الفهرس 
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Abstract
Lactic acid bacteria (LAB) are generally recognized as safe (GRAS microorganisms) and play an important role in food and feed fermentation and preservation either as the natural microflora or as starter cultures added under controlled conditions. LAB species with antagonistic activity for improving the quality and safety of meat and dairy products), lactic acid is able to inhibit the growth of many types of food spoilage bacteria, including gram-negative species of the families Enterobacteriaceae such as Citrobacter freundii , Proteus mirabilis and Salmonella sp.
The aim of the present study was to investigate the effect of applying two different Lactobacillus protective cultures, with the ability to produce bacteriocin-like inhibitory substances (BLIS), individually or in combination, on certain microbiological, chemical and sensory parameters during storage of refrigerated vacuum-packaged sliced poultry meat.
A total of 34 different food samples representing different food sources (poultry meat, dairy products and vegetables) are collected from local markets. All the samples were kept in sterile insulated bags. Out of 195 colonies were isolated on brilliant green and S-S media 49 colonies only had salmonella characters in triple sugar iron test. most of isolates were isolated from chickens specimens followed by vegetables and fruits. The lowest number of isolates were isolated from dairy products.
Based on color change in media, morphological characteristics and biochemical characteristics were studied and confirmed. A series of biochemical tests, namely tartrate, citrate, motility, H2S gas production, LIA tests were performed for forty-nine isolates. Results of biochemical tests indicated that out of forty-nine isolates, ten of them showed biochemical characteristics of Salmonella typhi like characters. Rest of isolates were suspected to be S. pullorum (9 isolates), S. gallinarum (5) , S . typhiumrium (6), S. chlorasous or S. paratyphi (7) Shigella (12) isolates . Chicken carcass was the most contaminated samples by Salmonella followed by milk, cheese, tomato, yogurt , orange, lettuce, carrots and herbs respectively. Salmonella is a gram negative, rod shaped bacteria, Non -spore forming, predominately motile, Peritrichous flagella, citrate positive, indole test positive, gas production, H2S , tartrate negative.
The antibiotic sensitivity test was done for the ten Salmonella typhi suspected isolates. The test was performed by the following seven antibiotics, ampicillin, amoxicillin, tetracycline, gentamycin, nalidixic acid, ciprofloxacin and streptomycin by using disc diffusion method . from results, two strains complete resistant to all seven antibiotics tested, hence highlighting the preponderance of multidrug-resistant isolates. 80% of isolates were resistant to ampicillin. seven isolates were resistant to amoxicillin and streptomycin, while six isolates were resistant to nalidixic acid and tetracycline. five and three isolates were resistant to gentamycin and ciprofloxacin respectively.
The multidrug-resistant isolates were chosen for molecular identification. The molecular identification was done by sigma laboratories which proved that the two Salmonella suspected isolates were Citrobacter freundii and Protues mirabilis not Salmonella. Both of the two resistant isolates sources were from chicken carcass.
Rest of the experimental part in this thesis was carried on the two organisms beside the standard Salmonella typhi ATCC 14028.
In this thesis the possibility of using Lactobacillus acidophilus and Streptococcus thermophilus against the antibiotic resistant Salmonella typhi 14028 , Citrobacter freundii and Protues mirabilis was studied .
It was found that Citrobacter freundii and Proteus mirabilis showed susceptibility against Streptococcus thermophilus & Lactobacillus acidophilus supernatants. Although the two supernatants had the same antimicrobial activity on Proteus mirabilis. Citrobacter freundii was more susceptible to the supernatants than Proteus mirabilis. The inhibition Zone of LAB supernatants were 20 mm and 15 mm on Citrobacter freundii respectively. S. typhis 14028 was the still completely resistant to Lactobacillus acidophilus supernatant rather than the other two pathogens, while the inhibition zone of S. thermophilus supernatant possessed antimicrobial substances inhibited S. typhi growth but the inhibition less than the other two pathogens. the S. thermophilus supernatant inhibition Zone on S. typhi was 10 mm.
In the thesis Gamma ray was used to activate the LAB cells to produce their antimicrobial substances and to increase the antimicrobial action of their supernatants on the 3 pathogens In this study, gamma irradiation doses used were 2.5, 5.0,10.0,15.0,20.0 Gray.
the net of the irradiation results showed that Five Gray was the best activated dose for LAB cells against the 3 pathogens except Lactobacillus acidophilus living cells against Citrobacter freundii in this case the best activation dose was15 Gray.
Twenty Gray had the greatest activation power on LAB supernatants. This dose increased the antimicrobial influence of L. acidophilus and S. thermophilus supernatants increased S.typhi inhibition zones 20mm and 15mm more than the non-irradiated respectively toward.
In this study the compression between acetic acid ,lactic acid, citric acid antimicrobial activity and the activated free cell supernatants of L. acidophilus and S. thermophilus against the three pathogens to evaluate the efficiency of the activated supernatants to use them parallel to acetic, citric and lactic acids in food preservation field. Results proved that the antimicrobial activity of the irradiated supernatant of S. thermophilus against C. freundii, and S.typhi were equivalent to the following concentrations ≥2.5%,≥3.5%and 4.0 % of acetic, citric and lactic acids respectively while the same supernatant had the same antimicrobial activity of ≥2.5% of the 3 acids for each against P. mirabilis. The antimicrobial activity of L. acidophlilus irradiated supernatant was equal to the antimicrobial activity of 4% of the 3 acids against the 3 pathogens except with lactic acid the previous supernatant was more stronger than acid.
The previous chapter in the thesis concerned about the antibacterial activities of the free cell supernatants of L. acidophilus and S. thermophilus This part in the thesis concerned about the antimicrobial activities of the viable cells of the two LAB strains. All the antimicrobial tests were done by Disk diffusion method but in order to know the reduction percentages exactly in the 3 pathogens the viable count must be determined. This step in the thesis was considered as preparation step for application on chicken carcasses to discover the potential for using them as protective culture in chicken and meat products. The result proved that the viable counts 108 of 24hr of both L. acidophilus and S. thermophiles could successfully reduce the viability of the 3 pathogens after 24hr but couldn’t completely killed them while 48 old cultures killed 103 of pathogens after 24hr.
The previous results proved that 48h controlled the presence of the 3pathogens in artificial medium. The goal of this part in the thesis investigation the antimicrobial activity of the 2 LAB strains against the 3 pathogens in chicken carcass
Chicken carcass are heavily involved in the food industry, also it is considered the major source of the 3 pathogens contamination. Consecutive is considered a basic source of human antibiotic resistance bacteria.
Over growth of LAB is a problem in food and fermentation industries. Production of excess acids causes sour taste, excess esters production added flavors sometimes are not desirable plus the change in the food texture. So determine the least lethal time for pathogens by the LAB was necessary .
C. freundii and S. typhi viable count decreased 2log cycles while P. mirabilis viable count decreased 1log cycle respectively when they artificially injected to pieces of chicken carcass with L. acidophilus cells (108) 30 minutes in refrigerator and decreased 3 log cycles after 60 minutes. Two hours was sufficient to eliminate the pathogens from the chicken pieces .
The antimicrobial S. thermophiles activated cells by 5Gy of gamma ray abled to kill after 2hours C. freundii and P. mirabilis after one hour and S. typhi thirty minutes. S. thermophilus reduced the3 pathogens viable counts 2 log cycles for C. freundii and P. mirabilis and one log cycle for after 60 minutes while 30 minutes was sufficient to decrease the 3 pathogens viable counts one log cycle.
LAB are involved in a large number of the spontaneous food fermentation. They are added in many cooked foods, salads and ready-made food in the form of yogurt beside used in the medical field as a dietary supplement as well as against pathogenic bacteria in the intestinal tract.
LAB or yogurt can’t be added to many food. Also they are not suitable to resist the antibiotic resistant biofilms which are formed by undesirable bacteria in machines of food manufactures. Moreover they can’t solve the problem of P.mirabilis antibiotic resistance which forms biofilm in ureteral stents and urethral catheters.
This part of the thesis compared between the antimicrobial activities of S. thermophilus and L.acidophilus mixed cultures and their supernatants for inhibiting pathogenic bacteria in food .
The mixtures of L. acidophilus and S. thermpohilus as well as their supernatants were applied on cubs of chicken carcass which was artificially contaminated by the three pathogens individually at 37oC for 4 hrs to determine the killing time for each mixture. According to the previous results the LAB mixture was activated by 5 Gy of gamma radiation while the supernatant mixture was activated by exposing 20 Gy. Results in figures 16, 17 and tables 17 & 18 indicated the irradiated viable cells mixture was stronger than the supernatant mixture for inhibiting pathogens. Irradiated bacterial mixture killed all the three pathogens during three hours while the supernatant mixture after four hours. The LAB mixture reduced the C.ferundii viable count 1.5 log cycles, P.mirobillis 2log cycles and S.typhi more the 3 log cycles after2 hours. In another side the supernatant mixture decreased the pathogens viable count less than one log cycle after 3hours.