Effects of several environmental factors on the anti-Listeria monocytogenes activity of an essential oil of Picea excelsa.

The effects of several environmental factors on the anti-Listeria monocytogenes activity of an essential oil of Picea excelsa were explored by determination of active concentrations using two methods and with survival curves. In trial conditions, the serovars 1/2c and 4b behaved similarly. A dose of 0.2-0.3% (v/v) of essential oil was bactericidal for 10(5)-10(7) cells contained in 1 ml of Tryptone Soy Broth Yeast Extract at pH 6 and 7 incubated at 13 and 37 degrees C and of medium supplemented with levan. Introduction of sodium caseinate, agar or fat into the test medium and use of a cheese medium decreased the bactericidal effects of the essential oil. Basic pH, addition of NaCl or use of Tryptone Soy Broth and saline solution increased its antilisterial activity. Serovar 1/2c survival curves exhibited an exponential death rate followed by a tailing effect in the presence of the minimal bactericidal concentration of the essential oil. A three log10 reduction of cell viability was obtained within 100 min in Tryptone Soy Broth Yeast Extract, within longer exposure in media supplemented with NaCl or at basic pH.