Studies. were undertaken to investigate the cause of the bacteriostatic activity of spinach leaves against Listeria monocytogenes. Listeria monocytogenes was inoculated at the concentration 104 cells/ml in raw, autoclaved and vacuum filtration sterilized spinach macerates as well as in tryptic soy broth used as control. The concentrations after 24 h at 28C were 105, 5 × l06 and 105 cells/ml in respective spinach macerates and 4 × l08 cells/ml in tryptic soy broth. The anti-Listeria activity was still present in spinach macerates sterilized by vacuum filtration but strongly reduced in autoclaved macerates. Serial dilutions of freeze-dried spinach were tested on the growth of Listeria monocytogenes in tryptic soy broth. The minimum concentration for maximum inhibition was 10 mg·ml–1. The growth of Listeria monocytogenes in tryptic soy broth in mixed cultures with different microbial species isolated from spinach was not inhibited. In conclusion, the bacteriostatic activity against Listeria monocytogenes might be due to antimicrobial compounds present in spinach leaves.
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