الفهرس 
Introduction
History and General characteristics of S. aureusOnly 14 pages are availabe for public view
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Abstract
In the present study, three hundred and fifty random samples were collected from Assuit city, Egypt including market milk, yoghurt, Domiati cheese, kareish cheese, Ras cheese, cooking butter and small scale ice cream, (50 samples each). The samples were examined for the presence of S. aureus and MRSA, and some samples were selected for staphylococcal enterotoxins screening using ELISA technique. The isolated strains were identified using conventional method and then by PCR using 16S rRNA gene. Additionally, the enterotoxogenicity of the identified S. aureus strains was evaluated using PCR. Last but not least, yoghurt starter culture and nickel oxide nanoparticles were applied to examine its inhibitory effect on the growth of the enterotoxogenic S. aureus and enterotoxin A production during manufacturing and storage of Domiati cheese.
The obtained results revealed that 21 (42%), 7 (14%), 19 (38%), 24 (48%), 14 (28%), 20 (40%) and 37 (74%) of market milk, Domiati cheese, kareish cheese, Ras cheese, cooking butter, yoghurt and small scale ice cream were contaminated with S. aureus, respectively using conventional method, with mean counts of 1.9 x 105 ± 2.8 x 104, 1.5 x 102 ± 50, 9.8 x 103 ± 2.8 x 103, 5 x 104 ± 2.6 x 104, 1.9 x 104 ± 6.9 x 103, 8.3 x 103 ± 1.6 x 103 and 4.9 x 104 ± 2.2 x 104 CFU/ml or g, respectively. Using PCR, the obtained results revealed that 2 (9.52%), 4(57.14%), 13 (68.42%), 18 (75.00%), 9 (64.29%), 6 (30.00%) and 4 (10.81%) of the isolates recovered from the same samples were confirmed to be S. aureus, respectively. The results of both conventional method and PCR were compared and illustrated that there was an obvious decrease in the incidence of the identified S. aureus isolated in case of PCR than the conventional method.
The identified S. aureus strains were then examined for the presence of SEA and SED enterotoxin genes by PCR. It was clear that 17 out of 56 strains (30.4%) were enterotoxigenic from which 8 (47%) and 7 (41.2%) harbored SEA and SED, respectively. While, only two strains (11.8%) were positive for both SEA and SED genes. The highest incidence of the enterotoxogenic S. aureus was observed in kareish and Ras cheeses.
For MRSA, 9 (18%), 2 (4%), 19 (38%), 7 (14%) and 17 (34%) of milk, kareish cheese, Domiati cheese, cooking butter and small scale ice cream, respectively were positive with a mean of 7.1 x 103 ± 1.7 x 103, 1.6 x 103 ± 6.6 x 102, 1.2 x 103 ± 4.6 x 102 and 7.8 x 103 ± 1.9 x 103 CFU/ml or g in milk, Ras cheese, cooking butter and ice cream, respectively. The organism failed to be detected in Domiati cheese and yoghurt. Only 4 (7.4%) out of 54 tested MRSA strains (recovered from Ras cheese) were enterotoxin producers (1 for SEA (25%) and 3 for SED (75%)).
Regarding to toxin screening, Ras cheese and small scale ice cream, 15 samples each were examined for the presence of staphylococcal enterotoxins using RIDASCREEN ELISA kit after preparation of the examined samples according to the manufacturer instructions. The obtained result revealed that 6 (40%) and 2 (13.33%) of the examined samples contained SEE, respectively. But the other enterotoxins (A, B, C and D) failed to be detected in such samples.
The results of the experimental part showed that there were changes in the chemical composition of cheese during the experiment in which salt % in moisture phase increased gradually and the moisture% decreased. While the microbial quality of cheese showed an increase in aerobic plate count and the number of the inoculated S. aureus increased during manufacture and storage of Domiatti cheese then decreased on the fourth week of maturation. On the other hand, the count decreased slightly earlier (3rd week) when a starter culture was used during the manufacture of such cheese. SEA was only detected in the control group that contained S. aureus only and failed to be detected in Domiati cheese inoculated with yoghurt starter culture.
The experimental study showed that the minimum inhibitory concentration of NiO NPs was 35 µg/ml using well diffusion method on brain heart infusion agar. The effect of such MIC on the life span of raw milk was determined, but there was no change in the result between the control and the tested sample. Furthermore, the effect of NiO NPs was evaluated on S. aureus in Domiati cheese and a significant decrease in the average count was observed till the third week of ripening where the organism could not be detected.