Stability of Dry Probiotic Bacteria in Relation to the Cellular Membrane and Genomic DNA

Research output: Book/ReportPh.D. thesisResearch

  • Marie-Louise Rittermann W Hansen
Lactic acid bacteria are used in a wide range of fermented dairy and meat products. Several strains of lactic acid bacteria are associated with health benefits upon digestion, and are known as probiotic bacteria. Probiotic bacteria can be produced for dietary supplements, but are also added as dry powders to infant formula and functional foods, e.g. cereal and chocolate bars. Part of production of the probiotic product often includes freeze-drying of the probiotic bacteria. Drying is necessary in the preparation of dry powders, and to increase shelf life of the product at ambient temperatures. Freeze-drying and storage of probiotic bacteria can, however, also have a negative effect on cell stability. It is important to understand the processes, which can lead to loss of cell stability during dry storage in order to develop more stable probiotic products.
The purpose of the present PhD thesis was to investigate the processes, which occur in the probiotic cell during dry storage and could lead to loss of cell culturability. Interest was more specifically on the cytoplasmic membrane and the genomic DNA.
The bacterial response to fatty acid supplementation was investigated with respect to the cytoplasmic membrane. In PAPER I it was found that, by supplementing fatty acids of different unsaturation to the fermentation medium, the fatty acid composition of the cytoplasmic membrane of L. acidophilus La-5 could be altered. In PAPER II the cytoplasmic membrane was investigated in greater detail, and the lipidome of L. acidophilus La-5 was characterized. Supplementation of saturated fatty acids altered the ratio between to lipid classes, cardiolipin (CL) and monolysocardiolipin (MLCL), compared to supplementation of different unsaturated fatty acids. The results indicated that L. acidophilus La-5 possesses a molecular mechanism for optimizing the fatty acid composition of CL and MLCL species, and changing the molar ratio of CL and MLCL.
Supplementation of fatty acids also had an effect on cell stability during dry storage, which was observed in PAPER I. Supplementing oleic acid was found to give the most robust bacteria. During storage the cytoplasmic membrane was also investigated with respect to loss of membrane integrity (leakage of cytoplasm) and lipid oxidation. The amount of leakage was greater for bacteria with a more saturated membrane. Products from lipid oxidation were identified, and a higher degree of lipid oxidation was found, together with a larger loss of cell stability, when the bacteria were stored with oxygen present. However, a direct connection between lipid oxidation and loss of cell stability was not found.
The results motivated the study of PAPER III where the integrity of the bacterial DNA of B. animalis subsp. lactis BB-12 and L. acidophilus La-5 was investigated. During dry storage the loss of cell culturability was found to correlate well with the degradation of DNA for both strains. Integrity of DNA was, similarly to the loss of culturability, negatively affected by the presence of oxygen and an elevated water activity. The extent of DNA degradation and loss of culturability was, furthermore, found to be strain dependent.
Original languageEnglish
PublisherDepartment of Food Science, Faculty of Science, University of Copenhagen
Number of pages172
Publication statusPublished - 2015

ID: 141017187