Ömer Şimşek

MarT Activates Expression of the MisL Autotransporter Protein of Salmonella enterica Serotype Typhimurium

MisL is a Salmonella enterica serotype Typhimurium fibronectin binding protein whose expression is induced during infection of mice. T-POP transposon mutagenesis identified marT as a positive regulatory element controlling expression of a misL::lacZYA transcriptional fusion. Gel shift analysis identified MarT as a transcriptional activator of the misL promoter.

Influence of growth conditions on the nisin production of bioengineered Lactococcus lactis strains

Nisin production of three bioengineered strains, (LAC338, LAC339 and LAC340) with immunity (nisFEG) and/or regulation (nisRK) genes of nisin biosynthesis on plasmids in the Lactococcus lactis LL27 nisin producer, was evaluated under pH-controlled and pH-uncontrolled batch fermentations. Optimization studies showed that fructose and yeast extract yielded the highest nisin activity. The strains LAC338, LAC339, and LAC340 produced 24, 45, and 44% more nisin, respectively, than wild-type L. lactis LL27 after 12-h incubation. However, sharp decreases in the yield of nisin were observed at the late phase of fermentation with LAC339 and LL27 in contrast to LAC340 and LAC338 strains for which the high level of nisin could be maintained longer. Obviously, increasing the copy number of the regulation genes together with immunity genes in the nisin producers retarded the loss of nisin in the late phase of the fermentation.

Immobilization of nisin producer Lactococcus lactis strains to chitin with surface-displayed chitin-binding domain

In this study, nisin producer Lactococcus lactis strains displaying cell surface chitin-binding domain (ChBD) and capable of immobilizing to chitin flakes were constructed. To obtain ChBD-based cell immobilization, Usp45 signal sequence with ChBD of chitinase A1 enzyme from Bacillus circulans was fused with different lengths of PrtP (153, 344, and 800 aa) or AcmA (242 aa) anchors derived from L. lactis. According to the whole cell ELISA analysis, ChBD was successfully expressed on the surface of L. lactis cells. Scanning electron microscope observations supported the conclusion of the binding analysis that L. lactis cells expressing the ChBD with long PrtP anchor (800 aa) did bind to chitin surfaces more efficiently than cells with the other ChBD anchors. The attained binding affinity of nisin producers for chitin flakes retained them in the fermentation during medium changes and enabled storage for sequential productions. Initial nisin production was stably maintained with many cycles. These results demonstrate that an efficient immobilization of L. lactis cells to chitin is possible for industrial scale repeated cycle or continuous nisin fermentation.

In vitro characterization of probiotic properties ofPediococcus pentosaceus BH105 isolated from human faeces

In this study, a new bacteriocin-producing strainPediococcus pentosaceus BH105 was isolated from human faeces and subsequentlyin vitro probiotic and antagonistic properties were investigated. This strain exhibited high viability at pH 3.0 and in the presence of pepsin, pancreatin and bile salts (0.3%). BH105 was sensitive to 19 of 25 antibiotics and showed no haemolytic activity. The bacteriocin produced by BH105 was active at wide pH range of 2.0 to 11.0 and was heat stable at 80, 90 and 100°C for 15 min. Bacteriocin activity was inhibited by pepsin, a-chymotrypsin, and proteinase K but not by trypsin,a-amylase, catalase, lysozyme and lipase. The Tricine-SDS-PAGE analysis allowed an approximation of bacteriocin BH105 size about 5 kDa.Pediococcus pentosaceus BH105 was able to adhere to Caco-2 cells (10.12±2.40%) and inhibited the adhesion ofEscherichia coli LMG3083 (88.72±5.53%) andSalmonella typhimurium SL1344 (60.64±10.97%).

Stress Response Kinetics of Two Nisin Producer Strains of Lactococcus lactis spp. lactis

The purpose of this study is to determine the survival and nisin production behaviors of two strains of Lactococcus lactis under different stress conditions that represent the food ecosystem. In this respect, the survival ratios of two nisin producers were determined under different pH, temperature, NaCl, and bile salt concentrations. Then, nisin production levels of the strains were determined at each stress conditions. Both strains had similar growth or inactivation patterns under the same stress conditions. NaCl and bile salt stresses on the survival ratio of the strains could be successfully described by the exponential decay function, whereas Gaussian function produced good fits for temperature and pH stresses. The nisin activity of two nisin producers (in their mid-exponential and/or early stationary phase) decreased dramatically under all stress conditions, except osmotic (NaCl) and low temperature applications. The results of this study showed that two nisin producers had similar adaptive responses under severe stress conditions, which could be described by appropriate mathematical equations. Moreover, the effect of harsh environment on the nisin activity of L. lactis strains depends on the stress factors applied.

Nisin production of Lactococcus lactis N8 with hemin‐stimulated cell respiration in fed‐batch fermentation system

In this study, nisin production of Lactococcus lactis N8 was optimized by independent variables of glucose, hemin and oxygen concentrations in fed‐batch fermentation in which respiration of cells was stimulated with hemin. Response surface model was able to explain the changes of the nisin production of L. lactis N8 in fed‐batch fermentation system with high fidelity (R2 98%) and insignificant lack of fit. Accordingly, the equation developed indicated the optimum parameters for glucose, hemin, and dissolved oxygen were 8 g L−1 h−1, 3 μg mL−1 and 40%, respectively. While 1711 IU mL−1 nisin was produced by L. lactis N8 in control fed‐batch fermentation, 5410 IU mL−1 nisin production was achieved within the relevant optimum parameters where the respiration of cell was stimulated with hemin. Accordingly, nisin production was enhanced 3.1 fold in fed‐batch fermentation using hemin. In conclusion the nisin production of L. lactis N8 was enhanced extensively as a result of increasing the biomass by stimulating the cell respiration with adding the hemin in the fed‐batch fermentation.

Diversity and Stability of Yeast Species During the Fermentation of Tarhana

Tarhana is a traditional cereal-based fermented food produced with a mixture of yoghurt and flour. The main microbiota in the fermentation of tarhana is yeast, together with lactic acid bacteria. In this study, the yeast microbiota of home-made tarhana (HMT) and plant-type tarhana (PTT) dough samples was evaluated and compared during fermentation. Culture-dependent LSU and ITS-5.8S rDNA sequence analysis of yeast isolates collected during the tarhana dough fermentation clarified 45 selected isolates representing different clusters. These yeast isolates displayed high homologywith species Pichia kudriavzevii (11), Candida glabrata (11), Candida humilis (10), Saccharomyces cerevisiae (7), Kluyveromyces marxianus (4), Kazachstania servazzi (1), and Kazachstania unispora (1). Additionally, both culture-dependent and PCR-Denaturated Gradient Gel Electrophoresis (PCR-DGGE) analyses showed that S. cerevisiae, P. kudriavzevii and K. marxianus were abundant in the fermentation of HMT dough samples whereas P. kudriavzevii, C. humilis, and C. glabrata dominated the PTT dough samples. It was concluded that tarhana fermentation was accomplished with the presence of a wide variety of yeast species that mainly included P. kudriavzevii in both HMT and PTT dough samples.

Comparison of lactic acid bacteria diversity during the fermentation of Tarhana produced at home and on a commercial scale

In this study, lactic acid bacteria diversity during the fermentation of homemade and commercially prepared Tarhana, a traditional fermented cereal food from Anatolia, was determined and compared. The isolates collected from Tarhana dough were differentiated according to their (GTG)5 profiles and then identified using 16S rDNA and pheS gene sequences. The variation of lactic acid bacteria during fermentation was also screened using PCR-DGGE. Commercially prepared Tarhana dough was fermented with higher Lactobacillus spp. diversity than homemade Tarhana dough. Lactobacillus casei, L. alimentarius, L. fabifermentas, and L. paralimentarius were identified differently from the fermentation of commercially prepared Tarhana dough. PCR-DGGE analysis revealed that L. plantarum was the main strain for homemade Tarhana, whereas L. brevis and L. alimentarius were observed in commercially prepared Tarhana dough fermentation. In conclusion, L. plantarum, L. brevis and L. alimentarius can be useful as a potential starter culture for the industrial production of Tarhana.