Natural yoghurt was subject to pressures of 200 and 250 MPa/4 and 18 • C/15 min, after which milkactivated inocula of Lactobacillus acidophilus and Bifidobacterium sp. were added. The yoghurts were stored for 4 weeks at refrigeration temperature. After preparation and each week of storage, the count of bacteria, acidity, antibacterial property and an organoleptic assessment was determined. The highest survival rate was demonstrated by the bacteria of Lactobacillus delbrueckii ssp. bulgaricus, Streptococcus thermophilus and Bifidobacterium sp. in the yoghurt pressurised 200 MPa/15 min at 4 • C. Acidity increases in the control yoghurts were higher than in the pressurised ones. Pressurised yoghurts demonstrated weaker antibacterial effect in comparison to control yoghurts. Slight changes in the smell and taste were observed after pressurisation. Yoghurts pressurised at 18 • C were characterised by more favourable organoleptic properties. Better conciseness of the curd and lower whey seep out were observed in pressurised yoghurt.

The study was aimed at determining the impact of higher pressures on the antibiotic resistance of selected strains of the Lactobacillus species. Cultures of 22 Lactobacillus strains were subjected to a high-pressure treatment at 30, 60, 90, and 300 MPa/1 min/18 • C. The examined strains of Lactobacillus acidophilus displayed the highest susceptibility to ampicillin and erythromycin. Strains of Lactobacilus casei exhibited lower susceptibility to kanamycin and erythromycin. For most of the L. casei strains examined, the resistance to antibiotics decreased with an increase in the pressure applied. A pressure of 300 MPa caused an increase in the susceptibility to the antibiotics applied. Pressurization over a pressure range of 30-90 MPa had no significant effect on the survivability of the strains examined. In contrast, a pressure of 300 MPa was found to reduce the number of Lactobacillus strains to a considerable extent.

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