## 3 Results  
### Assessment of Nickel Oxide Nanoparticles Cytotoxicity  
In the present study, HECs were exposed to NiO NPs for 48 h and the cytotoxicity was measured by MTT assays. Results showed that cell viability was reduced and the degree of reduction was dose-dependent. In the MTT assay, the HEC viability was decreased to 95, 93, 74 and 59% for concentrations of 17.5 (0.5 MIC), 35 (MIC), 70 (2 MIC) and 140 (4 MIC) \(\upmu\)g/mL, respectively (Figure 2). Effect of Yoghourt Starter Culture and NiO NPs on S. aureus during Manufacture and Storage of Domiati Cheese  
The average concentration of _S. aureus_ in milk was 6.3 \(\pm\) 0.02 log CFU/mL in the assayed treatments. After cheesemaking, the initial concentration of _S. aureus_ remained at the same levels in the control samples (6.6 \(\pm\) 1.02 log CFU/g) while a slight, non-significant decrease was found for cheese samples with the addition of yoghourt (5.8 \(\pm\) 0.15 log CFU/g). The microbial kinetics of _S. aureus_ and APC in control Domiati cheees are shown in Figure 3. At time 0 (the finished product), APC counts increased gradually till the 21 st day of cheese ripening (reaching, 9 log CFU/g). It was found that Domiati cheees cannot support _S. aureus_ growth since counts remained constant until the 21 st day of storage and decreased from day 28 th until reaching 3.5 log CFU/g at day 35 th of storage. pH values decreased over time until reaching 4.0 at the end of storage (Figure 3). In the case of cheese containing yoghourt starter culture, the average APC counts showed a 0.5 log reduction during manufacturing until the cheese was obtained, then increased gradually to reach its maximum growth (9.9 \(\pm\) 0.20 log CFU/g) at the end of cheese storage (Figure 4). On the 21 st day of ripening, the level of _S. aureus_ declined drastically to reach the final concentration of 2.5 \(\pm\) 0.06 log CFU/g (pH; 3.8) at the end of the experiment (Figure 4). After examining the antibacterial effect of different concentrations of the studied nanoparticles, 35 \(\upmu\)g/mL was the minimum inhibitory concentration (MIC) that could inhibit the enterotoxic _S. aureus_ strains. Then, Domiati cheese was manufactured from  
Figure 3: Total bacterial counts, _S. aureus_ survival and pH evolution during Domiati cheese ripening for 35 d storage in control samples. \(\bullet\) Observed growth of total bacterial counts (dashed line corresponds to the fitted Baranyi model); \(\square\) Observed survival of _S. aureus_ (solid line corresponds to the fitted Weibull model); \(\times\) pH evolution. Figure 2: Cytotoxicity assessment of NiO NPs over normal human epithelial cell (HEC) line using the standard MTT. cow's milk formulated with 35 \(\upmu\)g/mL NiO NPs and the periodical examination showed that there was a drastic decrease in the number of bacterial cells during the manufacture and ripening of cheeses with the added NiO NPs versus the control samples (\(p<0.05\)). In other words, the count of _S. aureus_ was reduced directly after manufacturing of NiO NPs Domiati cheese (Figure 5). In curd, the population of the microbe decreased by 0.5 log CFU/g, while one log reduction was observed in the finished product (zero time). Additionally, _S. aureus_ rapidly decreased during cheese ripening until becoming undetectable on the 21 st day of storage (Figure 5). For APC, there was a continuous increase in the total bacterial count from 6. 8 log CFU/g that reached 8.5 log CFU/g during the manufacturing and ripening of this kind of cheese with a clear decrease in pH (from 6.5 to 3.65) (Figure 5). Altogether, the bacterial load in the examined cheese samples was variable between groups. The initial concentration of _S. aureus_ in milk did not change in the control group (6.6 \(\pm\) 1.02 log CFU/g) after cheese making. A slight decrease in _S. aureus_ count was observed in cheese containing yoghourt starter culture and NiO NPs (5.8 \(\pm\) 0.15 log CFU/g & 5.3 log CFU/g, respectively) at time 0. During cheese ripening, the level of _S.