A rosemary extract commercially exploited (Oxy'less) as an antioxidant of lipids in foods was dissolved in ethanol (100 mg/ml), and the solution was tested against foodborne microorganisms. For gram-positive bacteria, the MIC of the ethanolic solution was 1% for Leuconostoc mesenteroides, 0.5% for Listeria monocytogenes, 0.5% for Staphylococcus aureus, 0.13% for Streptococcus mutans, and 0.06% for Bacillus cereus. It slowed the growth of Penicillium roquefortii and Botrytis cinerea. Up to 1% of the ethanolic solution had no activity on the gram-negative bacteria Escherichia coli, Salmonella Enteritidis, and Erwinia carotovora and on the yeasts Rhodotorula glutinis and Cryptococcus laurentii. Antibacterial activity of the rosemary extract was strongly influenced by the composition of the media. The MIC was reduced by low pH, high NaCl contents, and low temperatures. Low pH and high NaCl concentration had a synergistic effect on the MIC of the rosemary extract for S. aureus. Lipids, surface-active agents, and some proteins decreased its antibacterial activity, whereas pectin had no effect. The inhibitory effect was little modified by heat treatment (100 degrees C). The natural microflora of pasteurized zucchini broth was inhibited by 0.5% of the rosemary extract. The antibacterial activity was linked to the compounds extracted with hexane, which are presumably phenolic diterpenoids.

The anti-Listeria monocytogenes effects of 8 phenolic compounds, carnosol, carnosic acid, 12-methoxy carnosic, ferulic and caffeic acid, rosmarinic acid, luteolin and luteolin-7-glucoside were evaluated with a Plackett and Burman design in mixtures mimicking the phenolic composition of rosemary extract without essential oils. At 30°C carnosic acid was the most efficient compound during 24 h, whereas luteolin became more active after 72 h. The antibacterial effect of pure carnosic acid was modeled under a range of different pH and NaCl concentration, using a Doehlert design. Under moderately acidified conditions, carnosic acid displayed a bactericidal effect at low concentration (5.5 µg/ml). Its activity was not greatly influenced by NaCl.

Four Enterobacteriaceae (Enterobacter agglomerans and Rhanella aquatilis) and six pseudomonads (Pseudomonas fluorescens, Pseudomonas chlororaphis, Pseudomonas putida) isolated from minimally processed green endive were coinoculated at 10 degrees C with Listeria monocytogenes in a minimal medium. Pseudomonads did not modify the growth of L. monocytogenes, whereas Enterobacteriaceae reduced its maximal population by 2 to 3 log CFU/ml. The same effect was observed in a diluted yeast extract medium supplemented with amino acids and glucose, in which L. monocytogenes grown alone reached 10(9) to 10(10) CFU/ml. In the same diluted yeast extract medium, not supplemented with glucose and amino acids, the maximal population of L. monocytogenes in the presence of both Enterobacteriaceae and pseudomonads was only slightly reduced (less than 0.5 log CFU/ml). Culture filtrates of the Enterobacteriaceae had no inhibitory activity on L. monocytogenes. The effect of the Enterobacteriaceae on L. monocytogenes growth was presumably due to a competition for glucose and/or amino acids.

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