Antibacterial activity of Borago officinalis and Brassica juncea aqueous extracts evaluated in vitro and in situ using different food model systems

The present study was undertaken to characterize the antibacterial activity of the aqueous extracts (AEs) obtained from the leaves of Borago officinalis L. and Brassica juncea L. The antagonistic activity was evaluated against several bacteria (42 strains of Listeria monocytogenes, 35 strains of Staphylococcus aureus, 38 strains of Enterobacter spp. and 18 strains of Salmonella enterica) commonly associated with foodborne diseases by paper disc diffusion method. The susceptibility to the plant extracts was strain specific. Thirty-five strains (7 L. monocytogenes, 11 S. aureus, 1 S. Enteritidis, 1 S. Veneziana, 7 Enterobacter hormaechei, 5 Enterobacter cloacae, 1 Enterobacter sakazakii and 2 Enterobacter amnigenus) were sensitive to both AEs. The activity of B. juncea AE towards the Gram-positive strains was generally higher than that observed for B. officinalis (45 and 22 strains inhibited by B. juncea and B. officinalis, respectively), while an opposite trend was registered against the Gram-negative strains (22 and 35 strains inhibited by B. juncea and B. officinalis, respectively). The highest inhibition was displayed by B. juncea AE against E. sakazakii 23A. B. officinalis AE showed the same minimum inhibitory concentration (MIC) (10 mg/mL) for the majority of the most sensitive strains, while the MIC of B. juncea AE was different for each bacterial species and the lowest concentration was registered to inhibit enterobacteria (3.1 mg/mL). After 1-year storage in different thermal conditions (room temperature, 4 C and 20 C), both AEs lost their inhibitory power. The extracts did not show cellular toxicity when tested against sheep erythrocytes. Hence, B. officinalis and B. juncea AEs were effective as natural antibacterial substances. AEs were tested in situ in three food model systems (meat, fish and vegetable) at two concentrations, but only when added at a concentration 10-fold higher than that showing definite efficacy in vitro (100 and 31 mg/mL for B. officinalis and B. juncea, respectively), they inhibited the growth of the sensitive strains, even though the cells were still viable after 24 h. The influence of AEs on the volatile organic compounds (VOCs) composition of the food models was analysed by gas chromatography/mass spectrometry. The different levels of alcohols, aldehydes, esters, hydrocarbons, ketones and phenol registered, showed a consistent effect of B. officinalis and B. juncea AEs on the VOCs of the food models. However, the sniffing assay found only B. juncea AE impacting consistently the final aroma of the food models.