and for food irradiation (Indian facilities under BARC/KRUSHAK; approved under FSS regulations); non-ionising — UV at 254 nm (thymine dimers; poor penetration — surfaces and air only, used in LAF cabinets).
• Chemical agents: alcohols (60–70% ethanol/isopropanol — needs water for protein denaturation), aldehydes (glutaraldehyde 2% — a cold sterilant; formaldehyde fumigation), halogens (hypochlorite 1% for spills, iodophors), phenolics (phenol is the reference for the phenol coefficient), quaternary ammonium compounds, hydrogen peroxide and peracetic acid (sporicidal), ethylene oxide gas (plastics, catheters — alkylating, requires aeration), chlorhexidine.
• Antibiotics by target: cell-wall synthesis (penicillins, cephalosporins, vancomycin), protein synthesis (aminoglycosides and tetracyclines on 30S; chloramphenicol, erythromycin, clindamycin on 50S), nucleic acid (quinolones — DNA gyrase; rifampicin — RNA polymerase), folate pathway (sulfonamides, trimethoprim — competitive antimetabolites), membrane (polymyxin B, daptomycin).
• Resistance mechanisms: enzymatic inactivation (β-lactamases, ESBL, carbapenemases such as NDM-1), target modification (MRSA's PBP2a; ribosomal methylation), efflux pumps, reduced permeability, bypass pathways; spread by plasmids, transposons and integrons. India's AMR response: ICMR's National AMR Surveillance Network, the National Action Plan on AMR, and a schedule-H1 prescription-control category for key antibiotics.
• Susceptibility testing: Kirby–Bauer disc diffusion on Mueller–Hinton agar (zone diameter against CLSI breakpoints), broth microdilution for MIC, MBC, E-test gradient strips, automated systems (VITEK, BD Phoenix).
• Biosafety: BSL-1 to BSL-4 containment; India has BSL-4 at NIV Pune. GMOs and pathogen work are regulated under the Environment (Protection) Act, 1986 Rules, 1989 with the institutional IBSC, RCGM (DBT) and GEAC (MoEFCC); biomedical waste follows the Bio-Medical Waste Management Rules, 2016 (as amended) with colour-coded segregation — microbiology lab cultures must be pre-treated (autoclaved) before disposal in yellow/red streams.

Microbial genetics and viruses

• Mutation: spontaneous and induced; base substitution (transition/transversion), frameshift, point mutants (silent, missense, nonsense); mutagens — base analogues, alkylating agents, intercalators, UV, ionising radiation; Ames test for mutagenicity using histidine-auxotroph *Salmonella typhimurium*. Replica plating and the Luria–Delbrück fluctuation test showed mutations are pre-adaptive (random), not directed.
• Horizontal gene transfer (three mechanisms — a guaranteed exam question):
  • Transformation — uptake of free DNA by competent cells (Griffith's experiment; Avery, MacLeod, McCarty showed DNA is the transforming principle); artificial competence by CaCl₂ heat shock or electroporation.
  • Conjugation — cell-to-cell transfer via sex pilus, F plasmid; F⁺ × F⁻, Hfr strains used for interrupted-mating gene mapping (distances in minutes).
  • Transduction — bacteriophage-mediated; generalised (any gene, lytic phage packaging error, e.g. P22) and specialised (genes flanking the prophage site, λ *gal/bio*).
• Bacteriophage: head–tail structure (T4), lytic cycle (adsorption, penetration, biosynthesis, maturation, lysis — burst size measured by one-step growth experiment) and lysogeny (λ integrates as prophage; cI repressor; induction by UV). Plaque assay gives PFU/mL. Phage used as cloning vectors, in phage display and phage therapy.
• Virus general properties: obligate intracellular parasites, nucleic acid (DNA or RNA, ss or ds, linear or circular, segmented) plus a capsid of capsomeres (helical, icosahedral or complex symmetry), optional lipid envelope with glycoprotein spikes; no ribosomes or metabolism; measured in nanometres (20–300 nm); cultivated in cell culture, embryonated eggs or animals; Baltimore classification by genome type and replication strategy (Groups I–VII); retroviruses use reverse transcriptase; +ssRNA genome is directly infectious as mRNA, −ssRNA needs its own RNA-dependent RNA polymerase. Related entities: viroids (naked RNA, plant), prions (misfolded PrP protein, no nucleic acid), satellite viruses.
• Quantitation and effects: plaque assay, TCID₅₀, haemagglutination titre, cytopathic effect (CPE), inclusion bodies, one-step growth curve with eclipse and latent periods.

Fungi, algae, protozoa and the normal flora

• Fungi: eukaryotic, chitinous cell wall, ergosterol in the membrane (target of amphotericin B and azoles), heterotrophic absorptive nutrition; yeast (unicellular, budding — *Saccharomyces cerevisiae*), mould (hyphae, septate or coenocytic, forming mycelium), dimorphic (*Candida albicans*, *Histoplasma*). Classes examined: Zygomycota (*Rhizopus*, zygospore), Ascomycota (ascospores in ascus — *Aspergillus*, *Penicillium*, yeast), Basidiomycota (mushrooms), and the former "Deuteromycota" (no known sexual stage). Grown on Sabouraud dextrose agar (pH ~5.6) with antibiotic; identified by lactophenol cotton blue mounts and KOH mounts.
• Algae: photosynthetic eukaryotes (plus cyanobacteria, which are prokaryotes); *Spirulina* (Arthrospira) and *Chlorella* as single-cell protein, agar and carrageenan from red algae, alginate from brown algae, *Botryococcus/Chlorella* for biodiesel; diatoms give diatomaceous earth.
• Protozoa: classified by locomotion — Mastigophora (flagellates, *Giardia*, *Trypanosoma*, *Leishmania* — kala-azar is endemic in Bihar/Jharkhand/West Bengal), Sarcodina (amoebae, *Entamoeba histolytica*), Ciliophora (*Balantidium*), Apicomplexa/Sporozoa (*Plasmodium* — malaria, *Toxoplasma*, *Cryptosporidium*).
• Normal (resident) microflora and the microbiome: skin, mouth, gut (dominated by *Bacteroides*, *Firmicutes*; vitamin K and B₁₂ synthesis, colonisation resistance), vagina (*Lactobacillus*, low pH); sterile sites — blood, CSF, lower respiratory tract. Germ-free animals show reduced immune development. Probiotics are live organisms conferring benefit (*Lactobacillus*, *Bifidobacterium*); FSSAI regulates probiotic food claims and maintains a list of permitted strains.
• Host–microbe relations: symbiosis, commensalism, mutualism (*Rhizobium*–legume root nodules; rumen microbes), opportunistic pathogen, true pathogen; virulence factors — adhesins, capsule, invasins, exotoxins (protein, heat-labile, highly specific — tetanus, botulinum, cholera, diphtheria; toxoid vaccines possible) versus endotoxin (LPS lipid A, heat-stable, pyrogenic, causes septic shock; detected by the LAL/bacterial endotoxin test required by the Indian Pharmacopoeia for injectables).
• Water and food microbiology in Indian practice: *E. coli* and total coliforms as faecal indicators; IS 10500: 2012 (Drinking Water — Specification, with amendments) requires that coliforms be absent in 100 mL; tests by MPN, membrane filtration or H₂S strip; CPCB/State Boards use BOD and coliform counts for water-quality classes. Food microbial limits and testing come under FSSAI regulations with NABL-accredited (ISO/IEC 17025) labs; pharmaceutical microbiology follows Schedule M GMP (the revised Schedule M notified in 2023, with phased compliance) covering cleanroom grades A–D, environmental monitoring, media fills and water systems.

Key points

• Microbiology — study of bacteria, archaea, viruses, fungi, algae and protozoa; the base discipline for all bioprocessing.
• Prokaryote vs eukaryote — no nuclear membrane, 70S ribosomes, circular chromosome, binary fission versus true nucleus, 80S ribosomes, mitosis.
• Peptidoglycan — NAG–NAM chains cross-linked by transpeptidase; lysozyme cuts the glycan, penicillin blocks the cross-link.
• Gram-positive vs Gram-negative — thick peptidoglycan with teichoic acid, retains crystal violet (purple) versus thin wall plus LPS outer membrane, decolourised and counterstained pink.
• Endotoxin — LPS lipid A of Gram-negatives; heat-stable and pyrogenic, tested by LAL before any injectable is released.
• Endospore — dormant dehydrated survival form of *Bacillus*/*Clostridium*; the reason sterilisation means 121 °C, not boiling.
• Autoclave cycle — 121 °C, 15 psi, 15–20 min, moist heat, validated with *Geobacillus stearothermophilus* spores.
• Dry heat — 160–170 °C for 1–2 h; needs more heat and time than moist heat for the same kill.
• 0.22 µm filtration — sterilises heat-labile liquids by removal, not killing; viruses and mycoplasma can pass.
• Media classes — defined vs complex; enriched, enrichment, selective, differential, transport (MacConkey is both selective and differential).
• Growth curve — lag, log, stationary, death; log phase for kinetics and antibiotic sensitivity, stationary phase for most secondary metabolites.
• Generation time (g) — time to double; g = (t × log 2)/(log N − log N₀); µ = ln2/g.
• CFU — one colony assumed to arise from one viable cell; plate counts miss viable-but-non-culturable cells.
• Chemostat — at steady state the specific growth rate equals the dilution rate, µ = D = F/V.
• HGT — transformation (free DNA), conjugation (pilus/F plasmid), transduction (phage); the engine of antibiotic-resistance spread.
• Lysogeny — phage genome integrated as prophage, repressed, induced by stress; basis of specialised transduction and λ vectors.
• Baltimore classification — groups viruses by genome type and replication route; +ssRNA is infectious mRNA, −ssRNA carries its own RdRp.
• Fungal targets — ergosterol membrane and chitin wall explain why antibacterials do not work on fungi.
• Indian regulatory anchors — Schedule M GMP, Indian Pharmacopoeia microbial limit and endotoxin tests, IS 10500: 2012 for drinking water, IBSC/RCGM/GEAC for GMOs, Bio-Medical Waste Rules 2016, MTCC/NCIM as patent depositories.

Formulas and examples

Example 1 — generation time from a growth curve. A culture of *E. coli* grows from 5 × 10³ cells/mL to 4 × 10⁸ cells/mL in 5 hours at 37 °C. Find n, g and µ.

• log N = log(4 × 10⁸) = 8.6021; log N₀ = log(5 × 10³) = 3.6990.
• n = (8.6021 − 3.6990)/0.3010 = 4.9031/0.3010 = 16.29 generations.
• g = t/n = 300 min / 16.29 = 18.4 minutes.
• µ = 0.693/0.307 h = 2.26 h⁻¹ (g = 18.4 min = 0.307 h).
• Check: N₀ × 2^16.29 = 5 × 10³ × 80,000 ≈ 4 × 10⁸ ✓.

Example 2 — viable plate count from a serial dilution. 1 mL of milk is diluted through four successive 1:10 steps (total 10⁻⁴); 0.1 mL of the 10⁻⁴ tube is spread and gives 148 colonies.

• Dilution factor plated = 10⁻⁴; volume plated = 0.1 mL.
• CFU/mL = 148 / (0.1 × 10⁻⁴) = 148 / 10⁻⁵ = 1.48 × 10⁷ CFU/mL.
• The plate lies in the 30–300 countable range, so the count is valid; the 10⁻³ plate would have been ~1,480 colonies (too crowded) and the 10⁻⁵ plate ~15 (too few for statistical reliability).

Example 3 — thermal death time (D and Z values). A spore suspension has D₁₂₁ = 1.5 min and Z = 10 °C. (a) Time at 121 °C to take 10⁶ spores/mL down to a 10⁻⁶ survival probability. (b) D-value at 111 °C.

• (a) Required log reduction = log(10⁶) + 6 = 6 + 6 = 12 log. Time = 12 × 1.5 = 18 minutes — which is exactly why the standard cycle is 15–20 min, not 15 seconds.
• (b) A 10 °C fall equals one Z, so D rises tenfold: D₁₁₁ = 1.5 × 10 = 15 min; a 12-log kill at 111 °C would need 180 min.

Example 4 — chemostat and Monod. A chemostat of working volume V = 2 L is fed at F = 0.4 L/h. The organism has µmax = 0.5 h⁻¹ and Ks = 0.2 g/L glucose; feed S₀ = 10 g/L and Y(X/S) = 0.5 g cells per g glucose.

• D = F/V = 0.4/2 = 0.2 h⁻¹; at steady state µ = D = 0.2 h⁻¹ (below µmax, so no washout).
• From µ = µmax S/(Ks + S): 0.2 = 0.5 S/(0.2 + S) → 0.2(0.2 + S) = 0.5S → 0.04 = 0.3S → S = 0.133 g/L residual glucose.
• X = Y(S₀ − S) = 0.5 × (10 − 0.133) = 4.93 g/L dry cells.
• Volumetric productivity = D × X = 0.2 × 4.93 = 0.99 g L⁻¹ h⁻¹.
• Critical dilution rate D_crit = µmax S₀/(Ks + S₀) = 0.5 × 10/10.2 = 0.49 h⁻¹ — above this the culture washes out.

Example 5 — antibiotic dilution and MIC. A stock of ampicillin is 100 mg/mL. Prepare 500 mL of LB agar containing 100 µg/mL, then read an MIC series.

• Required mass = 100 µg/mL × 500 mL = 50,000 µg = 50 mg → volume of stock = 50 mg / 100 mg per mL = 0.5 mL, added after the agar cools to ~50 °C (ampicillin is heat-labile).
• In a two-fold broth microdilution series 64, 32, 16, 8, 4, 2 µg/mL, growth is seen at 8 µg/mL and below and no growth at 16 and above → MIC = 16 µg/mL. Subculturing the clear wells: 64 and 32 µg/mL give no growth, 16 does → MBC = 32 µg/mL. MBC/MIC = 2, so the drug is bactericidal for this isolate (a ratio ≥ 4 indicates tolerance/bacteriostatic action).

Where it is used

• Biopharmaceuticals and vaccines: *E. coli* and *Pichia*/CHO hosts for insulin, streptokinase, Hepatitis B and rotavirus vaccines at Biocon, Bharat Biotech, Serum Institute of India, Shantha Biotechnics — jobs in upstream process, media prep, QC microbiology, sterility assurance and environmental monitoring under Schedule M.
• Antibiotic and enzyme fermentation: penicillin (*Penicillium chrysogenum*), streptomycin and other actinomycete products, amylase and cellulase (*Aspergillus*, *Bacillus*), citric acid; roles in strain maintenance, inoculum development and contamination investigation.
• Food, dairy and beverages: starter cultures and probiotics (Amul, Mother Dairy, Yakult India, Nestlé), brewing and distilleries, shelf-life and spoilage studies, microbial limit testing in FSSAI-notified and NABL-accredited labs; food safety auditor and microbiologist roles.
• Clinical and diagnostic labs: culture and sensitivity, Ziehl–Neelsen smears and CBNAAT under the National TB Elimination Programme, blood culture systems; employers include Dr. Lal PathLabs, Metropolis, SRL, AIIMS and state medical colleges.
• Environmental and agricultural biotechnology: activated sludge and anaerobic digesters (biogas), bioremediation of oil and dye effluents, *Rhizobium*/*Azotobacter*/phosphate-solubilising biofertilisers and *Bacillus thuringiensis* biopesticides regulated under the Insecticides Act, 1968 and FCO specifications; CPCB-mandated effluent monitoring.
• Water testing and public health: coliform MPN/membrane filtration against IS 10500: 2012, packaged drinking water under BIS certification, municipal and Jal Jeevan Mission laboratories.

Interview questions

• Q: Why does Gram staining distinguish two groups of bacteria? A: The thick multilayered peptidoglycan of Gram-positives traps the crystal violet–iodine complex during alcohol decolourisation, while the thin wall and lipid-rich outer membrane of Gram-negatives lets it wash out, so they take the safranin counterstain.
• Q: Why is 121 °C for 15 minutes the sterilisation standard rather than boiling? A: Vegetative cells die near 100 °C but bacterial endospores survive boiling; saturated steam at 121 °C (15 psi) delivers enough lethality — about 12 log reductions — to inactivate spores.
• Q: How would you sterilise a heat-labile antibiotic solution? A: Filter it through a 0.22 µm membrane into a pre-sterilised container inside a laminar flow hood; it removes bacteria and fungi without heat, though mycoplasma and viruses need 0.1 µm or other treatment.
• Q: What does CFU mean and why is it not the same as cell count? A: A colony-forming unit is one colony assumed to come from one viable cell, but clumps and chains give one colony from many cells, and viable-but-non-culturable cells give none — so CFU underestimates total cells measured by OD or microscopy.
• Q: Which growth phase would you use to prepare competent cells or run an antibiotic sensitivity test, and why? A: Mid-log phase — cells are physiologically uniform, actively dividing and most susceptible to agents such as penicillin that act on wall synthesis.
• Q: Why do most antibiotics appear only in stationary phase in industrial fermentations? A: They are secondary metabolites, not needed for growth; their synthesis is switched on when a nutrient becomes limiting and growth slows, which is why fed-batch processes deliberately hold the culture in that state.
• Q: Distinguish exotoxin from endotoxin. A: Exotoxins are secreted proteins, heat-labile, highly potent and specific, and can be converted to toxoid vaccines; endotoxin is the lipid A part of Gram-negative LPS, heat-stable, pyrogenic and detected by the LAL test on injectables.
• Q: Name the three ways bacteria exchange genes and which matters most for antibiotic resistance. A: Transformation, conjugation and transduction; conjugative plasmids carrying resistance genes — often with transposons and integrons — spread resistance fastest, including between species.
• Q: Difference between generalised and specialised transduction? A: Generalised transduction packages any random host DNA fragment during a lytic cycle; specialised transduction transfers only genes flanking the prophage integration site after faulty excision of a temperate phage such as λ.
• Q: In a chemostat, what sets the growth rate? A: The dilution rate — at steady state µ = D = F/V; raising D above µmax washes the culture out.
• Q: Why does penicillin not affect *Mycoplasma*, and why not fungi? A: *Mycoplasma* has no cell wall and so no transpeptidase target; fungi have chitin walls and ergosterol membranes, which need azoles, amphotericin B or echinocandins instead.
• Q: A fermenter batch shows a sudden pH drop and a drop in product titre. How do you check for contamination? A: Take an aseptic sample for wet mount and Gram stain, streak on selective and non-selective plates, and compare colony and cell morphology with the production strain; check sterilisation records, filter integrity and seal/air-filter logs.
• Q: What validates that your autoclave actually sterilised the load? A: A biological indicator — *Geobacillus stearothermophilus* spore strips that show no growth after incubation — plus cycle charts (time–temperature–pressure) and chemical indicators; tape alone only shows the load was heated.

Common mistakes

• Treating disinfection, sanitisation and sterilisation as interchangeable; only sterilisation kills spores, and boiling or 70% alcohol never sterilises.
• Forgetting to multiply by the plated volume in a plate count — with 0.1 mL spread, CFU/mL is ten times the colony count divided by the dilution, not the count itself.
• Using log base e in n = (log N − log N₀)/log 2, or forgetting that the formula needs the same log base throughout; also quoting generation time in hours when the elapsed time was in minutes.
• Saying Gram-negatives have "no peptidoglycan" — they have a thin layer; and claiming the stain reflects pathogenicity rather than wall architecture.
• Autoclaving heat-labile additives (antibiotics, vitamins, urea, glucose at high concentration — which caramelises and causes the Maillard reaction) instead of filter-sterilising and adding after the medium cools.
• Reading plates outside the 30–300 countable range, or counting spreaders and confluent growth as colonies.
• Confusing CFU with PFU, or calling viruses and prions "microorganisms that can be cultured on nutrient agar" — viruses need living host cells.
• Writing species names wrongly (lower-case genus, no italics, or "*E.Coli*") — a mark lost in every university paper and noticed in interviews.

Related topics

Cell biology · Biochemistry: biomolecules and enzymes · Molecular biology: replication, transcription and translation · Genetics · Bioprocess engineering and fermentation technology · Immunology and vaccine technology