## 3 Results and Discussion  
### Effect of HP Treatment on the Reduction of Initial Microfora  
The initial microbial load (log CFU/g) for untreated sea bass fillets was 5.05 (\(\pm\)0.40) for TVC, 4.58 (\(\pm\)0.06) for _Pseudomonas_ spp., 3.25 (\(\pm\)0.11) for _Brochlor trix thermosphacta_, 2.52 (\(\pm\)0.09) for _Enterobacteriaceae_ spp., 4.09 (\(\pm\)0.10) for LAB, and 4.77 (\(\pm\)0.11) for H\({}_{2}\)S-producing bacteria (Figure 1b). The application of HP treatment did not significantly affect the initial TVC load of the treated sea bass fillets when low pressures (100 MPa) were applied in combination with low treatment temperatures (3-7 \({}^{\circ}\)C), regardless of the processing time, while an ambient temperature at low pressures resulted in a significant (\(p\) < 0.5) TVC load reduction of up to 0.7 log CFU/g, as depicted in Figure 1a. When fillets were processed at 200 and 300 MPa, a decrease of 1.4 and 2.1 log CFU/g, respectively, was measured. An increase in the process pressure to 400 MPa reduced microflora by more than 3 log cycles to non-measurable levels (<2.0 log CFU/g) of TVC, regardless of the processing temperature.  
Figure 1: (**a**) Initial microbial load of untreated European sea bass fillets and after HP processing at different pressure–temperature–time conditions. (**b**) Microbial load of all tested microorganisms of untreated European sea bass fillets and after HP processing at different pressures and 25 \({}^{\circ}\)C for 10 min. Symbol (*) refers to non-measurable levels of microorganisms, i.e., <2.0 log CFU/g for TVC, _Pseudomonas_ spp., and _Brochlor trix thermosphacta_ or <1 log CFU/g for _Enterobacteriaceae_ spp., LAB, and H\({}_{2}\)S producing bacteria. Each bar shows the mean and the standard deviation of four replicates (\(n\) = 4).  
With respect to the effect of HP processing on the mitigation of different microorganisms, the _Pseudomonas_ spp. load was significantly decreased when low pressures of 100-200 MPa were applied, whereas the application of higher pressures of 300-400 MPa decreased the load to non-measurable limits (Figure 1b). These findings are in agreement with those reported by [PERSON] et al. in which high pressure treatment at 500 MPa for 2 min at 4 \({}^{\circ}\)C reduced the microbial load of _Pseudomonas_ spp. to below measurable limits in fresh salmon and plaice fillets, exhibiting a lag phase of growth of 6-8 d when the fillets were stored at 4 \({}^{\circ}\)C [40]. Similar results were also observed for LAB in which applied pressures of above 200 MPa led to non-measurable levels of LAB at all temperatures tested. On the other hand, the _Brochothrix thermosphacta_ load was decreased to non-measurable limits when pressures equal to or higher than 200 MPa were applied. Previous studies demonstrated that _Brochothrix thermosphacta_ may be a dominant microorganism following treatment under HP conditions in food products of animal origin [41]. However, this was not confirmed in this study, and other microorganisms played a more crucial role in the shelf life determination of sea bass fillets. The most HP-resistant microorganisms, in descending order, were _Enterobacteriaceae_ spp. and H\({}_{2}\)S-producing bacteria, requiring a pressure above 300 MPa for a significant microbial load reduction (2.0-3.0 log CFU/g). These results are in accordance with those previously reported for HP-treated mackerel fillets at 100-500 MPa [42]. This study reported that the HP processing at low pressure levels (100 MPa) did not significantly affect the counts of H\({}_{2}\)S-producing bacteria, whereas the application of higher pressures (300-500 MPa for 2 or 5 min at 17-22 \({}^{\circ}\)C) led to non-measurable levels of H\({}_{2}\)S-producing bacteria.