In the master's thesis, we investigated whether depleted acid whey is a potential medium fort the cultivation of kefir grains. We observed the growth of biomass of 10 samples of kefir grains of Slovenian, English, German and Russian origin. Growth was monitored for 38 days, and kefir grains were fermented at 25 °C. We expected that depleted whey would be a potential medium for successful growth and production of kefir grain biomass. All of the kefir grains tested successfully produced biomass, some better and some worse, although the pH of the depleted whey was adjusted to 7 to optimize growth conditions. In biomass production, sample A Kombuchaorganic from the United Kingdom was the best and sample H from Arhel d.o.o. was the worst. We also evaluated the sensitivity of kefir grain isolates to antibiotics, as antibiotic use has been poorly controlled in the past and bacteria can develop antibiotic resistance. A total of 50 strains of lactic acid bacteria, previously isolated from kefir grains were evaluated: Leuconostoc mesenteroides (8), Lactobacillus helveticus (4), Lactobacillus kefiranofaci (8), Lactococcus lactis (9), Lentilactobacillus parakefiri (2), Lentilactobacillus kefiri (18), and one strain of Streptococcus thermophilus. In this analysis, a microdilution method was used in which specific concentrations of antibiotics were pre-applied to EUST and EUVENC plates. Particular attention was paid to clinically relevant antibiotics for which EFSA determines MIC values: Ampicillin, Gentamicin, Kanamycin, Streptomycin, Erythromycin, Clichamicin, Tetracycline, Chloramphenicol, and Vancomycin. Resistance to the following antibiotics was found most frequently: Kanamycin, Chloramphenicol, and Tetracycline. Based on the literature evaluated, it can be claimed that resistance to these antibiotics does not currently pose a threat to human health.
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