Processes for obtaining exoenzymes by culture
BACKGROUND OF THE INVENTION 
     The present invention relates to processes for continuously obtaining extracellular or secreted enzymes (exoenzymes) by cultivation of exoenzyme-producing bacteria in a continuous-flow fermenter. 
     Exoenzymes such as proteases, xylanases and amylases are obtained by cultivation of bacteria, in particular of strains of the genus Bacillus. To date, exoenzymes have been prepared only by discontinuous (batch) fermentation, because the secretion of these enzymes by the microorganisms rapidly diminishes in continuous culture (chemostat). See, generally, Priest, &#34;Extracellular Enzyme Synthesis in the Genus Bacillus,&#34; Bacteriol. Rev. 41 (3): (1977), the contents of which are hereby incorporated by reference. 
     The reasons for the diminution in exoenzyme secretion as soon as a batch fermentation is converted into a continuous fermentation have not yet been completely elucidated. The observation that exoenzyme production in batch cultures takes place mainly in the late logarithmic and stationary phases and before sporulation, and not, in contrast, in the logarithmic phase of growth (Dancer and Mandelstam, &#34;Criteria for Categorizing Early Biochemical Events Occurring During Sporulation of B. Subtilis,&#34; J. Bacteriol.: 411-415 (1975)), leads to the conclusion that the formation of exoenzymes, while generally constitutive, is subject to catabolite repression (Schaeffer, &#34;Sporulation and the Production of Antibiotics, Exoenzymes, and Exotoxins,&#34; Bacteriol. Rev.: 48-71 (1969)). Catabolite repression is also suggested by the observation that the production of exoenzymes increases with the difficulty of utilization by the organisms of the carbon source in the medium (Moses and Sharp, &#34;Intermediary Metabolite Levels in Escherichia coli,&#34; J. Gen. Microbiol. 71: 181-190)), e.g., the yield of enzyme is better when glycerol or lactose, rather than glucose is used as the carbon source. 
     In light of catabolite repression of this nature, there have been various proposals of multistage, continuous fermentation processes. See Tsaplina &amp; Loginova, &#34;Production of bacterial proteinase during continuous cultivation,&#34; in 5th INT. FERMENTATION SYMP. VERSUCHS-UND LEHRANSTALT FUR SPIRITUSFABRIKATION UND FERMENTATIONS-TECHNOLOGIE 262 (H. Dellweg ed., Berlin). But these proposed processes have proved unsatisfactory for industrial production. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a process for cultivating exoenzyme-producing microorganisms so as to obtain enhanced production of exoenzymes in culture. 
     It is also an object of the present invention to provide a process for producing exoenzymes by continuous fermentation. 
     These objects are achieved, in accordance with the present invention, by carrying out fermentation with an exoenzyme-producing microorganism in one step, during which (a) continuous limitation of growth appropriate for maximal exoenzyme productivity is effected or (b) repeated or periodic reductions are brought about in the concentration of culture biomass by at least 30% by inhibition of growth. 
     More specifically, there has been provided, in accordance with one aspect of the present invention, a process for obtaining exoenzymes by continuous-flow fermentation, which comprises cultivating an exoenzyme-producing microorganism in one step in a fermenter under conditions such that multiplication of the microorganism in culture declines during at least part of the growth-phase of the culture, whereby production of an exoenzyme by the microorganism is enhanced during a transient state of restricted multiplication after growth inhibition. In a preferred embodiment, multiplication of the exoenzyme-producing microorganism is actively inhibited repeatedly, over at least two periods, such that the culture undergoes a reduction in biomass concentration of at least 30% during each period and passing said transient state while increasing again. In another preferred embodiment, multiplication of the microorganism is subject to continuous limitation caused by continuously holding said transient state. 
     Other objects, features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is further described below by reference to the drawings, which are graphs plotting the variation over time of 
     FIG. 1: continuous xylanase formation according to the present invention; 
     FIG. 2: uncontrolled xylanase formation of a conventional nature in a continuous-flow fermenter; 
     FIG. 3: optical density, xylanase concentration and glycerol concentration during continuous fermentation with spontaneous biomass pulsation; 
     FIG. 4: optical density, exoenzyme concentration and pH during continuous fermentation (with repeated short-term growth inhibition) for xylanase, amylase and protease; 
     FIG. 5: optical density, xylanase concentration, glycerol concentration and pH during continuous fermentation, with repeated short-term growth inhibition according to the present invention; and 
     FIG. 6: quasi-continuous xylanase formation with repeated decreases in the pH, the graph of redox potential also being shown. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It has been discovered that, surprisingly, the production of exoenzymes, which in conventional processes substantially ceases when a steady state of fermentation is attained, can be maintained or restimulated if the culture is exposed to a particular deficiency state. As explained in greater detail below, a state of this type can be (1) continuously adjusted by a control system that responds sensitively to transition states of the culture or (2) achieved intermittently by repeatedly effecting in the culture a state of multiplication following a reduction characterized by changes in concentration of the biomass of at least 30%. 
     It has also been discovered that a repeated, in particular periodic, alternation of reduction and multiplication phases of this type will arise spontaneously in the culture if at least one of the conditions necessary for growth is cut back to a point where periodic increases and decreases in the biomass (&#34;biomass pulsations&#34;) take place. For purposes of this description, these spontaneous alternations in the biomass of a culture will be called the &#34;characteristic oscillations&#34; of that culture. The phenomenon of characteristic oscillations can be exploited, in accordance with the present invention, by adjusting a culture parameter crucial to growth, such as pH level, to a value near an extreme in the range of values acceptable to the cultured microorganism, thereby inducing biomass pulsations as described above. The resulting oscillations in biomass density are associated, in turn, with enhanced (repeatedly induced) exoenzyme production, as explained in greater detail below. 
     It is particularly advantageous, however, to initiate the repeated reduction by a repeated imposition, for relatively short periods, of a growth-inhibiting effect brought about, for example, by shifting the pH or the temperature out of the range which is tolerated by the cultured microorganism, or by cutting back on necessary nutrients and the like. The duration of this growth-inhibiting effect should be such as to bring about the desired reduction in biomass, where appropriate, after a time lag. For the purposes of this description, a single alteration of biomass increase and subsequent decrease will be called a &#34;period,&#34; whether the alteration is part of a characteristic oscillation or is actively induced by imposition of a growth-inhibiting effect (with constant or variable period as convenient). 
     The timing and nature of the inhibitory conditions to be applied depend on the culture conditions and, in particular, on flow rates (D) and/or residence times (τ) respectively, where &#34;residence time&#34; is the inverse of flow rate and refers to the interval during which a unit volume of culture virtually (statistically) remains in the reactor. It is particularly advantageous for the period to last 1 to 10, in particular 6 to 8, residence times, and for the growth-inhibiting effect to have a duration, respectively, of 0.01 to 5, particularly 0.1 to 1, and specifically for about 0.5, residence times. Advantageous flow rates, in turn, are in the range 0.01 to 2 h -1 , in particular 0.05 to 1 h -1 , and specifically 0.1 to 0.5 h -1 . 
     For continuous production it is possible simply to monitor continuously the optical density of the culture, since optical density corresponds to the biomass density, and to use it for control purposes. In this way, an approach close to the steady-state (when change in biomass density per residence time varies by 20% or less) can be detected in good time and used on each occasion to trigger the growth-inhibiting effect. Because of the resulting &#34;stress conditions,&#34; there occurs during each period a large decrease in the biomass density, which can extend beyond the duration of the stress phase, followed by a recovery and renewed steep increase in the production of exoenzymes. 
     It is also possible to monitor exoenzyme concentration for fine control. Thus, a decrease in the exoenzyme concentration at any particular time less than about 1/2 of the maximum concentration prior to that time is used as a signal for triggering the growth-inhibiting effect, which is maintained until the biomass density has fallen to less than about 2/3 of its steady-state value. 
     Detailed investigation of the individual periods have shown, surprisingly, that the production of the enzyme does not start until the level of available oxygen has decreased to a particular value. This phenomenon can be detected only by sensitive measurement of redox potential in culture, but there nevertheless is a close correlation between enzyme productivity and oxygen availability. 
     It is therefore possible to attain virtually continuous enzyme production if a decreased level of available oxygen corresponding to maximal exoenzyme production is maintained continuously in the culture. For this purpose, the redox potential in the fermenter, a very sensitive indicator of the oxygen content of the culture, is monitored and used as a control parameter for oxygen input, which can be varied, in particular, by the stirring speed. Stirring speeds ranging up to about 1,000 rpm with aeration at 0.3 to 3 unit volumes of air per unit volume of medium per minute (&#34;VVm&#34;), in particular 1 to 2 VVm, are preferred. 
     It is beneficial for the culture to contain a known antifoaming agent, for example, polypropylene glycol or silicone oil, to suppress, where appropriate, pronounced foam formation during the oxygen input controlled by the stirring speed. 
     The redox potential corresponding to maximal exoenzyme production depends on the parameters selected for the fermentation, such as the pH, the composition and concentration of nutrients, the aeration rate, the microorganism selected for culturing, and the temperature. Thus, the oxygen input which is to be adjusted in each instance (or the redox potential which is to be maintained in each instance) is most expediently determined right at the outset for a specific production in the particular fermentation system. 
     The control of oxygen input as a function of redox potential, according to the present invention, means that the bacterial culture is prevented from reaching a steady state, that is, the culture shows a continuous tendency for its concentration to increase or, alternatively, to &#34;wash out&#34; when loss of biomass in the fermenter due to flow-through outstrips the increase of biomass due to growth. It is therefore very beneficial in the present invention for biomass concentration to be kept within the specific limits. Wash-out can be counteracted, for instance, by reducing flow-through, by enhancing growth, or by retaining biomass in the fermenter, e.g., by immobilization of the cultured microorganism or by filtration or centrifugation of the culture medium. 
     For this purpose, a second control system can be provided that continuously monitors the biomass concentration in the fermenter (for example, via optical density) and controls it by variation of residence time in the fermenter and/or of oxygen availability, as indicated by the via the redox potential of the culture. In any event, the redox potential should remain in the range for optimal enzyme production. 
     During oxygen-limited exoenzyme formation in a continuous-flow fermenter, according to the present invention, it is preferable for biomass concentration in the reactor to be maintained as high as possible without being subject to any nutrient limitation (carbon, nitrogen, etc.). As previously indicated, it is also possible for the biomass to be retained in the fermenter by known procedures (immobilization, filtration, centrifugation, etc.), and for an optimal biomass concentration to be maintained by varying the proportion kept in the fermenter. 
     The microorganisms suitable for use in the present invention are exoenzyme-producing bacteria, particularly bacteria of the genus Bacillus. For example, bacteria of the species Bacillus amyloliquefaciens, as represented by deposited strains DSM 7 and DSM 1061, can be used to advantage in practicing the present invention. The temperature for culturing exoenzyme-producing bacteria in accordance with the present invention is conveniently in the range from about 30° to 37° C. 
     In each of the following examples, bacteria of the strain Bacillus amyloliquefaciens DSM 7 were cultivated in a stirred vessel reactor (fermenter) under the following conditions: 
     
         ______________________________________                                    
7-liter fermenter                                                         
                (Type BIOSTAT&#34;E&#34; supplied                                 
                by B. BRAUN, Melsungen)                                   
Operating Volume                                                          
                4 liters (Examples 1-3) or                                
                3 liters (Examples 4-5)                                   
Temperature     37°                                                
Aeration        6 l/min compressed air                                    
                [1.5 VVm (Examples 1-3)                                   
                or 2 VVm (Examples 4-5)]                                  
Stirrer type    Flat-blade turbo stirrer                                  
Speed           Examples 1-3: 0 rpm                                       
                (adequate mixing achieved                                 
                by air in-flow)                                           
                Examples 4 and 5: 300 rmp                                 
                (if not altered by the                                    
                the control system)                                       
______________________________________                                    
 
    
     The culture medium, which was sterilized beforehand, had the following composition: 
     
         ______________________________________                                    
20        g           glycerol                                            
6         g           (NH.sub.4).sub.2 SO.sub.4                           
1.67      g           NaNo.sub.3                                          
0.52      g           Mg(NO.sub.3).sub.2.7H.sub. 2 O                      
1         g           yeast extract                                       
2.72      g           KH.sub.2 PO.sub.4                                   
0.5       g           citric acid                                         
72        mg          CaCl.sub.2.2H.sub. 2 O                              
10        mg          FeSO.sub.4.7H.sub. 2 O                              
5         mg          MnSO.sub.4.H.sub.2 O                                
H.sub.2 O to 1,000 ml                                                     
______________________________________                                    
 
    
     For Examples 4 and 5 (with the stirrer operating), the medium additionally contained in 1 liter 
     0.083 ml polypropylene glycol 2000 and 
     0.083 ml silicone antifoam emulsion M 30 (supplied by Serva). 
     COMPARISON EXAMPLE 
     The fermenter was inoculated at time t=0 h with 200 ml of a preculture of B. amyloliquefaciens DSM 7 which had been cultured in a shaken flask, and was first operated as a batch culture (discontinuously) for 16 hours. 
     From t=16 h the fermentation was carried out continuously, with a constant volumetric flow of fresh medium pumped into the fermenter and the same amount of fermenter contents continuously drained off. The constant volumetric flow as 220 ml/h (D=0.055 h -1 , τ=18.2 h). The pH was controlled at a constant pH 6.0. 
     These conditions were kept constant for 13 residence times. During this period, the biomass concentration remained approximately constant (steady state), while the xylanase concentration decreased from 11 U/ml to below 0.5 U/ml (see FIG. 2). 
     EXAMPLE 1 
     After inoculation, the fermentation was carried out under the same conditions as described in the comparison example, first discontinuously for 15 hours and then continuously at 375 ml/h (D=0.094 h -1  ; τ=10.7 h). The pH was kept constant at pH 5.5 for the first 336 hours and thereafter kept constant at pH 5.1. 
     During continuous operation, pronounced biomass pulsations arose spontaneously at both pH values (see FIG. 3, optical density). The pulsations were characterized in particular by a steep, renewed rise in the biomass during each period. Throughout the duration of the test it was observed that xylanase production resumed during the renewed increase in the biomass. The pattern of variation in glycerol concentration was essentially 180° out of phase with the observed variation in the optical density. 
     EXAMPLE 2 
     After inoculation, the fermentation was carried out under the same conditions as described in the comparison example, continuously from the outset (t=0 h) at 400 ml/h (D=0.1 h -1  ; τ=10 h). The pH was initially maintained constant at pH 6.5. After the biomass concentration had settled into a steady state and the exoenzyme concentration had decreased, the pH control system was switched off at time t=107 h for 1 residence time (10 h). This resulted in the pH falling of its own accord to pH 5.0. At time t=117 h, the pH control system was switched on again, and this resulted in immediate correction of the pH back to pH 6.5. During the period when the pH had shifted outside the range tolerated by the bacterium (pH 6 to 8), a large reduction in biomass concentration was observed and, thereafter, a steep renewed increase in the biomass concentration. The latter was accompanied by a marked increase in the exoenzyme concentration (see FIG. 4 for xylanase, protease and amylase). 
     After the biomass had again settled into a steady state and the exoenzyme concentrations had again subsided, at time t=168 h the pH control system was again switched off for 1.5 residence times (15 h). 
     The pH decreased without intervention to pH 5.0, and from t=183 h was returned to a constant pH of 6.5 by switching on the pH control system again. 
     Once more, a renewed decrease in the biomass concentration was observed, followed by a steep increase in the biomass concentration, which was again accompanied by a marked increase in exoenzyme concentrations. 
     In a corresponding manner, the pH control system was switched off for 14 hours between t=215 h and t=229 h, resulting in a drop in pH to 4.7. The behavior of the biomass concentration and the exoenzyme concentrations, respectively, was analogous to what occurred during the first two shifts in pH. 
     At time t=325 h the pH was actively reduced to pH 5.0 by addition of acid by the pH control system, and the pH was maintained constant at 5.0 for 0.6 residence times (6 hours). The pH was raised thereafter to 6.0 by the pH control system, and was maintained constant at that value. The behavior of the biomass concentrations and the exoenzyme concentration during this procedure was again the same as during the spontaneous pH decrease. 
     EXAMPLE 3 
     After inoculation, the fermentation was carried out as described in the comparison example, continuously from time t=0.5 h at 370 ml/h (D=0.093 h -1 , τ=10.8 h). 
     The pH was actively reduced from a constant 6.5 to a constant 5.0 for 17 hours (1.6 residence times) from t=69 h onward, and for 8 hours each (0.75 residence times) from t=136 h and t=208 h onward, by means of the pH control system. The behavior of the biomass concentration and the xylanase concentration corresponded to the observations described in Example 2 (see FIG. 5.) 
     Other tests indicate that this procedure also works when other carbon sources than glycerol are used, for example, lactose and starch. 
     The species Bacillus subtilis, which is related to Bacillus amyloliquefaciens, showed a similar response to the growth-inhibition regimen of the present invention. 
     Although the growth-inhibiting effect chosen for biomass reduction in each of the above examples was a reduction in pH, which can be brought about in a very straightforward manner, other methods of inhibition, such as increasing pH or shifting temperature, are effective in the same way. 
     EXAMPLE 4 
     FIG. 6 shows the change in redox potential, in a continuous-flow fermenter, during xylanase formation that was initiated periodically by repeated pH reduction as described above. It is apparent that maximal enzyme concentrations in the culture coincide with moderate redox potentials in the decreasing regions of the redox curves, which redox potentials were around -50 mV in the present xylanase formation at pH 6 and a residence time of 5 hours. 
     EXAMPLE 5 
     The redox control shown in this example was brought about by use of a PID controller (supplied by Foxboro) which stabilizes the redox potential at a predetermined value by controlling the speed of rotation of the fermenter stirrer dependent on the monitored redox potential value. 
     The bacterial culture in the fermentation, which ran continuously for a prolonged period, was forced to a reduction in the biomass density by a decrease in the pH at time t=1960 h to 1964 h, and it started to increase substantially again from about t=1977 h onward (FIG. 1; plot C; the abscissa starts at t=1970 h). 
     At the same time the biomass increased, there was a large decrease in the redox potential up to about t=1985 h, and it then stayed in the region of about 0 to -50 mV (plot A). The redox control system was switched on, at time t=1988 h, before the redox potential could decline further, as is evident from FIG. 6. Whereas the speed of rotation of the fermenter stirrer had been constant at 300 rpm up to this time, the stirrer rotation speed was externally altered by the redox control system from t=1988 h in order to keep the redox potential at the preset redox target level in each case. 
     The first redox target level was selected at -15 mV from t=1988 to 1992 h. Since this target was slightly above the final uncontrolled redox figure, it was possible for the biomass concentration initially to increase further. In order to prevent an excessive increase or decrease in the biomass, the residence time of the continuous culture was varied at t=1989 h and at subsequent times (plot D). Primarily to likewise influence the biomass density, the redox target level was varied within the range from -15 to -50 mV. 
     Throughout the period when the redox was controlled, the enzyme concentration changed approximately in parallel with the biomass density, so that the enzyme yield stayed constant in the range from 2000 to 3000 U of xylanase per g of dry biomass (plot B). 
     The example described above relates to the formation of xylanase. Similar results were obtained for protease and amylase. 
     The end-product solution from the fermenter is worked up by known methods, for example, by centrifugation or crossflow filtration, with continuous removal of the biomass from the liquid phase. The enzyme or enzymes produced by the cultured microorganism can then be removed from the liquid phase, again via a conventional technique, such as by salting out, and thereafter isolated or purified.