## 3 Results and Discussion
### General Discussion  
Our results demonstrate the antimicrobial effect of cobalt citrate against important foodborne pathogens, both in an aqueous solution and incorporated into polylactic acid membranes. [PERSON] et al. [36] studied the antimicrobial properties of a cobalt complex ([Co(2-phenylimidazole)\({}_{2}\)(Acetate)\({}_{2}\)]) with similar molecular mass (463.35) but different structures against various pathogenic microorganisms, including _E. coli_, \(S\). Enteritidis, _S. aureus_, and _L. monocytogenes_. The antimicrobial activity of the complex was tested by the agar diffusion method on nutrient agar. The results of both studies showed the largest growth inhibition zone in _L. monocytogenes_. However, in the study of [PERSON] et al. [36], large inhibition zones were observed in both _E. coli_ and _S. aureus_, suggesting that the antimicrobial activity of the complex is not related to the Gram type and is probably strain-specific. The authors also examined the cobalt complex for the type of antimicrobial activity exerted, with the results indicating bacterieolytic action.  
According to [PERSON] [37], cobalt interferes with the functioning of the Krebs cycle, thereby inhibiting the production of energy through oxidative metabolism. This, in turn, can lead to inhibition of the growth of the microorganism.  
In this study, the strongest inhibition by Co-cit was observed in _L. monocytogenes_ and _P. aeruginosa_. Previous studies have examined the action of individual components of the complex (cobalt and citric acid) and have demonstrated their antimicrobial properties against _L. monocytogenes_, _P. aeruginosa_, and _E. coli_. The antimicrobial properties of cobalt oxide in combination with titanium dioxide (CoO-TiO\({}_{2}\)) were examined in liquid culture against _L. monocytogenes_[38]. The results showed that CoO-TiO\({}_{2}\) is bacteriostatic and prevents the proliferation of _L. monocytogenes_ cells.  
One possibility for future research is continuing the work on using Co-cit in combination with existing preservatives but incorporated into membranes. Combinations of preservatives could impart a broader spectrum of activity to antimicrobial packaging films and increase activity against a particular pathogen of concern. The incorporation of existing preservatives into biodegradable films for packaging fresh food is already a subject of research [39].  
In accordance with the general requirements set out in the Commission Regulation (EC) No 1935/2004 [40], food contact materials should not endanger human health or bring about an unacceptable change in the composition of the food or bring about deterioration in the organoleptic characteristics thereof.  
Figure 5: Aerobic plate counts (**A**) and presumptive _Listeria monocytogenes_ counts (**B**) on smoked salmon. Treatments: inoculated with _L. monocytogenes_, covered by PLA film with 23.3% \(w/w\) Co-cit (red, circles); inoculated with _L. monocytogenes_, covered by PLA film without Co-cit (blue, squares); inoculated with _L. monocytogenes_, no film (green, triangles); uninoculated, no film (yellow, Xs). Different letters within the same time point indicate significant differences (\(p<0.05\)) between inoculated samples (uninoculated samples not included in the calculation); where there are no letters, there were no significant differences (\(p>0.05\)). Error bars represent standard deviation.  
In accordance with Commission Regulation (EC) No 10/2011 [41] on plastic materials and objects intended to come into contact with food, the limit for cobalt migrating from plastic materials and objects is 0.05 mg/kg of food or stimulant. Further tests would be required to ensure that the films examined conform to this regulation.