Within the program many segments of research work were accomplished. The presence and characteristics of Bacillus species were analysed in milk and dairy products. All strains of B. cereus possessed identical fragments of 16S-ITS23S rDNK (720 bp and 960 bp). Moreover, 30 different groups were formed based on the restriction analyses of cerAB and hblA genes, which shows high variability of the toxin coding regions and suggests the use of PCR-RFLF method for further strain typisation. Selection and study of the methods for effective detection and tracing of the toxin producing B. cereus strains in milk and dairy products contribute to the knowledge of toxicology, spreading and possible transmission of these bacteria by food. The possible methods for the modulation of fatty acids composition of milk fat were studied. The recombinant lamb chymosin was analyzed and was proven especially useful in ewe's cheese production because of good technological properties and specific proteolytic activity. According to our knowledge, this is the first report on the recombinant lamb chymosin used as an alternative coagulation enzyme in cheese production. Over a period of 2 years, thirteen lactococcal phages were isolated from the whey samples. Phages were characterized by morphological properties, host range, restriction patterns, genomic size and similarity of phage protein genes. All phages belonged to the Siphoviridae family. Five phages had prolate heads and long non-contractile tails, and eight phages had small isometric heads and long non-contractile tails. The phages with the prolate heads showed a broader host range. Screening of the lactococci that might possess the natural defense mechanisms against phage contamination revealed certain strains resistant to a wide phage spectrum. The most common defense mechanisms are masking of the receptor and the mechanism of abortive infection (Abi). Both, type phages and indicator cells appropriate for rotation strategies in cheese plants, are held in our own collection. These results are of great importance in understanding the epidemiology of lactococcus phages and natural defense mechanisms of host cells. The most important part of our research work was focused on two intestinal isolates, Lactobacillus gasseri LF221 and K7. A novel two-peptide bacteriocin with the wide inhibitory spectrum was detected. Moreover, in-vivo and in-vitro researches on conventional and gnotobiotic piglets revealed these two strains as possible probiotic strains. The mechanisms of balancing the intestinal microflora consist of competitive exclusion, production of organic acids, production of bacteriocins and finally, the immune response stimulation. Both strains are well suited to cultivation and retain the probiotic activity after spray-drying and freeze-drying. The comparison of nucleotide sequences of genes for bacteriocins' production revealed homologies at the gene level between both strains. Result is interesting because of the similarity of strains in antimicrobial activity on the one hand and their diversity in the level and optimal growth condition and bacteriocin production, RAPD and plasmid profile as well as adhesion capability to the Caco-2 cells on the other hand. The research work contributed new knowledge in the field of bacteriocinogenic lactic acid bacteria with probiotic properties.