Patent Publication Number: US-11376315-B2

Title: Method for controlling Streptococcus pneumoniae polysaccharide molecular weight using carbon dioxide

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 15/493,806, filed Apr. 21, 2017, which is a continuation of U.S. application Ser. No. 14/845,065, filed Sep. 3, 2015, which is a continuation of U.S. application Ser. No. 12/640,666, filed Dec. 17, 2009, which claims the benefit of U.S. Provisional Application No. 61/138,570, filed Dec. 18, 2008, all of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to methods for increasing the molecular weight of isolated  Streptococcus pneumoniae  capsular polysaccharides having a phosphodiester linkage between saccharide repeat units. 
     BACKGROUND 
     In the preparation of multivalent conjugate pneumococcal vaccines directed to the prevention of invasive diseases caused by the organism  Streptococcus pneumoniae  (also known as pneumococcus), selected  Streptococcus pneumoniae  serotypes are grown to supply polysaccharides needed to produce the vaccine. The cells are grown in fermentors with lysis induced at the end of the fermentation by addition of sodium deoxycholate or an alternate lysing agent. The lysate broth is then harvested for downstream purification and the recovery of the capsular polysaccharide which surrounds the bacterial cells. After conjugation with a carrier protein, the polysaccharide is included in the final vaccine product and confers immunity in the vaccine&#39;s target population to the selected  Streptococcus pneumoniae  serotypes. 
     Polysaccharide size is a quality attribute assayed for in each preparation batch and must be appropriately controlled. Traditional processing has involved using NaOH (sodium hydroxide) as a base titrant during fermentation. The use of NaOH has the advantage of being able to lower the pH of the deoxycholate lysate without foaming to remove protein and improve filtration. This material is subjected to centrifugation followed by filtration to remove most of the solids down to a 0.45 μm nominal size. However, such traditional processing methods result in lower molecular weight polysaccharide (&lt;450 kDa) for serotypes having a phosphodiester linkage between saccharide repeat units (e.g., 6A, 6B, 19A, and 19F). 
     Accordingly, improved methods for the recovery of high molecular weight capsular polysaccharide from cellular  Streptococcus pneumoniae  lysates, in particular lysates containing  Streptococcus pneumoniae  serotype 6A, 6B, 19A, or 19F polysaccharides, are needed. 
     BRIEF SUMMARY OF THE INVENTION 
     Improved methods for the recovery of high molecular weight capsular polysaccharides from cellular  Streptococcus pneumoniae  lysates containing serotypes having a phosphodiester linkage between saccharide repeat units are provided. In one method, CO 2  is supplied to a fermentation culture of a  Streptococcus pneumoniae  serotype containing a phosphodiester linkage between saccharide repeat units. Accordingly, in one embodiment of the invention, the method includes the steps of: 1) preparing a fermentation culture of  Streptococcus pneumoniae  bacterial cells that produce capsular polysaccharides comprising a phosphodiester linkage between repeat units; 2) supplying CO 2  to the fermentation culture; 3) lysing the bacterial cells in the fermentation culture; and 4) isolating  Streptococcus pneumoniae  capsular polysaccharides from the fermentation culture, where a solution containing high molecular weight isolated  Streptococcus pneumoniae  capsular polysaccharides containing phosphodiester linkages between repeat units is produced. 
     In a particular embodiment, fermentation cultures of  Streptococcus pneumoniae  bacterial cells that produce polysaccharide serotypes 19A, 6A, 19F, 6B, and combinations thereof are prepared. In another particular embodiment, supplying CO 2  to the fermentation culture includes adding bicarbonate ion (HCO 3   − ) to the fermentation culture, for example, adding NaHCO 3  (sodium bicarbonate) to the fermentation culture. In a further embodiment, supplying CO 2  to the fermentation culture includes adding carbonate ion (CO 3   2− ) to the fermentation culture, for example, adding Na 2 CO 3  (sodium carbonate) to the fermentation culture. In another embodiment, supplying CO 2  to the fermentation culture includes a first addition of NaHCO 3  and a second addition of Na 2 CO 3 . In yet another embodiment, supplying CO 2  to the fermentation culture includes overlaying the fermentation culture with CO 2 . In another embodiment, the molecular weight of the isolated  Streptococcus pneumoniae  capsular polysaccharide is at least 700 kDa. In another embodiment, a solution containing high molecular weight isolated  Streptococcus pneumoniae  capsular polysaccharides in which the polysaccharides comprise phosphodiester linkages between repeat units is provided, where the solution is produced by the method described above. 
     In another embodiment of the present invention, a method is provided for producing a solution containing high molecular weight isolated  Streptococcus pneumoniae  serotype 19A capsular polysaccharides. The method includes the steps of: 1) preparing a fermentation culture of  Streptococcus pneumoniae  bacterial cells that produce serotype 19A capsular polysaccharides; 2) supplying CO 2  to the fermentation culture; 3) lysing the bacterial cells in the fermentation culture; and 4) isolating  Streptococcus pneumoniae  serotype 19A capsular polysaccharides from the fermentation culture; whereby a solution containing high molecular weight isolated  Streptococcus pneumoniae  serotype 19A capsular polysaccharides is produced. In another embodiment, a solution containing high molecular weight isolated  Streptococcus pneumoniae  serotype 19A capsular polysaccharides is provided, where the solution is produced by the method described above. 
     In another embodiment of the present invention, a method is provided for producing a solution containing high molecular weight isolated  Streptococcus pneumoniae  serotype 19F capsular polysaccharides. The method includes the steps of: 1) preparing a fermentation culture of  Streptococcus pneumoniae  bacterial cells that produce serotype 19F capsular polysaccharides; 2) supplying CO 2  to the fermentation culture; 3) lysing the bacterial cells in the fermentation culture; and 4) isolating  Streptococcus pneumoniae  serotype 19F capsular polysaccharides from the fermentation culture; whereby a solution containing high molecular weight isolated  Streptococcus pneumoniae  serotype 19F capsular polysaccharides is produced. In another embodiment, a solution containing high molecular weight isolated  Streptococcus pneumoniae  serotype 19F capsular polysaccharides is provided, where the solution is produced by the method described above. 
     In another embodiment of the present invention, a method is provided for producing a solution containing high molecular weight isolated  Streptococcus pneumoniae  serotype 6A capsular polysaccharides. The method includes the steps of: 1) preparing a fermentation culture of  Streptococcus pneumoniae  bacterial cells that produce serotype 6A capsular polysaccharides; 2) supplying CO 2  to the fermentation culture; 3) lysing the bacterial cells in the fermentation culture; and 4) isolating  Streptococcus pneumoniae  serotype 6A capsular polysaccharides from the fermentation culture; whereby a solution containing high molecular weight isolated  Streptococcus pneumoniae  serotype 6A capsular polysaccharides is produced. In another embodiment, a solution containing high molecular weight isolated  Streptococcus pneumoniae  serotype 6A capsular polysaccharides is provided, where the solution is produced by the method described above. 
     In another embodiment of the present invention, a method is provided for producing a solution containing high molecular weight isolated  Streptococcus pneumoniae  serotype 6B capsular polysaccharides. The method includes the steps of: 1) preparing a fermentation culture of  Streptococcus pneumoniae  bacterial cells that produce serotype 6B capsular polysaccharides; 2) supplying CO 2  to the fermentation culture; 3) lysing the bacterial cells in the fermentation culture; and 4) isolating  Streptococcus pneumoniae  serotype 6B capsular polysaccharides from the fermentation culture; whereby a solution containing high molecular weight isolated  Streptococcus pneumoniae  serotype 6B capsular polysaccharides is produced. In another embodiment, a solution containing high molecular weight isolated  Streptococcus pneumoniae  serotype 6B capsular polysaccharides is provided, where the solution is produced by the method described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the optical density (OD), base and glucose levels during the fermentation phase with Na 2 CO 3  as titration base from laboratory studies at 3 L scale. Base in grams is divided by 10 for plotting purposes. 
         FIG. 2  shows the optical density (OD), base and glucose levels during the fermentation phase with NaOH as titration base from laboratory studies at 3 L scale. Base in grams is divided by 10 for plotting purposes. 
         FIG. 3  shows total protein and polysaccharide results at different pH adjustments for alternate base feeds of Na 2 CO 3  or NaOH. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       Streptococcus pneumoniae  are Gram-positive, lancet shaped cocci that are usually seen in pairs (diplococci), but also in short chains or as single cells. They grow readily on blood agar plates with glistening colonies and display alpha hemolysis unless grown anaerobically where they show beta hemolysis. The cells of most pneumococcal serotypes have a capsule which is a polysaccharide coating surrounding each cell. This capsule is a determinant of virulence in humans, as it interferes with phagocytosis by preventing antibodies from attaching to the bacterial cells. Currently there are more than 90 known pneumococcal capsular serotypes identified, with the 23 most common serotypes accounting for approximately 90% of invasive disease worldwide. 
     As a vaccine, the pneumococcal polysaccharide coat can confer a reasonable degree of immunity to  Streptococcus pneumoniae  in individuals with developed or unimpaired immune systems, but a conjugated protein with polysaccharide allows for an immune response in infants and elderly who are also most at risk for pneumococcal infections. The pneumococcal cells are grown in fermentors with lysis induced at the end of the fermentation. The lysate broth is then harvested for downstream purification and the recovery of the capsular polysaccharides. 
     Polysaccharide size is a quality attribute assayed for in each preparation batch and must be appropriately controlled. The molecular weight for serotypes having a phosphodiester linkage between saccharide repeat units (e.g., 6A, 6B, 19A, and 19F) is affected by parameters of the fermentation process. The methods of the present invention allow for the recovery of high molecular weight capsular polysaccharides from cellular  Streptococcus pneumoniae  lysates containing serotypes having a phosphodiester linkage between saccharide repeat units, such as serotype 6A, serotype 6B, serotype 19A, serotype 19F, and combinations thereof. 
     In the development of the present methods, the concentration of HySoy and choice of base titrant were modified in an attempt to modify final polysaccharide yields and molecular weights. Four fermentation schemes were tested. The first used a baseline NaOH process with 28 g/L HySoy. The second used 20% sodium carbonate as the base titrant and 20 g/L HySoy. The third combined advantages of the first two approaches by introducing carbonate through the batching of sodium bicarbonate and using a mixed NaOH/carbonate base titrant. The fourth approach used carbonate as the base titrant with a 10 mM bicarbonate addition to bolster growth. 
     Using NaOH as the base titrant during fermentation had the advantage of being able to lower the deoxycholate lysate to pH 5.0 without foaming to remove protein and improve filtration, but resulted in lower molecular weight polysaccharide (&lt;450 kDa). Na 2 CO 3  provided higher molecular weight but had foaming issues if the pH of the deoxycholate lysate was lowered. At a higher pH hold step of 6.6, the fermentations using Na 2 CO 3  formed a gel-like material, with subsequent filtration problems. Minimizing the amount of Na 2 CO 3  by using a blend of NaOH and Na 2 CO 3  as a pH titrant provided the molecular weight size benefits of Na 2 CO 3  while eliminating foaming and filtration problems due to the sudden release of large amounts of CO 2 . The use of 20% Na 2 CO 3  (w/v) as the base titrant with a 10 mM NaHCO 3  addition to bolster growth (fourth approach) produced consistent, high molecular weight polysaccharides at high yield. 
     The present invention thus provides improved methods for the recovery of high molecular weight capsular polysaccharides from cellular  Streptococcus pneumoniae  lysates containing serotypes having a phosphodiester linkage between saccharide repeat units. In one method, CO 2  is supplied to a fermentation culture of a  Streptococcus pneumoniae  serotype containing a phosphodiester linkage between saccharide repeat units. Exemplary  Streptococcus pneumoniae  serotypes containing a phosphodiester linkage between saccharide repeat units include serotypes 6A, 6B, 19A, and 19F. Accordingly, in one embodiment of the invention, a method for producing a solution containing high molecular weight isolated  Streptococcus pneumoniae  capsular polysaccharides that comprise phosphodiester linkages between repeat units is provided, which includes the steps of: 1) preparing a fermentation culture of  Streptococcus pneumoniae  bacterial cells that produce capsular polysaccharides comprising a phosphodiester linkage between repeat units; 2) supplying CO 2  to the fermentation culture; 3) lysing the bacterial cells in the fermentation culture; and 4) isolating  Streptococcus pneumoniae  capsular polysaccharides from the fermentation culture; 
     whereby a solution containing high molecular weight isolated  Streptococcus pneumoniae  capsular polysaccharides with phosphodiester linkages between repeat units is produced. In another embodiment, the present invention relates to a solution containing high molecular weight isolated  Streptococcus pneumoniae  capsular polysaccharides with phosphodiester linkages between repeat units, where the solution is produced by the method described above. 
     The methods of the invention produce high molecular weight  Streptococcus pneumoniae  capsular polysaccharides that comprise phosphodiester linkages between repeat units (for example, serotypes 6A, 6B, 19A, and 19F). As used herein, “high molecular weight” refers to molecular weights that are at least about 480 kDa, about 490 kDa, about 500 kDa, about 510 kDa, about 520 kDa, about 525 kDa, about 550 kDa, about 575 kDa, about 600 kDa, about 625 kDa, about 650 kDa, about 675 kDa, about 700 kDa, about 725 kDa, about 750 kDa, about 775 kDa, about 800 kDa, about 825 kDa, about 850 kDa, about 875 kDa, about 900 kDa, about 925 kDa, about 950 kDa, about 975 kDa, or about 1000 kDa. 
     In certain methods, supplying CO 2  to the fermentation culture includes adding bicarbonate ion (HCO 3   − ) to the fermentation culture, for example, adding NaHCO 3  to the fermentation culture. NaHCO 3  additions of 5-50 mM can be used, such as 5 mM, 10 mM, 15 mM, 20 mM, 25 mM, 30 mM, 35 mM, 40 mM, 45 mM, or 50 mM. In other methods, supplying CO 2  to the fermentation culture includes adding carbonate ion (CO 3   2− ) to the fermentation culture, for example, adding Na 2 CO 3  to the fermentation culture. Na 2 CO 3  additions of 0.1 M-2.0 M can be used, such as 0.1 M, 0.2 M, 0.4 M, 0.6 M, 0.7 M, 0.9 M, 1.0 M, 1.5 M, 1.8 M, or 2.0 M. A weight/volume (w/v) equivalent can also be used, such as 5% (w/v) Na 2 CO 3 , 10% (w/v) Na 2 CO 3  or 20% (w/v) Na 2 CO 3 . In yet other methods, supplying CO 2  to the fermentation culture includes a first addition of NaHCO 3  and a second addition of Na 2 CO 3  to the fermentation culture. In further methods, supplying CO 2  to the fermentation culture includes overlaying the fermentation culture with CO 2 . CO 2  overlays of 5%-100% can be used, for example, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 100%. 
     Within the methods of the present invention, the bacterial cells may be lysed using any lytic agent. A “lytic agent” is any agent that aids in cell wall breakdown and release of autolysin which causes cellular lysis including, for example, detergents. As used herein, the term “detergent” refers to any anionic or cationic detergent capable of inducing lysis of bacterial cells. Representative examples of such detergents for use within the methods of the present invention include deoxycholate sodium (DOC), N-lauryl sarcosine (NLS), chenodeoxycholic acid sodium, and saponins. 
     In one embodiment of the present invention, the lytic agent used for lysing bacterial cells is DOC. DOC is the sodium salt of the bile acid deoxycholic acid, which is commonly derived from biological sources such as cows or oxen. DOC activates the LytA protein, which is an autolysin that is involved in cell wall growth and division in  Streptococcus pneumoniae . The LytA protein has choline binding domains in its C-terminal portion, and mutations of the lytA gene are known to produce LytA mutants that are resistant to lysis with DOC. 
     Although there is no evidence that the use of DOC during  Streptococcus pneumoniae  polysaccharide purification poses a health risk, the use of such biologically derived reagents could raise potential regulatory concerns. Accordingly, in one embodiment of the present invention, the lytic agent used for lysing bacterial cells is a non-animal derived lytic agent. Non-animal derived lytic agents for use within the methods of the present invention include agents from non-animal sources with modes of action similar to that of DOC (i.e., that affect LytA function and result in lysis of  Streptococcus pneumoniae  cells). Such non-animal derived lytic agents include, but are not limited to, analogs of DOC, surfactants, detergents, and structural analogs of choline, and may be determined using procedures as described in the Experimental section herein below. In one embodiment, the non-animal derived lytic agent is selected from the group consisting of decanesulfonic acid, tert-octylphenoxy poly(oxyethylene)ethanols (e.g. Igepal® CA-630, CAS #: 9002-93-1, available from Sigma Aldrich, St. Louis, Mo.), octylphenol ethylene oxide condensates (e.g. Triton® X-100, available from Sigma Aldrich, St. Louis, Mo.), N-lauryl sarcosine sodium (NLS), lauryl iminodipropionate, sodium dodecyl sulfate, chenodeoxycholate, hyodeoxycholate, glycodeoxycholate, taurodeoxycholate, taurochenodeoxycholate, and cholate. In another embodiment, the non-animal derived lytic agent is NLS. 
     Within the methods of the present invention,  Streptococcus pneumoniae  capsular polysaccharides are isolated using standard techniques known to those skilled in the art. For example, following fermentation of bacterial cells that produce  Streptococcus pneumoniae  capsular polysaccharides, the bacterial cells are lysed to produce a cell lysate. The capsular polysaccharides may then be isolated from the cell lysate using purification techniques known in the art, including the use of centrifugation, precipitation, ultra-filtration, and column chromatography (See, for example, U.S. Patent App. Pub. Nos. 20060228380, 20060228381, 20070184071, 20070184072, 20070231340, and 20080102498). 
     The process changes described above allow for the recovery of high molecular weight capsular polysaccharides from cellular  Streptococcus pneumoniae  lysates containing serotypes having a phosphodiester linkage between saccharide repeat units, such as serotype 6A, serotype 6B, serotype 19A, serotype 19F, and combinations thereof. This is a robust improvement of the fermentation/recovery process that can greatly enhance the production of pneumococcal polysaccharides. 
     The following examples are offered by way of illustration and not by way of limitation. 
     EXAMPLES 
     Selected  Streptococcus pneumoniae  serotypes were grown to supply polysaccharides needed to produce vaccines for active immunization against invasive disease caused by  Streptococcus pneumoniae  due to capsular serotypes included in the vaccine. The cells were grown in fermentors with lysis induced at the end of the fermentation. The lysate broth was then harvested for downstream purification and the recovery of the capsular polysaccharides. Because polysaccharide size is a quality attribute assayed for in each preparation batch, polysaccharide size must be appropriately controlled. The molecular weight for serotypes having a phosphodiester linkage between saccharide repeat units (e.g., 6A, 6B, 19A, and 19F) was found to be affected by parameters of the fermentation process. The following example describes studies relating to the supply of CO 2  during fermentation of  Streptococcus pneumoniae  serotypes having a phosphodiester linkage between repeat units to improve polysaccharide molecular weight. 
     Example 1: Carbon Dioxide Supply Effect on Polysaccharide Molecular Weight 
     Fermentation 
     Laboratory runs were performed in 3 L Braun Biostat B fermentors (B. Braun Biotech, Allentown, Pa.). They were filled with 1.8 L of HYS medium (20 g/L HySoy, 2.5 g/L NaCl, 0.5 g/L KH 2 PO 4 , 0.013 g/L CaCl 2 .2H 2 O, 0.15 g/L L-Cysteine HCl). The fermentors were then autoclaved for 60 minutes at 121° C. After cooling, either 40 or 60 mL/L of a 50% Glucose+1% Magnesium Sulfate (w/v) (DMS) solution was added to each unit. If required, sodium bicarbonate was added prior to inoculation. 
     Two 2 L seed bottles containing 1 L of HYS media were inoculated with Type 19A or Type 6A frozen seed stocks and incubated at 36° C. without shaking for approximately 6-8 hours. Inoculation of the fermentors was performed with a volume of 100 mL (˜5.2% v/v) aliquoted from a bottle with an OD 600  between 0.3-0.9 and pH between 4.75-5.60. The fermentation temperature and pH were controlled at the desired setpoints. The standard conditions of 36° C., 1 L/min air overlay, pH controlled to 7 and agitation of 75 rpm were used. Two impellers were placed at the low and middle positions on the agitator shaft. A bottle containing the appropriate base titrant (3 N NaOH, 3 N NaOH blended with various concentrations of NaHCO 3 , 3 N NaOH blended with various concentrations of Na 2 CO 3  and NaHCO 3 , and 20% Na 2 CO 3 ) was hooked up for automatic pH control. The fermentors were sampled at various time points for external pH, OD 600 , glucose, polysaccharide, and protein. The runs were terminated when the glucose concentration was near depletion, or no increase in OD over time was noted. 
     Optical Density (OD 600 ) Measurement 
     The cellular density of the fermentation broth was determined by reading the absorbance of the samples at 600 nm using a Shimadzu (Columbia, Md.) UV-1601 (2 nm bandwidth) or Spectronics (Westbury, N.Y.) Genesys 5 spectrophotometer (5 nm bandwidth). The unit was blanked with the HYS medium diluted with de-ionized (DI) water to match the dilution required of the sample. The sample was diluted to keep the absorbance below a reading of 0.4, which is well within the linear range of the spectrophotometer. 
     Glucose Concentration 
     Glucose levels were determined by centrifuging out the cells and using the supernatant straight or 3× diluted with DI water. The samples were analyzed on a Nova Biomedical (Waltham, Mass.) BioProfile 400. 
     Polysaccharide Analysis 
     Samples were taken at the final fermentation reading and treated with 12% sodium deoxycholate (DOC) to a concentration of 0.13% (w/v) and gently agitated. They were held between 8-24 hours at 5° C. then pH adjusted to 5.0 with 50% acetic acid to precipitate out most of the DOC and protein. After another hold interval of 12-24 hours at 5° C., the samples were centrifuged (14000 rpm, Sorvall (Thermo Fisher Scientific, Waltham, Mass.) SS34 rotor, 10 min at 15° C.). The pH of the supernatant was adjusted to 6.0. The supernatant was then filtered through 0.45 μm Pall (East Hills, N.Y.) HT Tuffryn Membrane syringe filters (low protein binding). The filtered product was analyzed by high-performance size exclusion chromatography (HPLC-SEC) using standard methodology well known in the art (see, e.g., Aquilar, M. “HPLC of Peptides and Proteins: Methods and Protocols” Totowa, N.J.: Humana Press (2004)). 
     Protein Analysis 
     Protein levels were analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) methods well known in the art (see, e.g., Walker, J. M. “The Protein Protocols Handbook” Totowa, N.J.: Humana Press (2002)). The filtered cell lysate (supernatant) as prepared above was aliquoted into microfuge tubes at 65 μL/tube. Additions of reducing agent (10 μL dithiothreitol (DTT)) and NuPAGE® (Invitrogen, Carlsbad, Calif.) 4× lithium dodecyl sulfate (LDS) sample buffer (25 μL) were made to each sample. The samples were vortexed and heated for 10 minutes prior to 10 μL/lane loading on NuPAGE® 4-12% Bis-Tris 12 well gels. The gels were run in NuPAGE® MES-SDS buffer at 150 V limiting for approximately 60 minutes and subsequently stained using the Zoion staining protocol (Zoion Biotech, Worcester, Mass.). Sample analysis was performed using an UVP Imager (UVP Inc., Upland, Calif.) with LabWorks™ (UVP Inc.) V.3 software to obtain approximate concentrations of specific protein bands of interest. Bovine Serum Albumin (BSA) Fraction V was used to develop a protein standard curve to calculate the approximate protein values of the samples (in lysed cell broth). 
     Molecular Weight Analysis 
     Fermentation samples of 1-2 liters were treated with 12% sodium DOC to a concentration of 0.13% (w/v) with agitation at 200 rpm. Samples were held between 8-24 hours at either 5° C. or 20° C. The samples were then pH adjusted to 5.0 or 6.6 with 50% acetic acid to precipitate out most of the DOC and protein. After another hold interval of 12-24 hours at 5° C., the samples were centrifuged (11000 rpm, Sorvall (Thermo Fisher Scientific, Waltham, Mass.) SLA-3000 rotor, 15 min at 10° C.). The supernatant samples were then pH adjusted to 6.0 with 3 N NaOH, and filtered using 0.45 μm Millipore (Billerica, Mass.) MP60 filters. The samples were then subjected to a modified purification process consisting of 100K molecular weight cut-off (MWCO) diafiltration (5× concentration followed by 7.5× diafiltration with DI water), 0.1% HB precipitation, and carbon filtration. The purified material was then subjected to Multi Angle Laser Light Scattering (MALLS) analysis. 
     Fermentation Process Study 
     Based on previous studies, the fermentation process was optimized by switching from Na 2 CO 3  to NaOH as the base titrant. Use of NaOH allowed the recovery pH to be lowered to 5.0 resulting in significant protein precipitation. Na 2 CO 3  will release CO 2  at low pH (&lt;6.6) creating foam formation. The impact of base titrant on Type 19A polysaccharide and protein levels was examined. Two 3 L fermentors were set up with one fermentor serving as the original process control, using 20% Na 2 CO 3  solution (w/v) as the base feed. The other fermentor used 3 N NaOH as the base feed. 
     During the recovery phase, cells were lysed in the fermentor with DOC (final concentration 0.13% (w/v)) with the fermentor held at 36° C. for 30 minutes. Following this step, the lysate was held overnight with agitation at ambient temperature (22° C.). After the lysate hold, the lysate was pH titrated through a range from unadjusted to 4.5 with samples pulled at various pH setpoints. These samples were held overnight at ambient temperature prior to being processed and analyzed for polysaccharide and protein concentrations. The OD, base and glucose levels during the fermentation phase are shown in  FIG. 1  and  FIG. 2 . The major difference was a higher final OD for the carbonate run. 
     The effect of post DOC lysate pH adjustment on total protein levels was also examined, and is shown in  FIG. 3 . The lower pH levels reduced the protein load in cell free broth for both the NaOH run and the Na 2 CO 3  run. The lower pH (&lt;6.6) had no negative impact on the polysaccharide yield. The fermentation analysis results served as an indication that the NaOH base feed was an acceptable alternative to the process using the Na 2 CO 3  base feed during fermentation, but produced lower yields than what was obtained with the Na 2 CO 3  feed. 
     Effect of Base Titrant on 19A and 6A Molecular Weight 
     A set of fermentations at the 3 L scale were performed to determine if the base titrant, HySoy concentration and pH hold step affected serotype 19A molecular weight. The molecular weight determination was performed using MALLS assay following the modified purification process. Results are shown in Table 1. For serotype 6A, only the base titrant was evaluated. Results are shown in Table 2. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Impact of base titrant on 19A molecular weight (L29331-94) 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 MALLS 
               
               
                 Run No. 
                 pH/Temp 
                 HySoy 
                 pH Hold 
                 Base 
                 (kDa) 
               
               
                   
               
               
                 D 
                 7.0/36° C. 
                 28 g/L 
                 5.0 
                 3N NaOH 
                 340 
               
               
                 E 
                 7.0/36° C. 
                 20 g/L 
                 5.0 
                 3N NaOH 
                 350 
               
               
                 F 
                 7.0/36° C. 
                 20 g/L 
                 5.0 
                 20% Na 2 CO 3   
                 713 
               
               
                 H 
                 7.0/36° C. 
                 20 g/L 
                 6.6 
                 20% Na 2 CO 3   
                 713 
               
               
                   
               
            
           
         
       
     
                     TABLE 2                  Impact of base titrant on 6A molecular weight                                         MALLS           Run No.   Base   (kDa)                                             Lab 1   3N NaOH   662           Lab 2   20% Na 2 CO 3     1189           Pilot 1   3N NaOH   500           Pilot 2   20% Na 2 CO 3     950                        
Effect of Bicarbonate and Mixed Base pH Titrant
 
     In the first study (Runs L29331-122 and -139), varying levels of initial sodium bicarbonate and base blends of sodium hydroxide and sodium carbonate were used in conjunction with a pH 5.0 hold step after the DOC hold step. The initial bicarbonate additions ranged from 10-50 mM and the sodium carbonate added to 3N sodium hydroxide for the base titrant ranged from 0.2-1.8 M. One run contained 50 mM initial bicarbonate and used NaOH as a base titrant. The carbonate levels at the end of these fermentations ranged from 14-111 mM. Serotype 19A molecular weights ranged from 520 to 713 kDa. Run parameters and results are shown in Table 3. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Na 2 CO 3  vs. mixed base as pH titrant 
               
            
           
           
               
               
               
               
               
            
               
                   
                 NaHCO 3   
                 Base 
                 MALLS 
                 PS Yield 
               
            
           
           
               
               
               
               
               
               
            
               
                 Run No. 
                 (mM) 
                 Na 2 CO 3   
                 NaOH 
                 (kDa) 
                 (mg/mL) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Part I 
                 E 
                 0 
                 20% 
                 0      
                 759 
                 0.836 
               
               
                 L29331-122 
                 F 
                 10 
                 0.2M 
                 3N 
                 520 
                 0.308 
               
               
                 20 g/L HySoy 
                 G 
                 10 
                 0.4M 
                 3N 
                 648 
                 0.538 
               
               
                   
                 H 
                 10 
                 0.9M 
                 3N 
                 563 
                 0.334 
               
               
                 Part II 
                 C 
                 20 
                 0.9M 
                 3N 
                 662 
                 1.027 
               
               
                 L29331-139 
                 D 
                 20 
                 1.8M 
                 3N 
                 611 
                 0.903 
               
               
                 28 g/L HySoy 
                 G 
                 50 
                 0.9M 
                 3N 
                 713 
                 0.924 
               
               
                   
                 H 
                 50 
                 0M   
                 3N 
                 713 
                 1.051 
               
               
                   
               
            
           
         
       
     
     A second study (L29331-159 and -185) used initial bicarbonate additions of 15-30 mM and base blends using 0.4-1.0 M Na 2 CO 3 . The carbonate levels at the end of fermentation ranged from 24-62 mM. Serotype 19A molecular weights ranged from 502 to 763 kDa. Run parameters and results are shown in Table 4. 
                     TABLE 4                  NaHCO 3  with mixed base as pH titrant                                             NaHCO 3         MALLS   PS Yield       Run No.   HySoy/DMS   (mM)   Na 2 CO 3 /NaOH   (kDa)   (mg/mL)               G2   28 g/L/60 mL/L   15   1.0M/3N   657   0.853       H2   28 g/L/60 mL/L   15   0.4M/3N   605   0.755       C   20 g/L/60 mL/L   20   0.4M/3N   571   0.386       E   20 g/L/60 mL/L   20   1.0M/3N   763   0.439       F   20 g/L/60 mL/L   25   0.7M/3N   462   0.382       G   20 g/L/60 mL/L   30   0.4M/3N   502   0.355       H   20 g/L/60 mL/L   30   1.0M/3N   594   0.415                    
Comparison of Mixed and Pure Carbonate Titration Base Fermentation Processes
 
     An experiment was performed to compare the base blend process (0.7 M Na 2 CO 3 /3 N NaOH) to the carbonate titrant process (20% Na 2 CO 3  solution, w/v). Results (Table 5) confirmed that the molecular weight from the carbonate titrant process was higher and more consistent (778, 781 kDa) than the molecular weight from the base blend process (561-671 kDa). Polysaccharide yield was also higher with the Na 2 CO 3  process. 
                     TABLE 5                  Run L29399-1 Na 2 CO 3  vs. mixed base                                     NaHCO 3     Base   MW   PS Yield                                     Run No.   (mM)   Na 2 CO 3     NaOH   (kDa)   (mg/mL)                                             C   25   0.7M   3N   565   1.106       D   25   0.7M   3N   561   0.908       E   25   0.7M   3N   612   0.894       G   25   0.7M   3N   671   0.873       F   0   20%   0        778   1.282       H   0   20%   0        781   1.249                    
Pilot Scale Runs
 
     Several serotype 19A pilot scale (100 L) runs with various base titrants were performed. The molecular weight determination was performed using MALLS assay following a complete purification process and is reported from the final purified batch. Results are shown in Table 6. 
                     TABLE 6                  Impact of base titrant on 19A molecular weight at pilot scale                             Fermentation       Purification   FBC MALLS       Batch   Titration Base   Batch   (kDa)               RRP19A-0008   3N NaOH   L26563-10   390       RRP19A-0009   3N NaOH   L26563-11   380       IPPPN19A-005   3N NaOH/0.6M Na 2 CO 3     L26260-37   492       IPPPN19A-006   3N NaOH/0.6M Na 2 CO 3     L26260-38   480       IPPPN19A-007   3N NaOH/0.6M Na 2 CO 3     L26260-39   490       IPPPN19A-014   20% Na 2 CO 3     L26260-49   580       IPPPN19A-016   20% Na 2 CO 3     L26260-50   559       IPPPN19A-017   20% Na 2 CO 3     L26260-51   599                    
Effect of Base Titrant and Overlay on 19A Molecular Weight
 
     A set of fermentations at the 3 L scale were performed to determine if the base titrant and atmospheric overlay affected the molecular weight. The molecular weight determination was performed using MALLS assay following the modified purification process. Results are shown in Table 7. 
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 Impact of base titrant and overlay on 19A molecular weight 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                   
                 MALLS 
               
               
                   
                 Run No. 
                 Base 
                 Overlay 
                 (kDa) 
               
               
                   
                   
               
               
                   
                 Control 
                 3N NaOH 
                 Air 
                 350 
               
               
                   
                 C 
                 0.7M Na 2 CO 3   
                 Air 
                 855 
               
               
                   
                 D 
                 1.5M Na 2 CO 3 / 
                 Air 
                 710 
               
               
                   
                   
                 1.5N NaOH 
               
               
                   
                 E 
                 3N NaOH 
                 100% CO 2   
                 634 
               
               
                   
                 F 
                 3N NaOH 
                 50% CO 2   
                 646 
               
               
                   
                 G 
                 3N NaOH 
                 20% CO 2   
                 567 
               
               
                   
                 H 
                 3N NaOH 
                 10% CO 2   
                 547 
               
               
                   
                   
               
            
           
         
       
     
     The article “a” and “an” are used herein to refer to one or more than one (i.e., to at least one) of the grammatical object of the article. By way of example, “an element” means one or more element. 
     All publications and patent applications mentioned in the specification are indicative of the level of those skilled in the art to which this invention pertains. All publications and patent applications are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. 
     Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, certain changes and modifications may be practiced within the scope of the appended claims.