## 4 Discussion  
The current study elucidated the inhibitory effect of yoghourt starter culture and NiO NPs on _S. aureus_ and enterotoxin production during the processing and ripening of Domiati cheese. Strikingly, adding NiO NPs to Domiati cheese could induce a higher antibacterial effect on _S. aureus_ than yoghourt starter culture. Hence, NiO NPs could be a useful candidate against _S. aureus_ and enterotoxin production in the dairy industry. Besides, using yoghourt starter culture during the processing of Domiati cheese could prevent enterotoxin production even in presence of a high level of contamination with the enterotoxicigenic _S. aureus_ in milk that will be used for cheese making. The present study revealed that _S. aureus_ decreased one week earlier in starter cultured cheese and in a rapid manner reaching a low level at the end of the experiment versus the control. This indicated that starter culture had some inhibitory effect on _S. aureus_ during the ripening process of Domiati cheese. Notably, the addition of yoghourt could interfere with the production of staphylococcal enterotoxins (SEA). Hence, cheese manufactured with the addition of yoghourt does not create risk for consumers even if it is contaminated with high counts of the enterotoxicigenic _S. aureus_. Figure 6 illustrated that the log reduction of the enterotoxicigenic _S. aureus_ in control and yoghourt starter culture Domiati cheese did not differ significantly. Although _S. aureus_ grows over a wide range of pH between four to 10 with an optimum of six to seven, and can survive salt concentrations from 0 to 20% with an optimum of 0% [40]. Herein, the decline in the mean count of _S. aureus_ may be due to the synergistic effect of low pH and high NaCl concentration that leads to shrinkage and death of the bacterial cells [41]. Similarly, [PERSON] et al. [42] reported that the combination of a starter culture, high salt concentration (15%), low temperature, and pH (\(\sim\)5.2) had inhibitory effects on the growth of _S. aureus_ and may prevent the enterotoxin production during white-brined cheese making. There are several studies, with different outcomes, evaluating the effect of starter culture, NaCl % and storage temperature on the development of _S. aureus_ during the manufacture and storage of different types of cheeses. The current result was in accordance with [PERSON] et al. [42] who examined the effect of 0.5% starter culture on _S. aureus_ during the manufacture of white cheese stored in pasteurized brine with different salt concentrations. The author reported that the addition of starter cultures did not significantly change the total number of _S. aureus_, but only increase the number of the organisms in the examined cheese from the 15 th to the 60 th day of cheese ripening and then declined at day 90 th of ripening. It is concluded from the study that the count of _S. aureus_ in cheese is the same in the presence or absence of the starter culture. In contrast, some of the previous studies stated that the microbial load of _S. aureus_ decreased during cheese ripening [6; 14], but only three out of nine examined cheese samples showed a decrease in _S. aureus_ counts. There are some differences between the obtained results in the present study and that reported before, this may be due to using different kinds of cheese, type and amount of starter culture, the level of _S. aureus_, NaCl % or the storage temperature. Overall, the outcome from the current study and some of the previous studies is that the starter culture had not a significant inhibitory effect on _S. aureus_ versus to control. This indicated that the decrease in the development of _S. aureus_ during cheese ripening is a normal mechanism of the pathogen to mimic the chemical and microbiological changes that occur during the ripening of cheese. Otherwise, the addition of yoghourt starter culture herein is recommended to prevent enterotoxin production and avoid food poisoning that may occur due to consumption of the contaminated cheese.