Pristinamycin production using Streptomyces pristinaespiralis and date sirup as substrate-process modeling, optimization, and scale-up.

Pristinamycin biosynthesis using Streptomyces pristinaespiralis and date sirup (DS) as substrates was optimized before scale-up. DS was filter sterilized as heat sterilization primes Maillard reactions having negative effects on antibiotic production. Multilinear regression modeling (MLR) predicted optimum medium composition, specifying components with positive and negative effects on production. The MLR showed that to maximize bacterial growth, DS, arginine, CaCl2, and KH2PO4 must be fixed at the highest concentration, but to maximize antibiotic production, these factors have to be fixed at a low level. A noticeable difference in productivity was observed in a shake flask experiments with 50.4 and 43.1 mg/L pristinamycin final concentration for the DS and the glucose substrates, respectively. In the 2 L bioreactor, the DS medium resulted in a 66.6 mg/L antibiotic, while the scale-up in the 100 L resulted in 39.0 mg/L. The low yield in the 100 L bioreactor could be attributed to the relatively high stirring rate applied which was the minimum possible in the bioreactor used. This high stirring rate prevented pellet formation by the cells, which is described as necessary for antibiotic formation by the bacterium. Hence, a successful scale-up to pilot-scale should consider the effect of stirring rate.

Corynebacterium glutamicum, a natural overproducer of succinic acid?

Corynebacterium glutamicum is well known as an important industrial amino acid producer. For a few years, its ability to produce organic acids, under micro-aerobic or anaerobic conditions was demonstrated. This study is focused on the identification of the culture parameters influencing the organic acids production and, in particular, the succinate production, by this bacterium. Corynebacterium glutamicum 2262, used throughout this study, was a wild-type strain, which was not genetically designed for the production of succinate. The oxygenation level and the residual glucose concentration appeared as two critical parameters for the organic acids production. The maximal succinate concentration (4.9 g L-1) corresponded to the lower kLa value of 5 h-1. Above 5 h-1, a transient accumulation of the succinate was observed. Interestingly, the stop in the succinate production was concomitant with a lower threshold glucose concentration of 9 g L-1. Taking into account this threshold, a fed-batch culture was performed to optimize the succinate production with C. glutamicum 2262. The results showed that this wild-type strain was able to produce 93.6 g L-1 of succinate, which is one of the highest concentration reported in the literature.

Pilot-scale biomethanation of cattle manure using dense membranes.

This study aimed at studying the biomethanation process using a 100 L pilot-scale digester equipped with a dense membrane for hydrogen injection. Hydrogen mass transfer was characterized and the impact of hydrogen flowrate, agitation rate and of the co-injection of CO2, on biogas production and composition, was precisely studied. A linear relationship between H2 flowrate and the CO2 and CH4 rates in biogas was found but no impact on biogas flowrate was shown. It was also noticed that, without exogenous CO2 injection, and for high H2 injection flowrates, residual H2 could be found at the digester outlet due to local CO2 limitation. Thus, this study suggested that biogas production in biomethanation process at the pilot scale was probably rather limited by the dissolved CO2 transport within the liquid phase than by the hydrogen mass transfer itself.

Impact of shear stress and impeller design on the production of biogas in anaerobic digesters.

Today, intensification of anaerobic digestion is still a scientific and technical challenge. The present study proposed combined experimental and computational fluid dynamics simulations to characterize the impact of shear stress and impeller design on the biogas production after sequential additions of substrate. Liquid phase (cattle manure digestate) rheological law was experimentally determined and input in numerical simulations. The results showed that the original use of a double helical ribbon in digester allowed a significantly faster dispersion of fresh substrate than the use of a classical Rushton turbine, leading to a 50% higher methane production rate. However, with both impellers, too high agitation rates entailed a clear slow-down of production rate and a decrease in CH4 content. To avoid this loss of productivity, it was shown that the maximal value of shear stress, determined by numerical simulations, was a consistent parameter to set the upper agitation conditions in digesters.

Lactate production as representative of the fermentation potential of Corynebacterium glutamicum 2262 in a one-step process.

The fermentative properties of thermo-sensitive strain Corynebacterium glutamicum 2262 were investigated in processes coupling aerobic cell growth and the anaerobic fermentation phase. In particular, the influence of two modes of fermentation on the production of lactate, the fermentation product model, was studied. In both processes, lactate was produced in significant amount, 27 g/L in batch culture, and up to 55.8 g/L in fed-batch culture, but the specific production rate in the fed-batch culture was four times lower than that in the batch culture. Compared to other investigated fermentation processes, our strategy resulted in the highest yield of lactic acid from biomass. Lactate production by C. glutamicum 2262 thus revealed the capability of the strain to produce various fermentation products from pyruvate.

Joint effect of nitrogen sources and B vitamin supplementation of date juice on lactic acid production by Lactobacillus casei subsp. rhamnosus.

The use of date juice as a substrate for lactic acid production was investigated. Various nitrogen sources were compared with yeast extract for efficient lactic acid production by Lactobacillus casei subsp. rhamnosus. Among different nitrogen sources added to date juice (yeast extract, ammonium sulfate, tryptic soy, urea, peptone, and casein hydrolysate), yeast extract was the most efficient. The effect of yeast extract could have been due to its B vitamin content. The addition of five B vitamins at less than 25 mg/l to date juice with any nitrogen source enhanced lactic acid production to some extent, except for date juice with yeast extract or urea or peptone. The most significant increase was obtained with ammonium sulfate. Half of the yeast extract content (10 g/l) in a supplemented date juice could be replaced by a mixture of B vitamins at less than 25 mg/l, and ammonium sulfate at 2.6 g/l with no significant decrease in lactic acid production.