Fredua-Agyeman, M; (2015) The application of isothermal microcalorimetry for studying mixed probiotic cultures. Doctoral thesis , UCL (University College London).

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Abstract

The main aim of this research was to explore the potential of the isothermal microcalorimeter to detect bacteria in mixed cultures; applied to investigate the antagonistic effect of commercial probiotics against pathogens and each other; and also the prebiotic potential of a substrate. Gastric tolerance of commercial probiotic products was also investigated with an improvement on current methods. An initial mixed culture study with Pseudomonas aeruginosa, Staphylococcus aureus and Escherichia coli in the microcalorimeter showed that the microcalorimeter could detect their growth in mixed cultures; S. aureus was always outcompeted in growth. Antagonistic activity of probiotic strains, Lactobacillus acidophilus, Bifidobacterium lactis, Bifidobacterium bifidum or commercial probiotic products against P. aeruginosa, E. coli, S. aureus and the clinically important gut pathogen, Clostridium difficile was demonstrated in the microcalorimeter and was shown to be pH-dependent using neutralized and unmodified cell free culture supernatant (CFS) produced by the probiotic strains. But concentrated CFS of the probiotics also inhibited the pathogenic species in a non pH-dependent manner, likely due to specific antimicrobial substances or bacteriocins. The result also demonstrated that probiotic strains could compete with each other in growth when put together. The prebiotic potential of inulin was demonstrated with the microcalorimeter using faecal slurry and pure probiotic strains. Gastric tolerance assay of commercial probiotic products in porcine gastric fluid, SGF (acidified NaCl solution) and FaSSGF (acidified NaCl solution with biorelevant amounts of bile salt, pepsin and lecithin) mimicking the fed and fasted states showed significant differences between the products and fluids. In conclusion, the research showed that the microcalorimeter is a useful in vitro tool for detecting bacterial growth in mixed cultures and studying functional characteristics of probiotics and prebiotics; overcoming some of the limitations of the conventional methods.

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