Aerobic cultivation of Streptococcus zooepidemicus and the role of NADH oxidase

Tapping on the Potential of Hyaluronic Acid: from Production to Application

The manufacture, purification, and applications of hyaluronic acid (HA) are discussed in this article. Concerning the growing need for affordable, high-quality HA, it is essential to consider diverse production techniques using renewable resources that pose little risk of cross-contamination. Many microorganisms can now be used to produce HA without limiting the availability of raw materials and in an environmentally friendly manner. The production of HA has been associated with Streptococci A and C, explicitly S. zooepidemicus and S. equi. Different fermentation techniques, including the continuous, batch, fed-batch, and repeated batch culture, have been explored to increase the formation of HA, particularly from S. zooepidemicus. The topic of current interest also involves a complex broth rich in metabolites and residual substrates, intensifying downstream processes to achieve high recovery rates and purity. Although there are already established methods for commercial HA production, the anticipated growth in trade and the diversification of application opportunities necessitate the development of new procedures to produce HA with escalated productivity, specified molecular weights, and purity. In this report, we have enacted the advancement of HA technical research by analyzing bacterial biomanufacturing elements, upstream and downstream methodologies, and commercial-scale HA scenarios.

Hyaluronic acid production by Klebsiella pneumoniae strain H15 (OP354286) under different fermentation conditions

BACKGROUND

Hyaluronic acid (HA) has gained significant attention due to its unique physical, chemical, and biological properties, making it widely used in various industries. This study aimed to screen bacterial isolates for HA production, characterize favorable fermentation conditions, and evaluate the inhibitory effect of bacterial HA on cancer cell lines.

RESULTS

A total of 108 bacterial isolates from diverse sources were screened for HA production using HPLC, turbidimetric, and carbazole determination methods. Among the HA-producing isolates, Klebsiella pneumoniae H15 isolated from an animal feces sample, was superior in HA production. The strain was characterized based on its morphological, cultural, and biochemical characteristics. Molecular identification using 16S rDNA sequencing and phylogenetic analysis confirmed its identity. Fermentation conditions, including pH, temperature, time, and agitation rate, were optimized to maximize HA production. The basal medium, comprising sucrose (7.0%) as carbon source and combined yeast extract with peptone (1.25% each) as nitrogen substrate, favored the highest HA production at pH 8.0, for 30 h, at 30 °C, under shaking at 180 rpm. The average maximized HA concentration reached 1.5 g L-1. Furthermore, bacterial HA exhibited a significant inhibitory effect on three cancer cell lines (MCF-7, HepG-2 and HCT), with the lowest concentration ranging from 0.98-3.91 µg mL-1.

CONCLUSIONS

K. pneumoniae H15, isolated from animal feces demonstrated promising potential for HA production. The most favorable fermentation conditions led to a high HA production. The inhibitory effect of bacterial HA on cancer cell lines highlights its potential therapeutic applications. These findings contribute to a broader understanding and utilization of HA in various industries and therapeutic applications.

Production of isotopically enriched high molecular weight hyaluronic acid and characterization by solid-state NMR

Hyaluronic acid (HA) is a naturally occurring polysaccharide that is abundant in the extracellular matrix (ECM) of all vertebrate cells. HA-based hydrogels have attracted great interest for biomedical applications due to their high viscoelasticity and biocompatibility. In both ECM and hydrogel applications, high molecular weight (HMW)-HA can absorb a large amount of water to yield matrices with a high level of structural integrity. To understand the molecular underpinnings of structural and functional properties of HA-containing hydrogels, few techniques are available. Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for such studies, e.g. ¹³C NMR measurements can reveal the structural and dynamical features of (HMW) HA. However, a major obstacle to ¹³C NMR is the low natural abundance of ¹³C, necessitating the generation of HMW-HA that is enriched with ¹³C isotopes. Here we present a convenient method to obtain ¹³C- and ¹⁵N-enriched HMW-HA in good yield from Streptococcus equi subsp. zooepidemicus. The labeled HMW-HA has been characterized by solution and magic angle spinning (MAS) solid-state NMR spectroscopy, as well as other methods. These results will open new ways to study the structure and dynamics of HMW-HA-based hydrogels, and interactions of HMW-HA with proteins and other ECM components, using advanced NMR techniques.

Comprehensive review on biotechnological production of hyaluronic acid: status, innovation, market and applications

The growing, existing demand for low-cost and high-quality hyaluronic acid (HA) needs an outlook of different possible production strategies from renewable resources with the reduced possibility of cross-infections. Recently, the possibility of producing HA from harmless microorganisms appeared, which offers the opportunity to make HA more economical, without raw material limitations, and environmentally friendly. HA production is mainly reported with Lancefield Streptococci A and C, particularly from S. equi and S. zooepidemicus. Various modes of fermentation such as batch, repeated batch, fed-batch, and continuous culture have been investigated to optimize HA production, particularly from S. zooepidemicus, obtaining a HA yield of 2.5 g L⁻¹ – 7.0 g L⁻¹. Among the different utilized DSP approaches of HA production, recovery with cold ethanol (4°C) and cetylpyridinium chloride is the ideal strategy for lab-scale HA production. On the industrial scale, besides using isopropanol, filtration (0.22 um), ultrafiltration (100 kDa), and activated carbon absorption are employed to obtain HA of low molecular weight and additional ultrafiltration to purify HA of higher MW. Even though mature technologies have already been developed for the industrial production of HA, the projections of increased sales volume and the expansion of application possibilities require new processes to obtain HA with higher productivity, purity, and specific molecular weights. In this review, we have put forth the progress of HA technological research by discussing the microbial biosynthetic aspects, fermentation and downstream strategies, industrial-scale scenarios of HA, and the prospects of HA production to meet the current and ongoing market demands.

The dilution effect of media culture on mixing time, Kla O2, and hyaluronic acid production in S. zooepidemicus fed-batch culture

OBJECTIVES

Microbial production of biopolymers is typically associated with high viscosity and suitable mixing plays an important role in their production. Due to the nature of Streptococcus strains in high production of lactic acid and consequently high consumption of NaOH, which is associated with increased viscosity and reduced mixing caused by hyaluronic acid production, the injected NaOH accumulates and causes cells loss, and decreases in quantity and quality of the produced hyaluronic acid.

RESULTS

In this study, the effect of increasing dilution of media culture of Streptococcus zooepidemicus fed-batch culture during pH control by NaOH on mixing time, volumetric oxygen transfer coefficient, and increasing hyaluronic acid production in a 2-L fermenter were studied. The results showed that significant increasing dilution causes reduction mixing time, remarkable improvement volumetric oxygen transfer coefficient, hyaluronic acid production enhancement from 6.6 to 8.4 g/L, and diminution the consumption of NaOH.

CONCLUSION

Dilution of media culture of S. zooepidemicus fed-batch culture by the pH controlling agent achieved one of the highest amounts of hyaluronic acid that was reported recently. This method does not require any automatic control and can be used at a low cost to produce other soluble extracellular biopolymers.

Enhanced hyaluronic acid production in Streptococcus zooepidemicus by an optimized culture medium containing hyaluronidase inhibitor

This study describes the hyaluronic acid (HA) production by S. zooepidemicus ATCC 43079, and the effect of the hyaluronidase enzyme on HA levels. The hyaluronidase production, glucose consumption, and lactate formation were recorded during fermentation. The HA production, and productivity at different amounts of glucose, yeast extract and pH were evaluated by response surface statistical approach in presence of 6-O-palmytoil-l-ascorbic acid as a chemical inhibitor for biocatalyst hyaluronidase. Under optimum conditions, HA production was increased two-fold from 190 ± 17 mg L^-1 in basal medium to 384.6 ± 7.5 mg L^-1 in the optimized medium containing enzyme inhibitor. Furthermore, the results indicated that the chemical inhibitor could suppress the biocatalyst activity and prevent the HA loss at the end of the exponential phase of S. zooepidemicus ATCC 43079.

Biomedical Applications of Bacteria-Derived Polymers

Plastics have found widespread use in the fields of cosmetic, engineering, and medical sciences due to their wide-ranging mechanical and physical properties, as well as suitability in biomedical applications. However, in the light of the environmental cost of further upscaling current methods of synthesizing many plastics, work has recently focused on the manufacture of these polymers using biological methods (often bacterial fermentation), which brings with them the advantages of both low temperature synthesis and a reduced reliance on potentially toxic and non-eco-friendly compounds. This can be seen as a boon in the biomaterials industry, where there is a need for highly bespoke, biocompatible, processable polymers with unique biological properties, for the regeneration and replacement of a large number of tissue types, following disease. However, barriers still remain to the mass-production of some of these polymers, necessitating new research. This review attempts a critical analysis of the contemporary literature concerning the use of a number of bacteria-derived polymers in the context of biomedical applications, including the biosynthetic pathways and organisms involved, as well as the challenges surrounding their mass production. This review will also consider the unique properties of these bacteria-derived polymers, contributing to bioactivity, including antibacterial properties, oxygen permittivity, and properties pertaining to cell adhesion, proliferation, and differentiation. Finally, the review will select notable examples in literature to indicate future directions, should the aforementioned barriers be addressed, as well as improvements to current bacterial fermentation methods that could help to address these barriers.

Enhancement of acetyl-CoA by acetate co-utilization in recombinant Lactococcus lactis cultures enables the production of high molecular weight hyaluronic acid

The molecular weight of hyaluronic acid (HA) is a critical property which determines its usage in various biomedical applications. This study investigates the correlation between the availability of a critical cofactor, acetyl-CoA, the concentration of a limiting precursor, UDP-N-acetylglucosamine (UDP-GlcNAc), and the molecular weight of HA (MW_HA) produced by recombinant Lactococcus lactis MKG6 cultures. This strain expressed three heterologous HA-pathway genes obtained from the has operon of Streptococcus zooepidemicus in an ldh-mutant host strain, L. lactis NZ9020. A flux balance analysis, performed using the L. lactis genome-scale metabolic network, showed a positive correlation of acetyl-CoA flux with the UDP-GlcNAc flux and the experimental data on HA productivity. To increase the intracellular levels of acetyl-CoA, acetate was supplemented as a pulse feed in anaerobic batch cultures. However, acetate is effectively utilized only in the presence of glucose and exhaustion of glucose resulted in decreasing the final MW_HA (1.5 MDa). Co-supplementation of acetate resulted in enhancing the acetyl-CoA and UDP-GlcNAc levels as well as the MW_HA to 2.5 MDa. This logic was extended to fed-batch cultures, designed with a pH-based feedback control of glucose feeding and pulse acetate supplementation. When the glucose feed concentration was optimally adjusted to prevent glucose exhaustion or accumulation, the acetate utilization was found to be high, resulting in significantly enhanced levels of acetyl-CoA and UDP-GlcNAc as well as a MW_HA of 3.4 MDa, which was sustained at this value throughout the process. This study provides the possibility of commercially producing high MW_HA using recombinant L. lactis strains.

Improvement Production of Hyaluronic Acid by Streptococcus zooepidemicus in Sugarcane Molasses

Microbial hyaluronic acid (HA) production has been preferred rather than extraction from animal tissue for medical and cosmetic applications. In this context, to obtain an economically competitive HA production by Streptococcus zooepidemicus, culture conditions were studied to improve the polymer production in sugarcane molasses. The highest HA production by S. zooepidemicus ATCC 39920 achieved was 2.825 g. L^-1 in a 4.5 L bioreactor with controlled pH (8.0) and medium containing molasses (85.35 g.L^-1 total sugar) pretreated with activated charcoal and yeast extract (50 g.L^-1). The HA produced exhibited a high molecular weight of 1.35 × 10³ kDa and the DPPH radical scavenging activity of the polymer at 1 g.L^-1 was 41 %. The FTIR and UV-Vis spectra showed no substantial differences in the spectral pattern between produced and standard HA. This study is a promising strategy for sugarcane molasses application by producing high value-added products such as hyaluronic acid.

Ratio of intracellular precursors concentration and their flux influences hyaluronic acid molecular weight in Streptococcus zooepidemicus and recombinant Lactococcus lactis

HA molecular weight variation in Streptococcus zooepidemicus and two recombinant Lactococcus lactis strains were investigated by chemostat experiments and metabolic flux analysis (MFA). The study showed that intracellular flux ratio of UDP-GlcUA to UDP-GlcNAc correlated directly with HA molecular weight, for all the three strains. The ratio of intracellular concentration of these HA precursors also exhibited a similar trend. Phosphoglucoisomerase activity and glucose flux towards lactic acid formation were found to be the major bottlenecks for HA production in all the three strains. The study suggests that environmental conditions and genetic manipulations that balance the intracellular flux and HA precursors concentrations will result in increased molecular weight.

Production Methods for Hyaluronan

Hyaluronan is a polysaccharide with multiple functions in the human body being involved in creating flexible and protective layers in tissues and in many signalling pathways during embryonic development, wound healing, inflammation, and cancer. Hyaluronan is an important component of active pharmaceutical ingredients for treatment of, for example, arthritis and osteoarthritis, and its commercial value far exceeds that of other microbial extracellular polysaccharides. Traditionally hyaluronan is extracted from animal waste which is a well-established process now. However, biotechnological synthesis of biopolymers provides a wealth of new possibilities. Therefore, genetic/metabolic engineering has been applied in the area of tailor-made hyaluronan synthesis. Another approach is the controlled artificial (in vitro) synthesis of hyaluronan by enzymes. Advantage of using microbial and enzymatic synthesis for hyaluronan production is the simpler downstream processing and a reduced risk of viral contamination. In this paper an overview of the different methods used to produce hyaluronan is presented. Emphasis is on the advancements made in the field of the synthesis of bioengineered hyaluronan.

Microbial production of hyaluronic acid: current state, challenges, and perspectives

Hyaluronic acid (HA) is a natural and linear polymer composed of repeating disaccharide units of β-1, 3-N-acetyl glucosamine and β-1, 4-glucuronic acid with a molecular weight up to 6 million Daltons. With excellent viscoelasticity, high moisture retention capacity, and high biocompatibility, HA finds a wide-range of applications in medicine, cosmetics, and nutraceuticals.

Traditionally HA was extracted from rooster combs, and now it is mainly produced via streptococcal fermentation. Recently the production of HA via recombinant systems has received increasing interest due to the avoidance of potential toxins. This work summarizes the research history and current commercial market of HA, and then deeply analyzes the current state of microbial production of HA by Streptococcus zooepidemicus and recombinant systems, and finally discusses the challenges facing microbial HA production and proposes several research outlines to meet the challenges.

Understanding the influence of phosphatidylcholine on the molecular weight of hyaluronic acid synthesized by Streptococcus zooepidemicus

The effect of phosphatidylcholine on the molecular weight properties of hyaluronic acid (HA) was studied in batch culture of Streptococcus zooepidemicus by adding phosphatidylcholine at the early stage of exponential phase. With the addition of 80 mg/L of phosphatidylcholine, maximum HA yield (2.47 g/L) and weight-average molecular weight (902.60 KDa) were achieved, increased by 17.4% and 67.1%, respectively, as compared to the control. Metabolic flux analysis was employed to study the mechanism of phosphatidylcholine on the molecular weight of HA. The normalized flux distribution maps based on fermentation data at phosphatidylcholine addition indicated that phosphatidylcholine resulted in higher flux flowing to the HA pathway and lower flux flowing to the glycolysis and biomass synthesis pathway, coupling with higher level of UDPNAG generation and extra regeneration of ATP. The GC-MS analysis of fatty acids in the plasma membrane showed that the addition of phosphatidylcholine could promote the mobility and permeability of the cell membrane, making the HA chain pass through the membrane more easily, thus decreasing the energy consumption. All these results led to higher molecular weight of hyaluronic acid.

Hyaluronan: From Biomimetic to Industrial Business Strategy

Hyaluronan (hyaluronic acid) is a naturally occurring polysaccharide of a linear repeating disaccharide unit consisting of β-(1→4)-linked D-glucopyranuronic acid and β-(1→3)-linked 2-acetamido-2-deoxy-D-glucopyranose, which is present in extracellular matrices, the synovial fluid of joints, and scaffolding that comprises cartilage.

In its mechanism of synthesis, its size, and its physico-chemical properties, hyaluronan is unique amongst other glycosaminoglycans. The network-forming, viscoelastic and its charge characteristics are important to many biochemical properties of living tissues. It is an important pericellular and cell surface constituent; its interaction with other macromolecules such as proteins, participates in regulating cell behavior during numerous morphogenic, restorative, and pathological processes in the body. The knowledge of HA in diseases such as various forms of cancers, arthritis and osteoporosis has led to new impetus in research and development in the preparation of biomaterials for surgical implants and drug conjugates for targeted delivery.

A concise and focused review on hyaluronan is timely. This review will cover the following important aspects of hyaluronan: (i) biological functions and synthesis in nature; (ii) current industrial production and potential biosynthetic processes of hyaluronan; (iii) chemical modifications of hyaluronan leading to products of commercial significance; and (iv) and the global market position and manufacturers of hyaluronan.

Proteomic investigation of the impact of oxygen on the protein profiles of hyaluronic acid-producing Streptococcus zooepidemicus

Hyaluronic acid (HA) is a linear and negatively charged polysaccharide regularly used in medicine and cosmetics. Recently Streptococcus zooepidemicus has been exploited in the fermentation industry to produce HA. Many studies showed that higher amounts of HA were produced under aerobic condition compared to anaerobic conditions. To explore the effect of oxygen on the HA synthesis in S. zooepidemicus, 2-DE was used to compare the proteomes of aerobically and anaerobically fermented bacteria to identify proteins, which might be associated with the influence of oxygen on the HA synthesis. Totally nine pairs of 2-DE gels collected from three batches were compared and nine overexpressed proteins were observed in aerobically fermented bacteria. These proteins were identified by LC/tandem MS as dihydrolipoamide dehydrogenase, UDP-acetyl-glucosamine pyrophosphoylase, dihydrolipoamide-S-acetyltransferase and acetoin dehydrogenase alpha and beta chains, respectively. These upregulated proteins were involved in acetoin dissimilation, the central carbon metabolism and the HA anabolic pathway, implicating that oxygen might augment the expression of genes that are involved in central energy metabolism, acetoin reutilization and HA biosynthesis to enhance the amount of acetyl-CoA as such that more acetyl-CoA can be diverged from the central carbon metabolism to replenish acetyl-CoA for the HA synthesis.

Hyaluronan molecular weight is controlled by UDP-N-acetylglucosamine concentration in Streptococcus zooepidemicus

The molecular weight of hyaluronan is important for its rheological and biological function. The molecular mechanisms underlying chain termination and hence molecular weight control remain poorly understood, not only for hyaluronan synthases but also for other beta-polysaccharide synthases, e.g. cellulose, chitin, and 1,3-betaglucan synthases. In this work, we manipulated metabolite concentrations in the hyaluronan pathway by overexpressing the five genes of the hyaluronan synthesis operon in Streptococcus equi subsp. zooepidemicus. Overexpression of genes involved in UDP-glucuronic acid biosynthesis decreased molecular weight, whereas overexpression of genes involved in UDP-N-acetylglucosamine biosynthesis increased molecular weight. The highest molecular mass observed was at 3.4 +/- 0.1 MDa twice that observed in the wild-type strain, 1.8 +/- 0.1 MDa. The data indicate that (a) high molecular weight is achieved when an appropriate balance of UDP-N-acetylglucosamine and UDP-glucuronic acid is achieved, (b) UDP-N-acetylglucosamine exerts the dominant effect on molecular weight, and (c) the wild-type strain has suboptimal levels of UDP-N-acetylglucosamine. Consistent herewith molecular weight correlated strongly (rho = 0.84, p = 3 x 10(-5)) with the concentration of UDP-N-acetylglucosamine. Data presented in this paper represent the first model for hyaluronan molecular weight control based on the concentration of activated sugar precursors. These results can be used to engineer strains producing high molecular weight hyaluronan and may provide insight into similar polymerization mechanisms in other polysaccharides.

Microbial production of low molecular weight hyaluronic acid by adding hydrogen peroxide and ascorbate in batch culture of Streptococcus zooepidemicus

Microbial production of low molecular weight hyaluronic acid (HA) by the addition of hydrogen peroxide and ascorbate during the batch culture of Streptococcus zooepidemicus was investigated. Hydrogen peroxide (1.0 mmol/g HA) and ascorbate (0.5 mmol/g HA) were added at 8h and 12h to degrade HA. With the redox depolymerization of HA, the HA molecular weight decreased from 1,300 kDa for the control to 80 kDa, and the average broth viscosity during 8-16 h decreased from 360 mPa s for the control to 290 mPa s. The average oxygen mass transfer coefficient K(L)a increased from 10h(-1) for the control to 35 h(-1) and the average dissolved oxygen level increased from 1% of air saturation in the control to 10%. HA production increased from 5.0 g/L for the control to 6.5 g/L, and contributed to the increased redox potential and energy charge. This novel process not only significantly enhanced production of low molecular weight HA, but also improved purification efficiency due to a decreased broth viscosity. Low molecular weight HA finds applications in biomedical and healthcare fields.

Binding of bivalent cations by hyaluronate in aqueous solution

The interaction between sodium hyaluronate and bivalent cations was investigated by conductometry, viscosimetry, circular dichroism and nuclear magnetic resonance spectroscopy. It is shown that the hyaluronate chains (Mn approximately 4.0 x 10(5)-1.7 x 10(6)g/mol) bind various bivalent cations (Ca2+, Mg2+, Mn2+, Fe2+, Cu2+, Zn2+, Cd2+ and Pb2+) at pH 6 in aqueous solutions. Hyaluronate deriving from Streptococcus equi was studied in comparison with dextran from Leuconostoc mesenteroides which was shown to develop no specific interactions with the bivalent cations. The molar relation between the bivalent cations and the disaccharide units of the resulting complex was determined with the result that one bivalent cation is bound by approximately five disaccharide units. Heavy metal ions (Cd2+, Pb2+) seem to bind stronger to the hyaluronate chain than their lighter counterparts (Ca2+, Mg2+). Circular dichroism spectra of the hyaluronate exhibit a cation-induced change in the n-pi* transition, indicating that the acetamide group of the aminoglucane unit is involved during the complexation. NMR spectra of hyaluronic acid in presence of paramagnetic manganese cations show strong interactions between the acetamide as well as the carboxylate groups and the cations. Based on these data, a structure of the binding complex is proposed which involves two disaccharide units.

Effect of expressing polyhydroxybutyrate synthesis genes (phbCAB) in Streptococcus zooepidemicus on production of lactic acid and hyaluronic acid

Hyaluronic acid (HA) has been industrially produced using the gram-positive bacterium Streptococcus zooepidemicus. Large amount of lactic acid formation was one of the important factors that restricted cell growth and HA productivity and lowered the substrate to HA conversion efficiency in a fermentor. In this study, polyhydroxybutyrate (PHB) synthesis genes (phbCAB) of Ralstonia eutropha were cloned from the plasmid pBHR68 and were inserted into the plasmid pEU308, an expression vector for gram-positive bacteria. The plasmid was transformed into S. zooepidemicus by electroporation. beta-Ketothiolase (PhbA), acetoacetyl-CoA reductase (PhbB), and polyhydroxyalkanoate (PHA) synthase (PhbC) activity assays were carried out to demonstrate the expression of these genes. The PhbA and PhbB activities were 3.13 and 1.23 U mg(-1), respectively. No PhbC activities were detected. In shake flask studies, there was no obvious difference between the wild-type and recombinant S. zooepidemicus harboring phbCAB genes in terms of lactic acid and HA formation. However, in fermentor studies, the recombinant produced only 40 g L(-1) lactic acid and 7.5 g L(-1) HA, whereas the wild type produced 65 g L(-1) lactic acid and 5.5 g L(-1) HA. These results suggested that expression of phbCAB genes in S. zooepidemicus could help regulate HA production metabolism. Because the lactic acid formation in S. zooepidemicus was sensitive to cellular oxidation/reduction potential, it is proposed that the PHB synthesis pathway could act as a regulator to adjust the cellular oxidation/reduction potential. This is the first study demonstrating that PHA synthesis related to energy and carbon metabolism could be employed as a pathway to regulate other cellular metabolism and possibly to regulate the production of other metabolic products.

Stable production of hyaluronic acid inStreptococcus zooepidemicus chemostats operated at high dilution rate

Hyaluronic acid is routinely produced through fermentation of both Group A and C streptococci. Despite significant production costs associated with short fermentations and removal of contaminating proteins released during entry into stationary phase, hyaluronic acid is typically produced in batch rather than continuous culture. The main reason is that hyaluronic acid synthesis has been found to be unstable in continuous culture except at very low dilution rates. Here, we investigated the mechanisms underlying this instability and developed a stable, high dilution rate (0.4 h-1) chemostat process for both chemically defined and complex media operating for more than 150 h of production. In chemically defined medium, the product yield was 25% higher in chemostat cultures than in conventional batch culture when arginine or glucose was the limiting substrate. In contrast, glutamine limitation resulted in higher ATP requirements and a yield similar to that observed in batch culture. In complex, glucose-limited medium, ATP requirements were greatly reduced but biomass synthesis was favored over hyaluronic acid and no improvement in hyaluronic acid yield was observed. The successful establishment of continuous culture at high dilution rate enables both commercial production at reduced cost and a more rational characterization and optimization of hyaluronic acid production in streptococci.

Microbial hyaluronic acid production

Hyaluronic acid (HA) is a commercially valuable medical biopolymer increasingly produced through microbial fermentation. Viscosity limits product yield and the focus of research and development has been on improving the key quality parameters, purity and molecular weight. Traditional strain and process optimisation has yielded significant improvements, but appears to have reached a limit. Metabolic engineering is providing new opportunities and HA produced in a heterologous host is about to enter the market. In order to realise the full potential of metabolic engineering, however, greater understanding of the mechanisms underlying chain termination is required.