## 4 Discussion  
It is well known that strains of psychotrophic _Pseudomonas_ are the main bacteria present in spoiled air-stored seafood originating from temperate waters [2; 3]. _Pseudomonas_ has been found to cause spoilage to various seafood, including the two most important fish of the Mediterranean aquaculture, the gilt-head sea bream and the European sea bass [5; 6; 7; 8; 9]. Different strains of _P. fluorescens_ and _P. fragi_ (also used in this study) dominated these products in most of cases. Based on our results, regarding the growth of the studied strains in FJA models, a similar profile between all strains in all studied storage conditions (0, 4 and 8 \({}^{\circ}\)C) was recorded, since all strains exhibited a remarkable growth rate and population levels in FJA models. In a previous study, [PERSON] et al. [15] found a \(\mu_{max}\) equal to 0.829 \(\pm\) 0.032 (d\({}^{-1}\)) for _Pseudomonas_ spp. in mixed cultures (composed by other strains, only strain 45 was common in both cases) in FJA models of gilt-head sea bream stored at 0 \({}^{\circ}\)C (higher than that recorded in mixed cultures at 0 \({}^{\circ}\)C, \(\mu_{max}\) = 0.641 \(\pm\) 0.071). This finding clearly indicates that different mixes of _Pseudomonas_ strains present different growth rates on FJA models under particular storage conditions might be due to interspecies symbiotic or antagonistic microbial interaction. Compared to real fish, the strains inoculated in FJA models presented much higher maximum specific growth rate (\(\mu_{max}\)) than those recorded on cetrimide/tucidin/cephalidine (CFC) agar (a selective culture medium for _Pseudomonas_ spp. enumeration) in gilt-head sea bream fillets at similar chilled temperatures [6]. On the other hand, \(\mu_{max}\) was found to be much lower than those observed for _Pseudomonas_ spp. Figure 2: Volatile organic compounds that increased during storage in inoculated FJA model substrates at 0 \({}^{\circ}\)C (**a**), 4 \({}^{\circ}\)C (**b**) and 8 \({}^{\circ}\)C (**c**). in gilt-head sea bream at 0, 4 and 8 \({}^{\circ}\)C in other studies [24]. The different findings in the aforementioned cases might be due to the domination of different microbial association (including _Pseudomonas_ spp. ), which is often influenced by microbial interaction in each case [25; 26]. Moreover, strains 35 (Bacterium SBF-B5-d0: _P. fluorescens_), 45 (Bacterium SBF-B19-dsp: _P. fragi_) and 55 (Bacterium SBF-B18-dsp: _P. fragi_), isolated from fresh and spoiled sea bream fillets stored at 0 \({}^{\circ}\)C [20], also reached higher population levels compared to the others in model systems stored at 0 \({}^{\circ}\)C herein, while strains 61 (Bacterium SBF-A12-dsp: _P. fragi_) and 71 (_P. fluorescens_ strain b84) isolated from spoiled sea bream stored at higher chilled temperatures [5; 20] were also found to reach higher population levels in models stored at 4 and 8 \({}^{\circ}\)C. These findings indicate that such model substrates can be exploited as a reliable in vitro tool to monitor the microbial behavior at strain level, being able to successfully simulate real fish flesh. The latter is also in agreement with the findings by [PERSON] et al. [15]. The majority of the compounds detected only in inoculated models in the present study (e.g., methanethiol, 2-methyl-1-butanol, isopropyl alcohol, dimethyl disulfide, 2-methylbutylaldehyde, 3-methylbutylaldehyde, 2-pentanone, 2-heptanone, n-nonanone, 2-undecanone, 2-nonanone, 3-pentanone, butyl butyrate, isobutyl isobutyrate, ethyl-2 methylbutyrate, ethyl isovalerate, ethyl isobutyrate, ethyl butyrate, ethyl acetate, ethyl octanoate, ethyl propionate, ethyl tglate, lauric aldehyde, etc.) have also been found to be produced by various _Pseudomonas_ strains in inoculated sterile seafood model substrates, for example, by _P. fragi_ in black rockfish at 5, 15, 25 \({}^{\circ}\)C [18] and in prawn extract at 5 \({}^{\circ}\)C [27], or by _P. fluorescens_ in sterile black rockfish at 0 \({}^{\circ}\)C [17], confirming that such metabolites come solely from bacterial activity.