Control of Microbial Growth & Industrial Fermentations
12 sub-topics · Pages 324–402
1. The Control of Microbial Growth
Controlling microbial growth is fundamental to medicine, food safety, and environmental engineering. The key terms — sterilisation, disinfection, sanitisation, and antisepsis — describe different levels of microbial reduction. The choice of method depends critically on what must be protected (the material), who must be protected (patients, consumers, the environment), and which organisms must be eliminated.
✏️ Fill in the Blank
1. The complete destruction of all microbial life, including endospores and viruses, is called _______.
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Sterilization🔘 Multiple Choice
1. The D-value (decimal reduction time) in heat sterilization refers to:
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Correct: B) The time required to reduce the viable microbial population by one log (90%) at a specific temperature💬 Open-Ended Questions
1. What is the 'antibiotic discovery gap' and why has the rate of new antibiotic discovery declined since the 1980s? What novel strategies are being explored to address the growing crisis of antimicrobial resistance?
Hint / Guidance
Gap: golden era (1940s–1960s) discovered most antibiotic classes (penicillins, tetracyclines, aminoglycosides, macrolides, glycopeptides) from soil Actinobacteria; since 1980s, few truly new classes. Reasons: most 'easy' compounds already discovered; rediscovery of known compounds from same soil bacteria; economic disincentives for industry; short treatment courses = low profit. Novel strategies: (1) Genome mining of uncultured bacteria (metagenomics); (2) Anti-virulence/quorum sensing inhibitors; (3) Phage therapy; (4) Antimicrobial peptides; (5) Antibody-antibiotic conjugates; (6) CRISPR-based antimicrobials; (7) Resensitisation strategies (β-lactamase inhibitors like clavulanate); (8) Exploring novel ecological niches (deep sea, cave systems).2. Discuss the mechanisms by which bacteria develop resistance to β-lactam antibiotics and the strategies used to overcome resistance.
Hint / Guidance
Mechanisms: (1) β-lactamase production (serine or metallo-) — hydrolyses β-lactam ring; (2) PBP modification — MRSA MecA gene encodes PBP2a with low penicillin affinity; (3) Reduced outer membrane permeability (loss of OmpF porin in Gram-negative); (4) Efflux pumps (AcrAB-TolC). Strategies to overcome: (1) β-lactamase inhibitors (clavulanate, sulbactam) combined with amoxicillin/ampicillin; (2) Methicillin-resistant strains treated with glycopeptides (vancomycin), linezolid; (3) New β-lactamase-stable carbapenems (meropenem) — but carbapenemase-producing Enterobacteriaceae (CPE) now emerging.2. Heat Killing
Heat killing is the most reliable and widely used sterilisation method. Moist heat (autoclave: 121°C, 15 psi, 15 minutes) denatures proteins and disrupts membranes far more efficiently than dry heat because water acts as a heat conductor and accelerates protein unfolding. Thermal death follows first-order kinetics, characterised by the D-value (decimal reduction time) and z-value (temperature sensitivity coefficient).
✏️ Fill in the Blank
1. Bacteria that cannot be cultured on standard laboratory media but remain alive and metabolically active are described as _______ (VBNC).
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Viable but non-culturable2. The process of using moist heat at 121°C for 15–20 minutes under pressure to sterilise materials is called _______.
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Autoclaving🔘 Multiple Choice
1. Ethylene oxide (ETO) is used for sterilizing heat-sensitive equipment because:
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Correct: B) It is a gas that penetrates packaging and inactivates microorganisms by alkylating nucleic acids and proteins at low temperatures (~50°C)💬 Open-Ended Questions
1. What is immobilised cell technology and what advantages does it offer for continuous industrial fermentation? Give two specific industrial applications.
Hint / Guidance
Immobilised cells: cells entrapped in/on solid support (alginate beads, polyurethane foam, membrane bioreactor); cells retained while product-containing medium flows through. Advantages: no cell washout at high dilution rates; reuse of expensive cells; higher cell density than suspension; simpler downstream processing (cells not in product stream); can use non-growing cells for biotransformations. Applications: (1) High-fructose corn syrup — glucose isomerase immobilised on ion-exchange resin; converts glucose → fructose continuously for years; (2) L-aspartate production — Escherichia coli cells in polyacrylamide gel; fumarate + NH₃ → L-aspartate; used as food/pharma amino acid.3. Filtration
Membrane filtration removes microorganisms by physical size exclusion. Filters with 0.22 µm pores retain bacteria; 0.45 µm filters remove most bacteria but may pass some small ones. Filtration is indispensable for sterilising heat-sensitive liquids — culture media, antibiotic solutions, enzymes, and parenteral drugs. It does not remove viruses, bacteriophages, prions, or endotoxins.
✏️ Fill in the Blank
1. A chemical agent that kills bacteria is described as _______ (adjective form).
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Bactericidal2. A chemical agent that kills or inhibits bacteria on living tissue is called an _______.
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Antiseptic🔘 Multiple Choice
1. In industrial antibiotic fermentation (e.g., penicillin production by Penicillium chrysogenum), antibiotics are considered 'secondary metabolites'. This means:
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Correct: B) They are produced primarily during or after the stationary phase and are not essential for the organism's primary metabolism💬 Open-Ended Questions
1. Compare dry heat and moist heat sterilisation. Explain why moist heat is generally more effective at lower temperatures.
Hint / Guidance
Dry heat (oven, 160–180°C, 1–2 h): kills by oxidation/protein denaturation; penetrates slowly; used for glassware, metal instruments, oils. Moist heat (autoclave, 121°C/15 min or 134°C/3 min): kills by protein denaturation/hydrolysis; water disrupts H-bonds and hydrophobic interactions; effective against spores. Why more effective: water has higher heat capacity and conductivity than air; moist conditions lower the activation energy for protein unfolding (water acts as a plasticiser for proteins); kills at 121°C vs 160°C dry = ~39°C difference. Endospores killed by moist heat in 15 min but require hours of dry heat.4. Commonly Used Agents for Disinfection and Antisepsis
Chemical disinfectants and antiseptics span a wide range of chemical classes. Halogens (chlorine, iodine) generate reactive oxidising species that attack proteins, nucleic acids, and lipids. Alcohols (70% ethanol) denature proteins and disrupt membranes. Glutaraldehyde cross-links proteins for high-level disinfection of endoscopes. Understanding the spectrum of activity, contact time, and concentration required is essential for designing effective infection control protocols.
✏️ Fill in the Blank
1. The minimum inhibitory concentration (MIC) is defined as the _______ concentration of an antimicrobial agent that prevents visible bacterial growth.
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Lowest2. The production of beer, wine, and spirits using microorganisms is carried out by _______ in industrial fermentation.
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Yeast (Saccharomyces cerevisiae)💬 Open-Ended Questions
1. Distinguish clearly between sterilization, disinfection, sanitization, and antisepsis. For each, state the target, level of microbial reduction achieved, and give a practical application.
Hint / Guidance
Sterilization: all life including endospores and viruses eliminated (autoclave 121°C/15 min for surgical instruments; ethylene oxide for heat-sensitive devices). Disinfection: reduces vegetative pathogens on inanimate surfaces; not necessarily kills spores (bleach 1000 ppm on benchtops; glutaraldehyde for endoscopes). Sanitization: reduces microbial load on food contact surfaces to levels safe by public health standards (quaternary ammonium compounds in food processing). Antisepsis: reduces/inhibits microorganisms on living tissue (70% ethanol on skin; iodine for wound care; chlorhexidine for hand scrub).5. Quaternary Ammonium Compounds
Quaternary ammonium compounds (quats) are cationic surfactants that insert into microbial membranes, disrupting membrane integrity and causing leakage of cellular contents. They are bactericidal, fungicidal, and viricidal against enveloped viruses, but less effective against Gram-negative bacteria (due to LPS barrier), endospores, and non-enveloped viruses. They are widely used in food service, healthcare, and veterinary disinfection.
✏️ Fill in the Blank
1. The term for the degree of reduction of microbial populations on inanimate surfaces to levels considered safe by public health standards (without achieving sterility) is _______.
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Sanitization🔘 Multiple Choice
1. UV radiation kills microorganisms primarily by:
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Correct: B) Forming pyrimidine dimers that block DNA replication💬 Open-Ended Questions
1. Explain the VBNC state in bacteria. What triggers it, how are VBNC cells detected, and why is this a concern in water treatment and food safety?
Hint / Guidance
Triggered by: nutrient starvation, temperature shock (too low/high), sub-lethal chlorination, osmotic stress, antibiotic exposure. Detection: direct viable count (nalidixic acid prevents division but not growth → enlarged cells detectable); CTC (5-cyano-2,3-ditolyl tetrazolium chloride) fluorescence for respiring cells; FISH with rRNA probes; flow cytometry with viability dyes (SYBR Green/PI). Concern: standard plate count misses VBNC population (overestimates safety); Vibrio cholerae and E. coli O157 confirmed VBNC states; can resuscitate in host gut; chlorinated drinking water and pasteurised food may contain VBNC pathogens invisible to regulatory monitoring.6. Microbial Characteristics and Microbial Control
Not all microorganisms are equally susceptible to antimicrobial agents. Endospore-forming bacteria (Bacillus, Clostridium) are the most resistant — requiring autoclaving or ethylene oxide for elimination. Mycobacteria (waxy cell wall) and Gram-negative bacteria (outer membrane barrier) show intermediate resistance. Understanding these resistance hierarchies guides the selection of appropriate agents for specific applications.
🔘 Multiple Choice
1. Pasteurization differs from sterilization in that pasteurization:
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Correct: B) Reduces the number of pathogens and spoilage organisms without achieving sterility2. Ethylene oxide is used as a sterilant because it:
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Correct: B) Alkylates proteins and DNA, inactivating all microbial forms including spores💬 Open-Ended Questions
1. You are designing a sterilization protocol for heat-sensitive medical instruments. What alternatives to autoclaving are available? Compare their mechanisms, effectiveness, and limitations.
Hint / Guidance
(1) Ethylene oxide (ETO): alkylates DNA/proteins; effective against spores and viruses; long aeration required to remove toxic residues; carcinogenic risk to operators; slow (8–12 h cycle). (2) Hydrogen peroxide plasma: ionised H₂O₂ produces free radicals; fast (~30–75 min); suitable for plastics/electronics; not for cellulose or liquids. (3) γ-irradiation (Co-60): ionising radiation → DNA double-strand breaks; excellent for bulk disposables; requires large facility; cannot be used in hospitals. (4) Filtration (0.22 µm): removes bacteria from liquids (IV fluids, media) but not viruses or endotoxins. (5) Formaldehyde gas: alkylating; irritant; limited use. Selection based on material compatibility, sterility assurance level required, and throughput.7. Large Scale Fermentor
Industrial fermentors (bioreactors) are precision-engineered systems for maximising microbial productivity. Key parameters — dissolved oxygen, pH, temperature, substrate feed rate, and foam control — must be monitored and controlled in real time. Scale-up from laboratory (litres) to industrial (10,000–200,000 L) scale is not linear: oxygen transfer rates and mixing become critical limiting factors at large scale.
🔘 Multiple Choice
1. The autoclave achieves sterilization using steam at 121°C under pressure. Why is this temperature required rather than boiling at 100°C?
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Correct: B) Bacterial endospores survive boiling water but are destroyed at 121°C under pressure2. The D-value in sterilisation refers to:
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Correct: B) The time required at a given temperature to reduce the microbial population by 90% (one log)💬 Open-Ended Questions
1. Explain the concept of D-value and z-value in thermal death kinetics. How are these values used to design safe sterilization or pasteurization processes?
Hint / Guidance
D-value: time (min) at a given temperature T to reduce population by 1 log (90%) — reflects organism resistance. z-value: temperature increase (°C) required to reduce D-value by 1 log (10-fold) — reflects temperature sensitivity of inactivation. F-value: total thermal lethality of a process in D-value equivalents. Application: for Clostridium botulinum (12D concept for food sterilization), D₁₂₁ ≈ 0.25 min → 12D process requires 12 × 0.25 = 3 min at 121°C. Pasteurization: HTST (72°C/15 s) designed to achieve 5D reduction of Coxiella burnetii (most heat-resistant non-spore-forming pathogen in milk).8. Continuously Stirred Tank Reactor (CSTR)
The Continuously Stirred Tank Reactor (CSTR) maintains a homogeneous environment throughout the vessel through mechanical agitation and aeration. It can be operated in batch (closed system), fed-batch (nutrients added periodically), or continuous mode. Fed-batch operation is the industrial standard for antibiotic and enzyme production, allowing substrate feeding strategies that overcome catabolite repression and maximise secondary metabolite yields.
🔘 Multiple Choice
1. A disinfectant that kills bacteria is bactericidal, while one that only inhibits growth is called:
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Correct: A) Bacteriostatic2. Secondary metabolites in industrial microbiology are characterised by being produced:
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Correct: B) After primary growth (stationary phase) and not directly essential for growth💬 Open-Ended Questions
1. What is the significance of biofilm in the context of antimicrobial resistance and infection control? How do biofilm bacteria resist disinfectants differently from planktonic cells?
Hint / Guidance
Biofilm resistance mechanisms: (1) Diffusion limitation: EPS matrix (polysaccharides, eDNA, proteins) retards penetration of disinfectants (e.g., chlorine reacts with EPS before reaching cells); (2) Persister cells: subpopulation (~1%) in metabolically dormant state (no growth targets for many antibiotics); (3) Altered gene expression: biofilm mode induces stress response genes; (4) Micro-environment gradients: inner cells experience anaerobiosis/nutrient starvation → slow growth → β-lactams ineffective. Infection control: hospital-acquired infections from catheter biofilms (Staphylococcus epidermidis), Pseudomonas on ventilators; require mechanical removal + high-concentration agents; prevention more effective than treatment.9. Microbial Growth in a Chemostat
The chemostat is an elegant tool for continuous culture: fresh medium is pumped in at a constant dilution rate (D), displacing culture at the same rate. At steady state, growth rate equals D — allowing precise, independent control of growth rate. This device is invaluable in research for studying growth physiology, selection experiments, and maintaining defined physiological states for extended periods.
🔘 Multiple Choice
1. UV radiation kills microorganisms primarily by:
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Correct: B) Forming thymine dimers in DNA, blocking replication and transcription2. HEPA filters are used in laminar flow hoods because they remove particles by:
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Correct: C) Physical filtration of particles ≥0.3 µm with ≥99.97% efficiency💬 Open-Ended Questions
1. Describe the industrial production of penicillin by Penicillium chrysogenum using submerged fermentation. Include bioreactor design, key process parameters, and downstream processing steps.
Hint / Guidance
Bioreactor: stirred tank, 10,000–200,000 L; temperature 25–26°C; pH 6.5; dissolved O₂ ≥ 40%; fed-batch mode. Carbon source: glucose (controlled low concentration to avoid catabolite repression) + phenylacetic acid (side-chain precursor). Growth phase: rapid biomass accumulation. Production phase: stationary phase; penicillin secreted. Yield: ~50 g/L modern strains. Downstream: mycelium separation by filtration; solvent extraction at pH 2 (n-butyl acetate or methyl isobutyl ketone); back-extraction into water at pH 6; crystallisation. Side-chain modification to produce semi-synthetic penicillins (ampicillin, amoxicillin).10. Tower Reactor
Tower (bubble column) reactors achieve mixing and aeration without mechanical stirrers by introducing compressed air at the base; rising bubbles drive circulation. They are mechanically simple, energy-efficient, and impose minimal shear stress — making them ideal for shear-sensitive organisms (filamentous fungi, mammalian cells) and large-scale gas-liquid reactions such as single-cell protein production.
🔘 Multiple Choice
1. Which of the following antimicrobial agents acts by disrupting cell membrane integrity?
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Correct: C) Polymyxin B2. The Maillard reaction during autoclaving can reduce sterilisation efficacy when:
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Correct: B) Media containing sugars and amino acids are heated together💬 Open-Ended Questions
1. A brewery uses Saccharomyces cerevisiae to produce beer. The yeast ferments glucose to ethanol and CO₂ via alcoholic fermentation. Explain the metabolic pathway, the conditions used, and why oxygen is initially provided before fermentation begins.
Hint / Guidance
Glycolysis: glucose → 2 pyruvate + 2 ATP + 2 NADH. Fermentation: pyruvate → acetaldehyde (pyruvate decarboxylase) → ethanol (alcohol dehydrogenase) + CO₂; NADH re-oxidised to NAD⁺. Initial oxygen: Pasteur effect — oxygen present initially; yeast uses aerobic respiration for rapid growth (biomass production) and synthesis of sterols/unsaturated fatty acids needed for membrane function; once oxygen depleted, yeast switches to fermentation. Flavour development: formation of esters (ethyl acetate), fusel alcohols (isoamyl alcohol) from amino acid metabolism; yeast strain selection determines flavour profile. Temperature control (10–20°C for lager, 18–24°C for ale) affects ester production.11. Laboratory Process Development Shake Flask Experiments
Shake flask experiments are the first and most cost-effective stage of fermentation process development. Variables such as medium composition, initial pH, carbon-to-nitrogen ratio, and incubation temperature can be screened in parallel across many flasks. Results guide the design of bioreactor runs, where more expensive monitoring and control capabilities are available.
🔘 Multiple Choice
1. In an industrial fed-batch fermentation for antibiotic production, nutrients are added gradually throughout the fermentation. The main reason for this strategy is:
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Correct: B) To maintain substrate concentrations that favour secondary metabolite production while avoiding catabolite repression or overflow metabolism💬 Open-Ended Questions
1. Compare the modes of action of penicillin, tetracycline, and ciprofloxacin as antibiotics. For each, identify the cellular target, mechanism of killing, and the main resistance mechanism bacteria use to overcome them.
Hint / Guidance
Penicillin: inhibits transpeptidase (penicillin-binding proteins) → prevents cross-linking of peptidoglycan → cell lysis (bactericidal); resistance: β-lactamase hydrolysis, altered PBP (MRSA). Tetracycline: binds 30S ribosomal subunit → blocks aminoacyl-tRNA binding → inhibits translation (bacteriostatic); resistance: efflux pumps (tet genes), ribosomal protection proteins. Ciprofloxacin: inhibits DNA gyrase (topoisomerase II) and topoisomerase IV → prevents DNA supercoiling/decatenation → DNA strand breaks (bactericidal); resistance: mutations in gyrA/parC genes, efflux pumps (MexAB-OprM in Pseudomonas).2. Explain the concept of minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC). How are these determined and what clinical significance do they have?
Hint / Guidance
MIC: lowest concentration of antimicrobial that prevents visible growth after 18–24 h incubation (determined by broth microdilution or E-test). MBC: lowest concentration that kills ≥99.9% of the initial inoculum (subculture MIC wells onto agar — no growth = MBC). Clinical significance: guides antibiotic selection and dosing; MBC/MIC ratio >4 = bacteriostatic drug; ratio ≤4 = bactericidal; bactericidal drugs preferred for endocarditis, meningitis, immunocompromised patients. MIC breakpoints set by EUCAST/CLSI define susceptible/resistant categorisation.12. Tray Fermentor
Solid-state fermentation in tray reactors grows microorganisms on moist solid substrates without free water, mimicking natural conditions in soil and decomposing plant material. This approach is used for traditional fermented foods (soy sauce, tempeh), industrial enzyme production (Aspergillus niger on wheat bran for glucoamylase), and biopesticide manufacture. It requires less water and energy than submerged fermentation but is more difficult to control at scale.
🔘 Multiple Choice
1. VBNC bacteria are a public health concern because:
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Correct: B) They cannot be detected by standard culture-based monitoring but can resuscitate and cause infection when conditions improve💬 Open-Ended Questions
1. Explain how the Kirby-Bauer disk diffusion test works and what its results mean. What are the limitations of this method compared to minimum inhibitory concentration (MIC) determination?
Hint / Guidance
Principle: standardised inoculum (0.5 McFarland = 10⁸ CFU/mL) spread on Mueller-Hinton agar; antibiotic-impregnated disks placed; incubate 18 h at 35°C; antibiotic diffuses creating gradient; zone of inhibition (mm) measured. Interpretation: zone diameter compared to CLSI/EUCAST breakpoint tables → S (susceptible), I (intermediate), R (resistant). Limitations: does not give exact MIC; affected by agar depth, medium composition, inoculum density; cannot be used for slow-growing organisms; zone size does not predict in vivo outcomes. MIC (broth microdilution): precise quantitative result; essential for serious infections, pharmacokinetic/pharmacodynamic dosing decisions, resistance surveillance.2. How is penicillin produced industrially? Describe the fermentation process including the organism, substrates, fermentation conditions, and downstream processing.