Patent Description:
Currently, there is no renewable or biologically derived source of o-aminobenzoate or the corresponding acid commercially available. Current production methods of aniline rely on chemical synthesis from petroleum-derived raw-materials. Such petroleum-derived raw materials are not renewable as opposed to raw materials which are renewable, such as the renewable resource "biomass". The chemical synthesis of aniline is a multi-step process. The several reaction steps involved in the production of aniline result in high production costs. Moreover, the conventional, i.e. chemical, synthesis of aniline is associated with hazardous intermediates, solvents, and waste products which can have substantial impacts on the environment. Non-specific side-reactions on the aromatic-ring result in the reduction of the product yield, thus further increasing the production costs. Petroleum-derived raw materials are influenced by cost fluctuations resulting from the global petroleum price.

o-aminobenzoate is a natural intermediate of the shikimate acid pathway and a precursor for the biosynthesis of the aromatic amino acid L-tryptophane. <CIT> discloses a concept of producing biologically-derived aniline in two process steps: (<NUM>) the fermentative production of o-aminobenzoate using recombinant bacteria and (<NUM>) the subsequent catalytic conversion of o-aminobenzoic acid into aniline. The recombinant bacteria used in said process belong to the family of Corynebacterium or Pseudomonas. Both bacteria produce o-aminobenzoate at pH values between <NUM> and <NUM>.

The following problem exists when producing o-aminobenzoate at pH in the range between <NUM> and <NUM>: Due to the fermentative production of o-aminobenzoate which is an acid, a base such as NH<NUM>OH, needs to be added in order to ensure a stable neutral pH. Thereby, a salt of e.g. NH<NUM>+/o-aminobenzoate- is produced. However, such o-aminobenzoate salts are toxic to microbial cells. According to <FIG>, the metabolic activity of bacterial cells (see OTR) is limited when NH4+/o-aminobenzoate concentrations of more than <NUM>/L o-aminobenzoate (not including the mass of the cation)are reached and cell growth (see dry weight) stops at higher concentration (><NUM>/L). This toxicity is not known for products such as glutamate or lysine.

This type of toxicity problem is typically solved by direct evolution of the applied microbial cells. First, microbial cells are exposed to increasing concentration of the toxic component (e.g. o-aminobenzoate) in repeated batch experiments or continuous fermentation trials. Thereby, the microbial cells evolve by random mutagenesis (which can be accelerated by adding mutagens) and the more resistant microbial cells survive. Secondly, the most resistant cells are isolated/selected and can be used for production.

However, many of the mechanisms underlying resistance in such microbial cells consume energy (<NPL>;<NPL>; <NPL>). Thus, a certain proportion of the fermentable substrate is consumed for maintenance metabolism leading to decreased yields of o-aminobenzoate. For this reason, the high titers of o-aminobenzoate which are required for a reasonable space-time yield of the process concomitantly decrease product yield.

Although biotechnological production of o-aminobenzoate from renewable sources as a precursor for aniline production offers potential benefits, the above-described factor of toxicity diminishes the potential benefits of this process. Therefore, there is a need for alternative methods for increasing the resistance of microbial cells towards o-aminobenzoic acid.

This problem is solved by the embodiments defined in the claims and the description below.

In a first embodiment, the present invention relates to a method for cultivating at least one microbial cell capable of converting a fermentable substrate into oAB in the presence of the fermentable substrate while maintaining its metabolic activity, wherein the culture medium is characterized by a concentration of ammonia which does not exceed <NUM>, a pH between <NUM> and <NUM> and a concentration of ortho-aminobenzoic acid (oAB) of at least <NUM>/l to <NUM>/l.

Preferably, the concentration of oAB in this embodiment is <NUM>/l to <NUM>/l and more preferably <NUM>/l to <NUM>/l.

In this embodiment, the metabolic activity is preferably oxygen consumption as determined by the oxygen transfer rate (OTR) of the culture. The "culture" is the suspension consistting of the at least one microbial cell and the culture medium. A maintained metabolic activity of the cell or cells is indicated by an OTR of the culture which does not decrease. Preferably, a maintained metabolic activity is an increasing OTR of the culture.

Under the culture conditions set forth above, said microbial cells converts at least a proportion of the fermentable substrate into oAB. Therefore, disclosed is a method for producing ortho-aminobenzoic acid (oAB) by cultivating a microbial cell capable of converting a fermentable substrate into oAB in the presence of the fermentable substrate and under conditions suitable for the conversion of the fermentable substrate into oAB, wherein the concentration of ammonia in the culture medium does not exceed <NUM> and the concentration of oAB is at least <NUM>/l.

Preferably, the concentration of oAB in this embodiment is <NUM>/l to <NUM>/, more preferably <NUM>/l to <NUM>/l and more preferably <NUM>/l to <NUM>/l.

The microbial cell is, preferably, a cell which is capable of biologically converting a fermentable substrate into oAB. The term "biologically converting" refers to the biochemical processes which transform one or more molecules of the fermentable substrate into one or more molecules oAB. These processes are predominantly mediated by enzymes expressed by the bacterial cell.

The term "o-aminobenzoic acid" (or oAB) as referred to in the present application relate to <NUM>-aminobenzoic acid. This compound is also known as anthranilic acid. The person skilled in the art knows that an acid may be present in its protonated form as neutral substance or deprotonated as anion. In aqueous solution a part of the acid is protonated and a part is present as anion. The ratio between protonated acid and anion depends on the pH of the solution and the dissociation constant Ka of the acid in question. Unless indicated otherwise, the term "o-aminobenzoic acid" as used in this application always refers to both the protonated acid as well as the corresponding anion.

The microbial cell used in the present invention may be a naturally occurring strain, i.e. a microbial strain which is without any further human interaction, particularly without genetic manipulation, capable of converting a fermentable substrate into oAB. However, in a preferred embodiment of the present invention, it is a microbial cell which gained the aforementioned capability in the process of genetic manipulation or a microbial cell, where such methods were used to improve a pre-existing capability.

The term "genetic modification" within the meaning of the invention refers to changes in nucleic acid sequence of a given gene of a microbial host as compared to the wild-type sequence. Such a genetic modification can comprise deletions as well as insertions of one or more deoxyribo nucleic acids. Such a genetic modification can comprise partial or complete deletions as well as insertions introduced by transformations into the genome of a microbial host. Such a genetic modification can produce a recombinant microbial host, wherein said genetic modification can comprise changes of at least one, two, three, four or more single nucleotides as compared to the wild type sequence of the respective microbial host. For example, a genetic modification can be a deletion or insertion of at least one, two, three, four or more single nucleotides or a transformation of at least one, two, three, four or more single nucleotides. A genetic modification can have the effect of e.g. a reduced expression of the respective gene or of e.g. an enhanced expression of the respective gene.

The microbial cell is a prokaryotic cell or an eukaryotic cell. Preferably, the prokaryotic cell is a bacterial cell. Preferred bacterial cells belong to genera Corynebacterium, Mycobacterium, Bacillus, Pseudomonas, Escherichia, and Vibrio. More preferred are Corynebacterium glutamicum and Pseudomonas putida. Most preferred is Corynebacterium glutamicum ATCC <NUM>. Preferred eukaryotic cells belong to the order Saccharomycetales or the genus Aspergillus. More preferably, they belong to the species Ashbya gossypii, Pichia pastoris, Hansenula polymorpha, Yarrowia lipolytica, Zygosaccharomyces bailii, Kluyveromyces marxianus and Saccharomyces cerevisiae. Most preferably, the yeast is Saccharomyces cerevisiae.

It is particularly preferred that said microbial cell is characterized by a genetic modification of the trpD gene which prevents or decreases the expression of said gene and/or which leads to a gene product with decreased or without enzymatic activity. The person of average skill in the art can easily generate such microbial cells using conventional genetic methods.

Recombinant bacterial cells which are particularly suitable for the method of the present invention are disclosed in <CIT>.

The term "cultivating" refers to the incubation of the microbial cell under conditions which facilitate metabolic activity. Such conditions are known to the person skilled in the art. Said conditions minimally encompass presence of the microbial cell in a medium suitable for growth of the cell at temperatures which allow cell proliferation, presence of a fermentable substrate and presence of oxygen. Preferably, said metabolic activity is oxygen consumption. More preferably, the metabolic activity is measured as one or more parameter selected from the group consisting of oxygen consumption, glucose consumption, oAB-production and growth rate. Most preferably, the metabolic activity is the biological conversion of the fermentable substrate to oAB or the growth rate.

Preferably, the cultivation is performed in a culture medium having a pH between <NUM> and <NUM>.

A fermentable substrate as understood by the present application is any organic compound or mixture of organic compounds which can be utilized by the microbial cell to produce o-aminobenzoic acid in the presence or absence of oxygen. Preferred fermentable substrates additionally serve as energy and carbon sources for the growth of the microbial cell. Preferred fermentable substrates are processed sugar beet, sugar cane, starch-containing plants and lignocellulose. Also preferred as fermentable substrate are glycerol and C1-compounds, preferably CO, and fermentable sugars. A preferred fermentable sugar is glucose.

The term "culture medium" is generally understood in the art. It refers to an aqueous solution which provides conditions which allow metabolic activity of the microbial cell. Said conditions are physical or physicochemical such as temperature, concentration of dissolved oxygen, ion strength and pH. They are also chemical and include the concentration of the different nutrients required by the microbial cell for its activity. The person skilled in the art can adapt these conditions to the needs of a particular microbial cell based on the common knowledge available for the particular microbial cell.

The term "ammonia" refers to ammonia in its neutral form, i.e. NH<NUM>, as well as to ammonium, i.e. NH<NUM>+. "Ammonia concentrations", hence, always refer to the sum of the concentrations of both species.

Aqueous ammonia is a popular buffer in biotechnological applications. In the production of organic acids such as oAB it can be used to maintain a stable pH despite increasing concentrations of the acidic fermentation product. However, in the study underlying the present invention it was surprisingly found that the absence of high ammonia concentrations due to the use of sodium hydroxide as buffer substance lead to a marked increase of the oAB-concentration tolerated by the microorganism in question.

Therefore, the concentration of ammonia during the production of oAB by a microbial cell must be limited in order to concomitantly limit product toxicity of oAB. The concentration of ammonia present during the production of oAB does not exceed <NUM>. More preferably it reaches a maximum of <NUM> and most preferably of <NUM>. It is to be understood that the addition of aqueous ammonia or ammonium salts from stock solutions as nitrogen source or as component of a buffer system may lead to concentrations which exceed the aforementioned values locally and for a limited period of time because the even distribution of the added liquid in a fermentation vessel requires a certain amount of time. The same applies for the addition of ammonium salts in solid form, where the dissolution of the salt may lead to local peaks of ammonia concentration before the dissolved salt is diluted.

Thus, the term "locally" refers to a proportion of not more than <NUM> % of the total volume of the culture medium. Any peak of the ammonia concentration in the culture medium does not last for more than <NUM> minutes before the mixing of the culture medium leads to a distribution of the added compounds in the vessel so that the above-defined threshold levels are not exceeded.

Since smaller concentrations of ammonia do not cause excessive toxicity of oAB, the culture medium may contain <NUM> to <NUM> and most preferably <NUM> to <NUM> ammonia. Preferably, the lower limit of the concentration of ammonia is <NUM>. Thus, ammonia can still be used as a part of the buffer system and it can be used as a cheap and convenient nitrogen source.

However, in another preferred embodiment of the present invention no ammonia is actively added to the culture medium so that the concentration of ammonia does not exceed <NUM>, preferably <NUM> and more preferably <NUM>. In this embodiment, nitrogen sources other than ammonia must be used. Alternative nitrogen sources are well known to the person skilled in the art. Preferred alternative nitrogen sources are urea, amino acids, peptides as well as complex mixtures which comprise the aforementioned compounds. Mixtures comprising alternative nitrogen sources are, preferably, yeast extract, tryptone, casamino acids and corn steep liquor. Preferred amino acids are glutamine, threonine, alanine, asparagine and serine.

In a preferred embodiment of the present invention low concentrations of ammonia are supplemented with alternative nitrogen sources. This is done preferably if the total ammonia concentration at any point in time of the cultivation is in the range between <NUM> and <NUM>, more preferably <NUM> to <NUM>. A preferred alternative nitrogen source to be used for supplementing the aforementioned ammonia concentration is urea.

Under the above-defined culture conditions, oAB-concentrations of at least <NUM>/l, preferably at least <NUM>/l and most preferably at least <NUM>/l are reached.

Preferably, under the above-defined culture conditions the microbial biomass reaches a biomass of at least <NUM>/l, more preferably at least <NUM>/l.

In a particular preferred aspect of the disclosure the microbial biomass reaches a biomass of at least <NUM>/l, more preferably at least <NUM>/l, under the above-defined culture conditions and in the presence of at least <NUM>/l oAB, more preferably at least <NUM>/l oAB.

Advantageously, the microbial cell maintains its metabolic activity up to concentrations of at least <NUM>/l oAB, more preferably at least <NUM>/l oAB and most preferably at least <NUM>/l oAB if the concentration of ammonia in the culture medium most preferably does not exceed <NUM>. The cell maintains its metabolic activity if said metabolic activity is at least <NUM> % of the activity in the absence of oAB. Preferably, metabolic activity is measured as one or more parameter selected from the group consisting of oxygen consumption, glucose consumption, oAB-production and growth rate.

It is to be understood that any sudden increase of oAB concentration causes a transient decrease of the metabolic activity of the microbial cell (see examples). During the process of oAB production this effect is unlikely to be encountered because oAB concentrations increase gradually so that the microbial cell has time to adapt. However, addition of oAB for testing purposes may have this effect. Therefore it is preferred that the metabolic activity of a microbial cell is measured at least two hours after any sharp increase of oAB concentration, e.g. caused by the addition of oAB to the culture medium. Otherwise the true and lasting effect of oAB on the activity may be overestimated for the given conditions.

In one preferred embodiment of the present invention aqueous ammonia is replaced by the base of an alkali metal ion as buffering agent during the cultivation of the microbial cell. Said base of an alkali metal is preferably sodium hydroxide.

In another preferred embodiment of the present invention no buffering agent is added during the cultivation of the microbial cell. A buffering agent can be omitted if the culture medium contains sufficient concentrations of CaCO<NUM> from the beginning of the fermentation process or if urea is used as alternative nitrogen source. Based on the amount of CaCO<NUM> which is required to neutralize a given amount of oAB and the acceptable maximum drop in pH which is acceptable, the initial concentration of oAB can be calculated. If the addition of a buffering agent during the fermentation is to be omitted, <NUM> to <NUM>/l, more preferably <NUM> to <NUM>/l CaCO<NUM> are added to the culture medium before the start of the fermentation.

In another preferred embodiment the present invention relates to the use of a culture medium with an ammonia concentration not exceeding <NUM> for the biological conversion of a fermentable carbon source into oAB by a microbial cell capable of said conversion.

All definitions given above for the method of the invention also apply to this embodiment unless otherwise specified.

The ammonia concentration in the culture medium does not exceed <NUM>. Even more preferably, it does not exceed <NUM>. Most preferably, it does not exceed <NUM>.

Disclosed is the use of a culture medium with an ammonia concentration not exceeding <NUM> in order to increase the tolerance of microbial cells towards oAB.

The term "use of a culture medium in order to increase the tolerance of microbial cells towards oAB".

The following examples are only intended to illustrate the invention. They shall not limit the scope of the claims in any way.

The maximum tolerable oAB concentration in the presence of sodium ions in the medium was tested with a modified C. glutamicum production strain. This strain was derived from Corynebacterium glutamicum ATCC <NUM> by evolutionary engineering in order to increase its resistance against oAB. This strain was then genetically modified to make it capable of oAB-production and comprised the following modifications. The incorporated modifications have the effect of reduced expression of the trpD gene, encoding anthranilate phosphoribosyl transferase, knock out of gene ppc, encoding PEP Carboxylase, constitutive overexpression of heterologous aroGD146N and trpEGS40F genes from E. coli, encoding feedback resistant DAHP synthase and anthranilate synthase, respectively, constitutive overexpression of the gene aroL from E. coli, encoding shikimate kinase.

One bioreactor with a nominal volume of <NUM> was filled with sterile cultivation medium including an initial amount of <NUM>/L glucose, <NUM>/L (NH<NUM>)<NUM>SO<NUM>, <NUM>/L KH<NUM>PO<NUM>, <NUM>/LK<NUM>HPO<NUM>, <NUM>/L MgSO<NUM>·<NUM><NUM>O, <NUM>/L CaCl<NUM>·<NUM><NUM>O, <NUM>/L biotin (vitamin B7), <NUM>/L protocatechuic acid (<NUM>,<NUM>-Dihydroxybenzoic acid), <NUM>/L MnSO<NUM>·H<NUM>O. <NUM>/L FeSO<NUM>·<NUM><NUM>O, <NUM>/L ZnSO<NUM>·<NUM><NUM>O, <NUM>/L CuSO<NUM>·<NUM><NUM>O and <NUM>/L NiCl<NUM>·<NUM><NUM>O.

The preculture medium for the cultivation in shake flasks contained additionally <NUM>/L MOPS buffer, <NUM>/L brain heart infusion broth, <NUM>/L urea (CH<NUM>N<NUM>O) and <NUM>/L (NH<NUM>)<NUM>SO<NUM> (instead of <NUM>/L). The preculture was cultivated in <NUM> shake flasks with a liquid volume of <NUM> at a temperatur of <NUM> and a shaking frequency of <NUM> rpm until OD<NUM> > <NUM> was reached.

The cultivation was performed in a lab scale bioreactor with an initial cultivation volume of <NUM>. Temperature was controlled at <NUM> and pH was kept constant at pH=<NUM> by adding aqueous NH<NUM>OH solution (<NUM> w% NH<NUM>) during the fermentation. The gassing rate was adjusted to <NUM>/min air and the dissolved oxygen tension was controlled at <NUM>% air saturation by controlling the stirrer speed between <NUM> rpm and <NUM> rpm. Results of the cultivation are shown in <FIG>. The oAB concentration was increased stepwise by adding <NUM> of a <NUM>/L oAB stock solution (oAB dissolved with NaOH at pH <NUM>) after <NUM>, <NUM>, <NUM> (two times), <NUM> (two times) and <NUM> (two times). Glucose was added as stock solution after <NUM>, <NUM>, <NUM>, <NUM> and <NUM> to prevent a glucose limitation. An increase in biomass concentration was observed even at an oAB concentration of <NUM>/L as shown in <FIG>. Increasing the oAB concentration from <NUM>/L to <NUM>/L after <NUM> resulted in a decrease of the metabolic activity (indicated by the declining OTR signal) and no further increase in biomass was observed at that point. With this experiment it was demonstrated that growth of the C. glutamicum production strain in the presence of <NUM>/L oAB can be achieved by adding oAB as sodium salt instead of ammonium salt to the bioreactor and, in this way, avoiding high ammonium concentrations.

glutamicum strain derived from ATCC <NUM> was used to compare the metabolic activity after NH4-oAB and Na-oAB addition during the cultivation. This strain was derived from Corynebacterium glutamicum ATCC <NUM> by evolutionary engineering in order to increase its resistance against oAB. It was not further genetically modified. Thus it was not capable of oAB-production. For this purpose, two <NUM> bioreactors were filled with sterile cultivation medium including the following initial concentrations: <NUM>/L glucose, <NUM>/L (NH<NUM>)<NUM>SO<NUM>, <NUM>/L KH<NUM>PO<NUM>, <NUM>/L K<NUM>HPO<NUM>,<NUM>/L MgSO<NUM>·<NUM><NUM>O, <NUM>/L CaCl<NUM>·<NUM><NUM>O, <NUM>/L biotin (vitamin B7), <NUM>/L protocatechuic acid (<NUM>,<NUM>-Dihydroxybenzoic acid), <NUM>/L MnSO<NUM>·H<NUM>O, <NUM>/L FeSO<NUM>·<NUM><NUM>O, <NUM>/L ZnSO<NUM>·<NUM><NUM>O, <NUM>/L CuSO<NUM>·<NUM><NUM>O and <NUM>/L NiCl<NUM>·<NUM><NUM>O.

Results for dry biomass, oAB and NH<NUM> concentrations and the Oxygen Transfer Rate (OTR) signals are shown in <FIG>. After addition of <NUM> of a <NUM>/L oAB stock solution (oAB dissolved with NaOH at pH <NUM>) at a cultivation time of <NUM> and <NUM> to reactor <NUM>, the biomass concentration and OTR signal continued to increase (filled symbols in <FIG>). In contrast to that, the addition of NH<NUM>-oAB (added by injection of <NUM> of a <NUM>/L oAB stock solution containing oAB dissolved with NH<NUM>OH at pH <NUM>) to reactor <NUM> after <NUM> resulted in a constant OTR signal and a reduced biomass accumulation (open symbols in <FIG>). This experiment shows that the replacement of ammonia by sodium as counter ion for oAB increases the tolerance of C. glutamicum ATCC <NUM> towards oAB.

The influence of ammonium was tested with C. glutamicum ATCC <NUM> (without further genetic modifications, not modified by evolutionary engineering) using two <NUM> bioreactors filled with sterile cultivation medium including the following initial concentrations: <NUM>/L glucose, <NUM>/L (NH<NUM>)<NUM>SO<NUM>, <NUM>/L KH<NUM>PO<NUM>, <NUM>/L K<NUM>HPO<NUM>, <NUM>/L MgSO<NUM>·<NUM><NUM>O, <NUM>/L CaCl<NUM>·<NUM><NUM>O, <NUM>/L biotin (vitamin B7), <NUM>/L protocatechuic acid (<NUM>,<NUM>-Dihydroxybenzoic acid), <NUM>/L MnSO<NUM>·H<NUM>O, <NUM>/L FeSO<NUM>·<NUM><NUM>O, <NUM>/L ZnSO<NUM>·<NUM><NUM>O, <NUM>/L CuSO<NUM>·<NUM><NUM>O and <NUM>/L NiCl<NUM>·<NUM><NUM>O.

During the fermentation the temperature was controlled at <NUM> and pH was kept constant at pH=<NUM> by adding aqueous NH<NUM>OH solution (<NUM> w% NH<NUM>). The gassing rate was adjusted to <NUM>/min air and the dissolved oxygen tension was controlled at <NUM>% air saturation by adjusting the stirrer speed between <NUM> rpm and <NUM> rpm.

Results for dry biomass and oAB concentrations and the related signals for the Oxygen Transfer Rate (OTR) are shown in <FIG>. <NUM> of a <NUM>/L oAB stock solution (oAB dissolved with NaOH at pH <NUM>) was added to reactor <NUM> after <NUM> and <NUM> (filled symbols) and <NUM> of a <NUM>/L oAB stock solution (oAB dissolved with NH<NUM>OH at pH <NUM>) was added after <NUM> and <NUM> to reactor <NUM> (open symbols).

As shown in <FIG>, the biomass accumulation continued after Na-oAB was added to reactor <NUM> (filled symbols in <FIG>). In contrast to that, the addition of NH<NUM>-oAB to reactor <NUM> caused a growth inhibition after <NUM> (open symbols in <FIG>). From this it follows that the toxicity of oAB on C. glutamicum ATCC <NUM> in presence of high sodium concentrations is reduced as compared to the toxicity in presence of high ammonium concentrations.

The strain was modified as explained in example <NUM>. To evaluate whether an increased tolerance towards oAB is caused by the absence of high ammonium concentrations or by the presence of sodium in the medium, the influence of NaCl and NH<NUM>Cl addition on the toxicity of oAB was tested. Two bioreactors with a nominal volume of <NUM> were filled with sterile cultivation medium including the following initial concentrations: <NUM>/L glucose, <NUM>/L (NH<NUM>)<NUM>SO<NUM>, <NUM>/L KH<NUM>PO<NUM>, <NUM>/L K<NUM>HPO<NUM>, <NUM>/L MgSO<NUM>·<NUM><NUM>O, <NUM>/L CaCl<NUM>·<NUM><NUM>O, <NUM>/L biotin (vitamin B7), <NUM>/L protocatechuic acid (<NUM>,<NUM>-Dihydroxybenzoic acid), <NUM>/L MnSO<NUM>·H<NUM>O. <NUM>/L FeSO<NUM>·<NUM><NUM>O, <NUM>/L ZnSO<NUM>·<NUM><NUM>O, <NUM>/L CUSO<NUM>·<NUM><NUM>O and <NUM>/L NiCl<NUM>·<NUM><NUM>O.

The cultivation was performed in lab scale bioreactors with an initial cultivation volume of <NUM>. Temperature was controlled at <NUM> and the pH value was kept constant at pH <NUM> by adding aqueous NH<NUM>OH solution (<NUM> w% NH<NUM>) during the fermentation. The gassing rate was adjusted to <NUM>/min air and the dissolved oxygen tension was controlled at <NUM>% air saturation by adjusting the stirrer speed between <NUM> rpm and <NUM> rpm. Two bioreactors were inoculated with the production strain. The cultivation results are shown in <FIG>. The oAB concentration was increased stepwise in both bioreactors by adding <NUM> of a <NUM>/L oAB stock solution (Na-oAB) after <NUM>, <NUM> and <NUM> (two times). Glucose was added after <NUM>, <NUM>, <NUM> and <NUM> to a prevent glucose limitation. After a cultivation time of <NUM>, a volume of <NUM> water containing <NUM>/L NaCl was added to bioreactor <NUM> (solid lines in <FIG>) resulting in an added amount of <NUM> mmol Na+ ions. An equimolar amount of NH<NUM>Cl was added to reactor <NUM> by adding <NUM> of a <NUM>/L NH<NUM>Cl stock solution after <NUM> (dashed lines). The online signals for the Oxygen Transfer Rates (OTR) for both bioreactors are shown in <FIG>. The addition of NaCl had no effect on the OTR signal of reactor <NUM>. In contrast to that, the OTR signal of reactor <NUM> decreased by about <NUM> % after NH<NUM>Cl solution was added (<FIG>). The drop in the metabolic activity affected the final dry weight concentration that reached <NUM>/L in reactor <NUM> and <NUM>/L in reactor <NUM> after a cultivation time of <NUM> hours. The results demonstrated that increased ammonium concentrations lead to a reduced metabolic activity of the C. glutamicum production strain.

Claim 1:
A method for cultivating at least one cell of Corynebacterium glutamicum capable of converting a fermentable substrate into oAB in the presence of the fermentable substrate while maintaining its metabolic activity, wherein the culture medium is characterized by a concentration of ammonia which does not exceed <NUM>, a pH between <NUM> and <NUM> and a concentration of ortho-aminobenzoic acid (oAB) of <NUM>/l to <NUM>/l.