الفهرس 
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Abstract
The aim of this study was to: 1) isolate and identified of lactic acid bacteria from some Egyptian dairy products, 2) test the acid, bile salts- resistant, and antibiotics, enzymes- tolerant of the active lactic acid bacterial isolates, 3) study their antibacterial and antifungal activities against some pathogenic bacteria and fungi, 4) study the antimicrobial effect of some selected isolates against some spoilage and pathogenic bacteria in milk and 5) study the suitability of production of probiotic red beetroot juice by some selected isolates. from the different dairy products, 85 isolates were obtained, purified and milk clotting test was done. Among these, 30 isolates showed better clotting properties were chosen. Among the 30 LAB isolates tested for acid production, only nine reduced the pH of reconstituted skim milk (RSM) below 5.3. By morphological, cultural, physiological and biochemical characteristics as well as molecular identification using 16S-rRNA gene three isolates were identified as Streptococcus thermophilus with sequence identity 99%, one isolate as Lactobacillus acidophilus and one as L. casei with sequence identity 98% for each. Eleven organic acids were detected in the different LAB filtrates. Among them, lactic and acetic acids were the major acids produced by the five strains. Factors affecting LAB activity under stress conditions including pH, bile salt concentrations, antibiotics and digestive enzymes on the viability of the selected cultures of lactic acid bacteria was evaluated. All the strains examined exhibited variable resistance to the stress conditions. The inhibitory effect of LAB cell free extract CFSs (CFF and HCFF) were detected against seven strains of Gram-positive and Gram-negative bacteria and one yeast strain as well as some food-contaminating fungi. CFSs of L. acidophilus showed high antimicrobial activity against all the indicator pathogens. The effect of lactic acid bacterial isolates on some food-borne microbes in milk was evaluated. The mixture of L. acidophilus and Str. thermophilus KF724888 has the highest ability to inhibit most of indicator pathogens followed by a single culture of Str. thermophilus then L. acidophilus. The suitability of larger beets as a raw material for production of probiotic beet juice by lactic acid bacterial strains was studied. L. acidophilus, L. casei and Str. thermophilus strains were capable of utilizing beet juice for growth and lactic acid production, respectively. Regarding to organoleptic properties, L. casei recorded the high score in taste, odor, color, appearance, sweeteners and overall acceptability being 5, 5.2, 5 and 4.8 and 8.5, respectively in compared with control sample.