Comparison of the effects of various attenuation methods on cell permeability and accessibility of intracellular enzymes in Lactococcus lactis strains

Attenuated Lactococcus lactis and Surface Bacteria as Tools for Conditioning the Microbiota and Driving the Ripening of Semisoft Caciotta Cheese

This study aimed at establishing the effects of attenuated starters and surface bacteria on various features of caciotta cheese. The cheese undergoes a ripening period during which the house microbiota contaminates the surface. Conventional cheese (the control cheese [CC]) is made using only primary starters. Primary starters and attenuated (i.e., unable to grow and synthesize lactic acid) Lactococcus lactis (Lc. lactis) subsp. lactis were used to produce caciotta cheese without (ATT cheese) or with an inoculum of surface bacteria: (i) Leuconostoc lactis (Le. lactis) (LL cheese), (ii) Vibrio casei (VC cheese), (iii) Staphylococcus equorum (SE cheese), (iv) Brochothrix thermosphacta (BX cheese), and (v) a mixture of all four (MIX cheese). Attenuated Lc. lactis increased microbial diversity during cheese ripening. At the core, attenuated starter mainly increased indigenous lactococci and Lactobacillus delbrueckii group bacteria. At the surface, the main effect was on Macrococcus caseolyticus Autochthonous Le. lactis strains took advantage of the attenuated starter, becoming dominant. Adjunct Le. lactis positively affected Lactobacillus sakei group bacteria on the LL cheese surface. Adjunct V. casei, S. equorum, and B. thermosphacta did not become dominant. Surfaces of VC, SE, and BX cheeses mainly harbored Staphylococcus succinus Peptidase activities were higher in cheeses made with attenuated starter than in CC, which had the lowest concentration of free amino acids. Based on the enzymatic activities of adjunct Le. lactis, LL and MIX cheeses exhibited the highest glutamate dehydrogenase, cystathionine-γ-lyase, and esterase activities. As shown by multivariate statistical analyses, LL and MIX cheeses showed the highest similarity for microbiological and biochemical features. LL and MIX cheeses received the highest scores for overall sensory acceptability.IMPORTANCE This study provides in-depth knowledge of the effects of attenuated starters and surface bacterial strains on the microbiota and related metabolic activities during cheese ripening. The use of attenuated Lc. lactis strongly impacted the microbiota assembly of caciotta cheese. This led to improved biochemical and sensory features compared to conventional cheese. Among surface bacterial strains, Le. lactis played a key role in the metabolic activities involved in cheese ripening. This resulted in an improvement of the sensory quality of caciotta cheese. The use of attenuated lactic acid bacteria and the surface adjunct Le. lactis could be a useful biotechnology to improve the flavor formation of caciotta cheese.

Control of oxidation-reduction potential during Cheddar cheese ripening and its effect on the production of volatile flavour compounds

In cheese, a negative oxidation-reduction (redox) potential is required for the stability of aroma, especially that associated with volatile sulphur compounds. To control the redox potential during ripening, redox agents were added to the salted curd of Cheddar cheese before pressing. The control cheese contained only salt, while different oxidising or reducing agents were added with the NaCl to the experimental cheeses. KIO3 (at 0·05, 0·1 and 1%, w/w) was used as the oxidising agent while cysteine (at 2%, w/w) and Na2S2O4 (at 0·05 and 0·1%, w/w) were used as reducing agents. During ripening the redox potential of the cheeses made with the reducing agents did not differ significantly from the control cheese (E h ≈ -120 mV) while the cheeses made with 0·1 and 0·05% KIO3 had a significantly higher and positive redox potential in the first month of ripening. Cheese made with 1% KIO3 had positive values of redox potential throughout ripening but no starter lactic acid bacteria survived in this cheese; however, numbers of starter organisms in all other cheeses were similar. Principal component analysis (PCA) of the volatile compounds clearly separated the cheeses made with the reducing agents from cheeses made with the oxidising agents at 2 month of ripening. Cheeses with reducing agents were characterized by the presence of sulphur compounds whereas cheeses made with KIO3 were characterized mainly by aldehydes. At 6 month of ripening, separation by PCA was less evident. These findings support the hypothesis that redox potential could be controlled during ripening and that this parameter has an influence on the development of cheese flavour.

Statistical optimization of cell disruption techniques for releasing intracellular X-prolyl dipeptidyl aminopeptidase from Lactococcus lactis spp. lactis

X-prolyl dipeptidyl aminopeptidase (PepX) is an intracellular enzyme from the Gram-positive bacterium Lactococcus lactis spp. lactis NRRL B-1821, and it has commercial importance. The objective of this study was to compare the effects of several cell disruption methods on the activity of PepX. Statistical optimization methods were performed for two cavitation methods, hydrodynamic (high-pressure homogenization) and acoustic (sonication), to determine the more appropriate disruption method. Two level factorial design (2FI), with the parameters of number of cycles and pressure, and Box-Behnken design (BBD), with the parameters of cycle, sonication time, and power, were used for the optimization of the high-pressure homogenization and sonication methods, respectively. In addition, disruption methods, consisting of lysozyme, bead milling, heat treatment, freeze-thawing, liquid nitrogen, ethylenediaminetetraacetic acid (EDTA), Triton-X, sodium dodecyl sulfate (SDS), chloroform, and antibiotics, were performed and compared with the high-pressure homogenization and sonication methods. The optimized values of high-pressure homogenization were one cycle at 130 MPa providing activity of 114.47 mU ml(-1), while sonication afforded an activity of 145.09 mU ml(-1) at 28 min with 91% power and three cycles. In conclusion, sonication was the more effective disruption method, and its optimal operation parameters were manifested for the release of intracellular enzyme from a L. lactis spp. lactis strain, which is a Gram-positive bacterium.

Using physical approaches for the attenuation of lactic acid bacteria in an organic rice beverage

A wild strain of Lactobacillus plantarum, isolated from an Italian sourdough, was inoculated in an organic rice drink; however, it caused a strong acidification. Thus, it was preliminary processed through homogenization (single or multiple passes) or sonication (US) and then inoculated in the beverage. The samples were stored at 4 °C and analyzed to assess pH, production of lactic acid, viable count and sensory scores. A US-2-step process (power, 80%) could control acidification; viability and sensory traits were never affected by sonication. This result was confirmed on two commercial probiotics (Lactobacillus casei LC01 and Bifidobacterium animalis subsp. lactis Bb12). In the 2nd step samples inoculated with attenuated strains were also stored under thermal abuse conditions (25 or 37 °C for 4 or 24 h, then at 4 °C) and the results showed that US could control acidification for a short thermal abuse. Finally, US-attenuated starter cultures were inoculated in the rice drink containing β-glucans as healthy compounds; the targets did not cause any significant change of prebiotic.

Comparative study of fungal cell disruption--scope and limitations of the methods

Simple and effective protocols of cell wall disruption were elaborated for tested fungal strains: Penicillium citrinum, Aspergillus fumigatus, Rhodotorula gracilis. Several techniques of cell wall disintegration were studied, including ultrasound disintegration, homogenization in bead mill, application of chemicals of various types, and osmotic shock. The release of proteins from fungal cells and the activity of a cytosolic enzyme, glucose-6-phosphate dehydrogenase, in the crude extracts were assayed to determine and compare the efficacy of each method. The presented studies allowed adjusting the particular method to a particular strain. The mechanical methods of disintegration appeared to be the most effective for the disintegration of yeast, R. gracilis, and filamentous fungi, A. fumigatus and P. citrinum. Ultrasonication and bead milling led to obtaining fungal cell-free extracts containing high concentrations of soluble proteins and active glucose-6-phosphate dehydrogenase systems.

CTAB, Triton X-100 and freezing-thawing treatments of Candida guilliermondii: effects on permeability and accessibility of the glucose-6-phosphate dehydrogenase, xylose reductase and xylitol dehydrogenase enzymes

Cells of Candida guilliermondii (ATCC 201935) were permeabilised with surfactant treatment (CTAB or Triton X-100) or a freezing-thawing procedure. Treatments were monitored by in situ activities of the key enzymes involved in xylose metabolism, that is, glucose-6-phosphate dehydrogenase (G6PD), xylose reductase (XR) and xylitol dehydrogenase (XD). The permeabilising ability of the surfactants was dependent on its concentration and incubation time. The optimum operation conditions for the permeabilisation of C. guilliermondii with surfactants were 0.41 mM (CTAB) or 2.78 mM (Triton X-100), 30°C, and pH 7 at 200 rpm for 50 min. The maximum permeabilisation measured in terms of the in situ G6PD activity observed was, in order, as follows: CTAB (122.4±15.7U/g(cells)) > freezing-thawing (54.3 ± 1.9U/g(cells))>Triton X-100 (23.5 ± 0.0U/g(cells)). These results suggest that CTAB surfactant is more effective in the permeabilisation of C. guilliermondii cells in comparison to the freezing-thawing and Triton X-100 treatments. Nevertheless, freezing-thawing was the only treatment that allowed measurable in situ XR activity. Therefore, freezing-thawing permeabilised yeast cells could be used as a source of xylose reductase for analytical purposes or for use in biotransformation process such as xylitol preparation from xylose. The level of in situ xylose reductase was found to be 13.2 ± 0.1 U/g(cells).

Responses of Escherichia coli, Listeria monocytogenes, and Staphylococcus aureus to simulated food processing treatments, determined using fluorescence-activated cell sorting and plate counting

Three common food pathogenic microorganisms were exposed to treatments simulating those used in food processing. Treated cell suspensions were then analyzed for reduction in growth by plate counting. Flow cytometry (FCM) and fluorescence-activated cell sorting (FACS) were carried out on treated cells stained for membrane integrity (Syto 9/propidium iodide) or the presence of membrane potential [DiOC2(3)]. For each microbial species, representative cells from various subpopulations detected by FCM were sorted onto selective and nonselective agar and evaluated for growth and recovery rates. In general, treatments giving rise to the highest reductions in counts also had the greatest effects on cell membrane integrity and membrane potential. Overall, treatments that impacted cell membrane permeability did not necessarily have a comparable effect on membrane potential. In addition, some bacterial species with extensively damaged membranes, as detected by FCM, appeared to be able to replicate and grow after sorting. Growth of sorted cells from various subpopulations was not always reflected in plate counts, and in some cases the staining protocol may have rendered cells unculturable. Optimized FCM protocols generated a greater insight into the extent of the heterogeneous bacterial population responses to food control measures than did plate counts. This study underlined the requirement to use FACS to relate various cytometric profiles generated by various staining protocols with the ability of cells to grow on microbial agar plates. Such information is a prerequisite for more-widespread adoption of FCM as a routine microbiological analytical technique.