At the heart of industrial biotechnology is the bioreactor: a controlled vessel in which microorganisms grow and produce the desired product under optimised conditions. Understanding how fermentation works, what the key process parameters are, how yield and productivity are maximised, and what the downstream processing steps look like is essential knowledge for industrial microbiologists, bioprocess engineers, and pharmaceutical manufacturing professionals.
Batch, Fed-Batch, and Continuous Fermentation
The three fundamental fermentation modes differ in how nutrients are supplied and product is harvested:
Batch fermentation: all nutrients are added at the start, and the fermentation proceeds through lag, exponential, stationary, and decline phases without addition of further medium. Simple to set up, easy to validate, low contamination risk. Disadvantage: once nutrients are depleted, growth stops. Primary and secondary metabolite production typically peaks at different phases (primary metabolites produced during exponential growth, secondary metabolites during stationary phase). Penicillin fermentation historically used batch culture.
Fed-batch fermentation: the most common mode in pharmaceutical and high-value fermentation. Nutrients are fed continuously or intermittently during the fermentation, extending the productive phase and allowing higher cell densities than batch culture. The feed rate can be controlled to maintain substrate at a low concentration (preventing catabolite repression and overflow metabolism). Recombinant protein production (e.g., recombinant insulin in E. coli or yeast) typically uses fed-batch.
Continuous fermentation (chemostat): medium is continuously added and culture removed at equal rates, maintaining a constant culture volume at a defined growth rate (the dilution rate D = flow rate/culture volume). At steady state, growth rate equals dilution rate. The chemostat provides a precisely controlled environment ideal for research but has higher contamination risk and is rarely used in pharmaceutical manufacturing. Used for: research into growth physiology, single-cell protein production, and some brewing applications.
Key Bioreactor Parameters and Control
The bioreactor maintains optimal conditions for microbial growth and product formation through monitoring and control of several critical process parameters (CPPs):
Temperature: each organism has an optimal temperature for growth and for product formation (which may differ from growth optimum). Mesophilic bacteria (E. coli, Saccharomyces cerevisiae): 30 to 37 degrees Celsius. Thermophilic organisms: 50 to 80 degrees Celsius (used in bioethanol production from lignocellulose). Temperature is controlled by a heating jacket and cooling water coils.
pH: most bacteria prefer pH 6.5 to 7.5. Acid or base addition (controlled by pH probes) maintains pH within the set point range. In penicillin fermentation, pH regulation is critical: acid production from glucose metabolism tends to lower pH; base addition maintains pH at approximately 6.5, which is optimal for penicillin production by Penicillium chrysogenum.
Dissolved oxygen (DO): monitored by polarographic or optical DO probes. Maintained above a critical level (typically 30 per cent air saturation for aerobic organisms) by agitation (impeller stirring) and aeration (sparging sterile air or oxygen through the culture). The oxygen transfer rate (OTR = kLa × C*-Cx) is a key engineering parameter in scale-up: maintaining adequate oxygen supply at large scale (where surface-to-volume ratio decreases) requires careful agitator and sparger design.
Substrate (carbon source) concentration: often maintained at low concentrations in fed-batch to avoid catabolite repression (excess glucose represses the synthesis of enzymes for other carbon sources in many bacteria) and overflow metabolism (aerobic production of lactate, acetate, or ethanol at high glucose concentrations in E. coli).
Foaming: surface-active compounds (proteins, phospholipids) produced during fermentation cause foaming, which can reduce oxygen transfer and cause culture overflow. Mechanical foam control (foam breaker impeller) and chemical antifoam agents (silicone-based, polypropylene glycol) are used.
Antibiotic Fermentation: Penicillin as the Example
Penicillin G (and V) are produced by Penicillium chrysogenum (the original source of Fleming's penicillin). The industrial organism has been extensively developed by classical mutation and selection programmes over 80 years: current industrial strains produce approximately 50 to 70 g/L penicillin, compared to approximately 0.002 g/L from the original Fleming strain. This represents a 25,000-fold improvement through mutagenesis, selection, and metabolic engineering.
Penicillin is a secondary metabolite: produced not during exponential growth (primary metabolite) but during the stationary and decline phases, when cells are nitrogen-limited. The biosynthetic gene cluster for penicillin (pcbAB, pcbC, penDE) is upregulated by nitrogen limitation and pH. The precursor phenylacetic acid (PAA) must be supplied in the feed to produce penicillin G (PAA is incorporated into the side chain).
Fed-batch culture is the standard production mode: glucose and PAA are fed continuously, maintaining low substrate concentrations. Batch culture lasts approximately 5 to 7 days, with penicillin production occurring in the last 3 to 5 days. The broth is then harvested by filtration, and penicillin is extracted into organic solvent, purified, and crystallised.
Recombinant Protein Production
Recombinant proteins (insulin, erythropoietin, monoclonal antibodies, vaccines) are produced by microorganisms or mammalian cells engineered to express the gene of interest. The choice of expression system determines post-translational modifications, product folding, and yield:
E. coli: the simplest and fastest system. Produces high yields of bacterial proteins and small recombinant proteins. Does not perform glycosylation (so cannot produce correctly glycosylated human proteins). Recombinant insulin is produced in E. coli as inclusion bodies (aggregated misfolded protein), then denatured, refolded, and processed to produce active insulin.
Saccharomyces cerevisiae and Pichia pastoris: yeast expression systems that perform glycosylation (though yeast glycosylation patterns differ from mammalian patterns). Used for some therapeutic glycoproteins including hepatitis B surface antigen (the HBV vaccine component) and recombinant erythropoietin (some forms).
Chinese Hamster Ovary (CHO) cells: the dominant expression system for complex therapeutic glycoproteins and monoclonal antibodies, because CHO cells perform human-compatible glycosylation, which is critical for antibody effector function and pharmacokinetics. Over 70 per cent of approved therapeutic proteins are produced in CHO cells. Lower productivity and more expensive culture conditions than microbial systems but required for correctly glycosylated products.
Frequently Asked Questions
What is a bioreactor?
A bioreactor is a controlled vessel in which microorganisms or animal cells are cultivated under precisely controlled conditions (temperature, pH, dissolved oxygen, substrate concentration, agitation) to produce a desired product at scale. Bioreactors range from 1-litre laboratory-scale vessels to 200,000-litre industrial fermenters used for antibiotic or amino acid production.
What is the difference between batch and fed-batch fermentation?
Batch fermentation adds all nutrients at the start and allows the culture to proceed until nutrients are exhausted. Fed-batch fermentation adds nutrients continuously or intermittently during the run, extending the productive phase and allowing higher cell densities and product yields. Fed-batch is the dominant mode in pharmaceutical fermentation.
Why is penicillin a secondary metabolite?
Secondary metabolites are produced not as part of the primary growth-essential metabolism (amino acids, nucleotides: primary metabolites) but as specialised products, often during nutrient-limited (stationary) growth phases. Penicillin production by P. chrysogenum is induced by nitrogen limitation and increases as glucose is consumed. The ecological role of antibiotic secondary metabolites is competitive exclusion of other microorganisms in the natural environment.
What is catabolite repression?
Catabolite repression is the suppression of genes encoding enzymes for alternative carbon source utilisation when a preferred carbon source (usually glucose) is present in excess. In E. coli, excess glucose represses the lac operon (preventing lactose utilisation) via cAMP-CRP transcriptional regulation. In fermentation processes, high glucose concentrations cause catabolite repression of secondary metabolite biosynthetic genes, reducing antibiotic or enzyme yields. Low glucose concentration in fed-batch culture prevents catabolite repression.
Why is dissolved oxygen critical in aerobic fermentation?
Dissolved oxygen is the terminal electron acceptor for aerobic respiration, required for ATP generation and for many biosynthetic reactions. If DO falls below the critical level (typically 10 to 30 per cent air saturation depending on the organism), growth rate and product formation decline, and fermentative by-products (lactate, acetate) may accumulate in organisms capable of mixed-acid fermentation. Maintaining adequate DO requires careful agitation and aeration control, particularly challenging at large scale.
What is E. coli used for in industrial biotechnology?
E. coli is one of the most widely used industrial organisms because of its fast growth rate (doubling time approximately 20 minutes), well-understood genetics, ease of genetic manipulation, and high recombinant protein yields. It is used for: recombinant insulin production, amino acid synthesis (threonine, tryptophan), industrial enzymes, and small molecule biosynthesis. It does not perform glycosylation, limiting its use for complex mammalian glycoproteins.
What are CHO cells and why are they used for monoclonal antibodies?
Chinese Hamster Ovary (CHO) cells are mammalian cells derived from the ovary of the Chinese hamster, engineered to express therapeutic proteins. They are used for monoclonal antibody production (and other complex glycoproteins) because they perform glycosylation that is compatible with human immune receptors, which is critical for antibody effector function, pharmacokinetics, and immunogenicity. Over 70 per cent of approved biotherapeutic proteins are produced in CHO cells.
What is downstream processing in fermentation?
Downstream processing (DSP) refers to the steps that recover, purify, and formulate the product after fermentation. Typical DSP sequence: harvest (centrifugation or filtration to separate cells from broth), cell disruption (if product is intracellular), initial purification (precipitation, filtration), chromatographic purification (affinity, ion exchange, size exclusion), viral clearance steps (for mammalian cell products), final formulation (buffer exchange, concentration, excipient addition), and fill and finish (aseptic filling into vials or syringes).
What is the kLa (volumetric mass transfer coefficient)?
kLa is the volumetric oxygen mass transfer coefficient, measuring the rate at which oxygen is transferred from the gas phase to the liquid phase in a bioreactor. It depends on agitation rate (increasing kLa by dispersing gas bubbles and increasing turbulence), aeration rate (sparging more air), bubble size (smaller bubbles have higher surface area), and liquid viscosity. Maximising kLa is a key design objective in aerobic fermentation scale-up.
What is the Pasteur effect?
The Pasteur effect is the observation that the presence of oxygen inhibits fermentation (anaerobic glycolysis) in facultative anaerobes such as yeast and E. coli, directing carbon flux toward oxidative phosphorylation. Under anaerobic conditions, yeast produces ethanol and CO2 (fermentation). Under aerobic conditions, ethanol production is suppressed and oxidative catabolism predominates. The Crabtree effect (or overflow metabolism) is the complementary phenomenon: even under aerobic conditions, at high glucose concentrations, yeast and E. coli produce ethanol or acetate respectively due to saturation of the TCA cycle.