الفهرس 
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Abstract
The aim of present study : Isolation and identification of antimicrobial producing lactic acid bacteria isolated from local Egyptian dairy products including yoghurt, Ras chesse , Kariesh cheese and Laban Rayeb. It also determination of antimicrobial metabolites including Lactic acid, hydrogen peroxide, diacetyl and Bacteriocin-Like substances produced by lactic acid bacteria isolates recovered from local Egyptian dairy products and stuyding some factors affecting the activity of enteriocins producing Enterococcus isolates recovered from local Egyptian dairy products. The obtained results and conclusions could be summarized as follows: Part ”I”: Isolation And Identification Of Antimicrobial Producing Lactic Acid Bacteria from Some Local Egyptian Dairy Products:1- One hundred fifty samples of local Egyptian dairy products were randomly collected from the markets in Dakahlia governorate.These samples included 40 samples of yoghurt, 30 of Ras chesse, 48 of Kariesh cheese and 32 samples of Laban Rayeb.2-Only 16 out of 226 LAB isolates could inhibit the growth of the examined indicator organisms.3-A PCR protocol was performed to identify the antibacterial substances-producing lactic acid bacteria isolates (16 isolates) to the species level.Part ”II”:Quantitative Production Of Antimicrobial metabolites by LActic Acid Bacteria Isolates Cultured from Some Local Egyptian Dairy Producs 1.The concentration of lactic acid ranged between 0.15 to 1.74mg/L. Lb. delbrueckii, subsp. bulgaricus has the highest producing lactic acid at 48 hrs after incubation.2.Five isolates included 2 of E. gallinarum , E. faecalis and 1 isolate of E. facium produced H2o2 more than 300 ppm that were still capable to inhibit the indicator organisms in neutralized to pH 6.8 medium. The E. gallinarum 60 strain was of the highest production of H2O2, compared with the other isolates.3.The highest diacetyl yield was detected by Lb.delbrueckii subsp. Bulgaricus, compared with other isolates.4.The activity of the bacteriocins-like Ef14 (Ef14), Ef41 (Ef41) and Em72 (Em72) were maximum against En. feacalis, followed by Staph. aureus, B.cereus, S.typhi and E.coli after final step of bacteriocin like substances purification (gel filtration). Gram positive bacteria (En. feacalis and Staph. aureus) were more sensitive to all types of bacteriocin like substances, compared to spore forming bacteria or Gram negative bacteria.5.It could also be seen that the highest antimicrobial effect observed in the third fraction by spectrophotometer,which synchronized a low A280 value. The highest antimicrobial effect was associated with low concentration of proteins containing aromatic amino acids, given that aromatic amino acids varies greatly between different bacteriocins.6.A combination of antibacterial activity and Tricine-SDS-PAGE revealed that the molecular weight of the enteriocin 14, 41 and 72 are around 70, 70 and 80 kDa, in the same order. Tricine- SDS-PAGE followed by in situ antibacterial effect testing showed that the purified peptide of enterocin 14, 41 and 72 are of antibacterial activity.7.The PCR amplification protocol of enterocin L50A/B are found in three strains, whereas the enterocin A, enterocin Q, bacteriocin 31 and enterocin 1017A/B are not found in strains. The E. faecium 14 contains ent AS48 gene.E. malodoratus exhibits enterocin B, and either enterocin P or enterocin L50A/B.Part ”III”: Some Factors Affecting The Activity Of Enterocins:1.The optimum initial pH for E. faecalis and E. faecium were 6.5. However, the optimum initial pH for E. malodaratus was 7.5.2.The incubation period from 24-72 h was considered as the optimum incubation period for enterocin producing strains.3.There was no negative effect in the activity of entrocin when it was pre-adjusted to pH values over the range 3-9.0. This suggests that while acidic pH does not affect the activity of enterocins , alkaline pH value, above 9.0 decreases its activity. It could also be seen that the enterocins were still stable, following heating at 65ºC, 80ºC and 100ºC for 30 min. However, a decline activity was observed with exposure to 121ºC to 15 min. Exposing enterocins to 121ºC for 15 or 30 min at different pH values was associated with a complete loss of activity.In conclusion, These antibacterial substances producing lactic acid bacteria such as lactic acid, hydrogen peroxide , diacetyl and Enterocin 14, Enterocin 41 and Enterocin 72 produced by E. faecium 14, E. faecalis 41 and E. malodoratus 72,in the same order, and could be suitable for appliction in food industry preservation of dairy products to improve quality and safety.