Characterization of the response of GFP microbial biosensors sensitive to substrate limitation in scale-down bioreactors

in Biochemical Engineering Journal (2011), 55(2), 131-139

The dynamics of microbial stress response in intensive cultivation conditions remains not completely understood. In this work, two green fluorescent protein (GFP) transcriptional reporters have been used ... [more ▼]

The dynamics of microbial stress response in intensive cultivation conditions remains not completely understood. In this work, two green fluorescent protein (GFP) transcriptional reporters have been used as biosensors of the heterogeneities generated in a two-compartment scale-down reactor. The stress promoters have been chosen for their responsiveness to carbon limitation corresponding to the global substrate profiles encountered in intensive fed-batch cultures. From our results, it can be concluded that the exposure of microbial cells to substrates heterogeneities tends to decrease the GFP expression level in fed-batch mode. Fluorescence intensities have been monitored at the single cell level by using flow cytometry. During the course of the fed-batch culture, a drop at the level of the intracellular GFP content has been observed for the two scale-down operating conditions and for the two promoters sensitive to substrate limitation (rpoS and csiE). The fluorescence drop can be attributed to the repression of these promoters but also to the release of GFP to the extracellular medium according to the increase of the fluorescence level of the supernatant. This leakage has been observed for all the operating conditions, i.e. the scale-down reactors and the culture operating in the normal mode, i.e. in a well-mixed bioreactor. Interestingly, GFP leakage is more pronounced in the case of the cultures operated in the normal mode. Indeed, staining by propidium iodide tends to be more elevated for the microbial cells cultured under the normal mode by comparison with those cultured in scale-down conditions, indicating a higher permeability of the membrane. These results suggest that GFP microbial biosensors could be used to detect simultaneously mixing imperfections and their impact on the viability of microorganisms. [less ▲]

Elaboration of a biased radnom walk model with a high spatial resolaution for the simulation of microorganisms exposure to gradient stress in scale-down reactors

in Biochemical Engineering Journal (2008), 39

The effect of heat stress on freeze-drying and conservation of Lactococcus

in Biochemical Engineering Journal (2005), 24(2), 141-145

The production of lactic acid bacteria isolated from Tunisian fermented milk (Lben): Lactococcus lactis subsp. lactis var. diacetylactis (SLT6) was investigated in fed-batch process. The final biomass ... [more ▼]

The production of lactic acid bacteria isolated from Tunisian fermented milk (Lben): Lactococcus lactis subsp. lactis var. diacetylactis (SLT6) was investigated in fed-batch process. The final biomass production was 2.2710(11) cfu/ml. The effect of heat stress, after fermentation, on the survival rate after freeze-drying and during storage was studied. The survival rate after freeze-drying of SLT6 subject of a thermal treatment was 38.8% whereas it was only 21.9% for unprocessed SLT6. The accelerated shelf life testing (ASLT) method was used to study the effect of heat stress on conservation of Lactococcus. The bacterium was stored at four different temperatures of 4, 15, 25 and 37 degrees C. Lactococcus cell kinetic behaviour suggested a first order reaction, and the effect of storage temperature was quantified by determination of the half-life period and the activation energy. The beneficial effect of heat stress is clearly associated with a half-life period of 86 days for SLT6T and only 48 days four SLT6NT stored at 4 degrees C. (c) 2005 Elsevier B.V. All rights reserved. [less ▲]