Browsing by Subject "nisin"

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  • Fu, Y; Qiao, W; Zhu, D; Wang, X; Liu, F; Xu, H; Saris, Per Erik Joakim; Kuipers, Osacar; Qiao, Mingqiang (2018)
    Nisin, an important bacteriocin from Lactococcus lactis subsp., is primarily active against various Gram-positive bacteria. Leucocin C, produced by Leuconostoc carnosum 4010, is a class IIa bacteriocin used to inhibit the growth of Listeria monocytogenes. Because two bacteriocins have different modes of action, the combined use of them could be a potential strategy for effective inhibition of foodborne pathogens. In this study, L. lactis N8-r-lecCI (N8 harboring lecCI gene) coexpressing nisin–leucocin C was constructed based on the food-grade carrier L. lactis N8. Production of both bacteriocins was stably maintained. Antimicrobial measurements showed that the recombinant strain is effectively against Listeria monocytogenes and Staphylococcus aureus and moderately against Salmonella enterica serovar Enteritidis and Escherichia coli because of its stronger antibacterial activity than the parental strain, this result first demonstrated that the co-expression of nisin and leucocin C results in highly efficient antimicrobial activity. The checkerboard assay showed that the antibacterial activity of L. lactis N8-r-lecCI supernatant was enhanced in the presence of low concentration of EDTA. Analysis of the scanning electron microscope image showed the biggest cellular morphology change in L. monocytogenes treated with a mixture of EDTA and L. lactis N8-r-lecCI supernatant. The practical effect was verified in pasteurized milk through time-kill assay. The L. lactis N8-r-lecCI strain expressing both nisin and leucocin C has a promising application prospect in pasteurized milk processing and preservation because of its strong antibacterial activity.
  • Chandrasekar Rajendran, Suresh Chander (Helsingfors universitet, 2014)
    Idli is a popular cereal-legume fermented food of Indian origin. It is steam cooked from fermented (lactic acid-yeast) batter of rice (cereal) and black gram (legume). Idli preparation process includes three major steps – soaking of rice and black gram, grounding and fermentation. The idli preparation process is laborious, as the whole procedure takes about 20 hours. Further, the fermented batter has a shelf life of 4-5 days at 4 ºC. Literature studies reveal less efforts has been taken to improve shelf life and nutritional quality of idli. The overall aim of this thesis was to improve the quality of idli batter by mild heat treatment (Objective 1) and through microbial applications (Objective 2-4). First, the fermented idli batter was mild heat (MH) treated (57, 60, 63, 66 and 70 ºC ) to reduce the high (10.5 log cfu/g) lactic acid bacteria and yeast counts for enhancing the shelf stability at refrigerated storage. MH treatment (at 70 ºC) induced the highest reduction (3.6 log cfu/g) without affecting the pasting profile of idli batter. During storage study (upto 10 days at 4 ºC) the microbial counts further decreased without change in pH.The second objective was to monitor the changes in physicochemical properties and B-vitamin (riboflavin, folate and vitamin B12) levels in idli batter fermentation on addition of starters - Lactococcus lactis N8 (SAA1) and Saccharomyces boulardii (YEA1). Fermentation profiles were recorded individually and in combination of starters. SAA1 and YEA1 were able to enhance or retain riboflavin and folate levels, but no change in vitamin B12 levels were observed during fermentation. Further, YEA1 individually and in combination with SAA1 significantly improved the idli batter volume, implying high gas production. The third objective was to produce nisin in idli batter by addition of SAA1 (nisin producer). The results highlighted SAA1 was capable of producing nisin in idli. However, the produced nisin was degraded by the activity of indigenous LAB and yeast in idli batter. The final objective of this thesis was to determine the viability of probiotic Bacillus coagulans (BAC1) spores after cooking (steaming and microwaving) and during storage (at 4 ºC) of idli batter. Microwave cooking resulted in higher reduction of BAC1 than steam cooking. However, 5.4 log cfu/g of BAC1 spores were still viable in steamed idli from the initial added amount (8.2 log cfu/g). The BAC1 spores were not stable in idli batter suggesting spore outgrowth during storage. In summary, these results present different strategies and information for future process and product developments in idli.
  • Wambui, Joseph; Eshwar, Athmanya K.; Aalto-Araneda, Mariella; Pöntinen, Anna; Stevens, Marc J. A.; Njage, Patrick M. K.; Tasara, Taurai (2020)
    Nisin is a commonly used bacteriocin for controlling spoilage and pathogenic bacteria in food products. Strains possessing high natural nisin resistance that reduce or increase the potency of this bacteriocin against Listeria monocytogenes have been described. Our study sought to gather more insights into nisin resistance mechanisms in natural L. monocytogenes populations by examining a collection of 356 field strains that were isolated from different foods, food production environments, animals and human infections. A growth curve analysis-based approach was used to access nisin inhibition levels and assign the L. monocytogenes strains into three nisin response phenotypic categories; resistant (66%), intermediate (26%), and sensitive (8%). Using this categorization isolation source, serotype, genetic lineage, clonal complex (CC) and strain-dependent natural variation in nisin phenotypic resistance among L. monocytogenes field strains was revealed. Whole genome sequence analysis and comparison of high nisin resistant and sensitive strains led to the identification of new naturally occurring mutations in nisin response genes associated with increased nisin resistance and sensitivity in this bacterium. Increased nisin resistance was detected in strains harboring RsbUG77S and PBPB3V240F amino acid substitution mutations, which also showed increased detergent stress resistance as well as increased virulence in a zebra fish infection model. On the other hand, increased natural nisin sensitivity was detected among strains with mutations in sigB, vir, and dlt operons that also showed increased lysozyme sensitivity and lower virulence. Overall, our study identified naturally selected mutations involving pbpB3 (lm0441) as well as sigB, vir, and dlt operon genes that are associated with intrinsic nisin resistance in L. monocytogenes field strains recovered from various food and human associated sources. Finally, we show that combining growth parameter-based phenotypic analysis and genome sequencing is an effective approach that can be useful for the identification of novel nisin response associated genetic variants among L. monocytogenes field strains.