الفهرس 
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Abstract
This study aimed to : The polymerase chain reaction (PCR) is a modern method that could differentiate microorganisms from each other by detecting certain DNA sequences uniquely associated with each species. This study aimed to optimize an effective PCR assay to detect Salmonella ser. Typhimurium in synthetic culture media, milk, and dairy products. The results of the study could be summarized as follows :
1.The conditions of both DNA extraction and PCR reactions were developed in terms of the boiling time for DNA extraction and primer concentration for the PCR assay.
2.Results showed that all of the three examined boiling times of 10, 15 and 20 min were equally effective for carrying out the efficient DNA extraction from Salmonella cells.
3.PCR reaction mixture involving a primer concentration of 1300 nM was the most efficient concentration, compared to 650 nM, and 325 nM for detecting Salmonella ser. Typhimurium.
4.The sensitivity of the developed PCR method to detect different viable numbers of Salmonella ser. Typhimurium in tryptone soya broth (TSB) and reconstituted skim milk (RSM) was examined.
5.The PCR assay was able to detect 109, 108, 107, and 106 cfu mL-1 of Salmonella in TSB. Whereas, it could detect 109, and 108 cfu mL-1 of Salmonella in RSM.
6.These results suggested that the developed PCR method had higher sensitivity to detect Salmonella in TSB, compared to RSM. This was attributed to some milk components that could be inhibitory to PCR reactions.
7.A pre-enrichment step was used to improve the sensitivity of the end point PCR method to detect various viable numbers of Salmonella in reconstituted skim milk.
8.Pre-enrichment was carried out using tryptone soya broth (TSB), as a nutritious, non-selective culture medium for a prolonged time of 24 h and shorter time of 8 h.
9. A 24 h culture of Salmonella was serially diluted in RSM to provide viable numbers ranging from 109 to 10 cfu/ ml. The dilutions were then pre-enriched in TSB for 24 h and 8 h at 37°C.
10.The results showed that pre-enrichment in TSB for 24 h and 8 h could improve the sensitivity of the PCR assay from 108 cfu/ml to 10 cfu/ml.
11.Pre-enrichment for 8 h was more time-efficient than 24 h, it was thus selected for pursuing the rest of experiments described in this work.
12.The effect of different contentsof fat inbuffalo milk (0.1, 3.0 and 6.5% fat)on the sensitivity of the end point PCR method combined with 8 h pre-enrichment to detect Salmonella was studied.
13.The results suggested that the sensitivity of the end point PCR combined with pre-enrichment for 8 h was not affected by different contents of fat.
14.The effect of different contents of milk protein(3, 5 and 8%)in RSM on the sensitivity of the end point PCR method combined with 8 h pre-enrichment to detect Salmonella was examined.
15.It was found that there was no effect of the different contents of protein on the sensitivity of the end point PCR assay combined with 8 h pre-enrichment.
16.The sensitivity of the pre-enrichment/PCR method was not affected by the presence of different concentrations of NaCl (3, 5 and 7 %) in RSM.
17.The end point PCR assay combined with pre-enrichment for 8 h was applied to the detection of different numbers of Salmonella ser. Typhimurium in yoghurt prepared from full fat buffalo milk (6.5 % fat).
18.Results showed that the end point PCR analysis combined with pre-enrichment for 8 h could successfully detect different viable numbers of Salmonella ser. Typhimurium ranging from 107to 10 cfu/g in yoghurt.
19.The end point PCR assay combined with pre-enrichment for 8 h was also used to detect Salmonella ser. Typhimurium in Domiati cheese prepared from buffalo’s milk (6.5% fat), salted with 7% (w/w%) of milk weight.
20.The method was also able to detect various numbers of Salmonella ser. Typhimurium ranging from 107to10 cfu/g in Domiati cheese. Recommendation : This study recommends using end point PCR assay to be combined with pre-enrichment in tryptone soya broth for 8 h for the detection of Salmonella ser. Typhimurium in milk and dairy products, at dairy plants and the laboratories of food inspection authorities.