Growth of Listeria monocytogenes restricted by native microorganisms and other properties of fresh-cut spinach

A study was undertaken to investigate the cause of the bacteriostatic activity of fresh-cut spinach leaves against Listeria monocytogenes. L. monocytogenes was cultivated in pure tryptic soy broth for use as a monoculture, in tryptic soy broth containing 10 mg ml(-1) of autoclaved or nonautoclaved freeze-dried spinach powder, and in tryptic soy broth in mixed cultures with various microorganisms isolated from fresh-cut spinach, including Pseudomonas fluorescens biovar I, P. fluorescens biovar III, Staphylococcus xylosus, and an undefined culture of mesophilic aerobic microorganisms (MAMs) isolated from freeze-dried spinach pow der. These microorganisms were inoculated at 4.3 log CFU ml(-1) and L. monocytogenes was inoculated at 2.4 and 4.4 log CFU ml(-1) After 24 h of incubation at 30 degrees C, the populations of the two inoculum levels L. monocytogenes increased to 9.0 and 9.6 log CFU ml(-1) in the tryptic soy broth control, to 5.4 and 7.5 in nonautoclaved spinach powder cultures, and to 8.8 and 9.1 log CFU ml(-1) in autoclaved spinach powder cultures; In mixed cultures with biovar I of Fl fluorescens, L. monocytogenes increased to 7.4 and 8.6 log CFU ml(-1); with biovar III to 7.7 and 9.1, with S. xylosus to 7.8 and 9.2, and with the MAMs to 7.1 and 8.0 CFU ml(-1) in the low and high listerial inoculum cultures respectively. The LSD(0.05), of the means were 0.5 and 0.6, respectively. The freeze-dried spinach powder had an inhibitory effect on the growth of L. monocytogenes. The inhibitory effect was greatly decreased when the native microorganisms were almost eliminated by heating or irradiation. These results indicate that if L. monocytogenes is present as a contaminant on fresh-cut spinach, its growth probably will be restricted by native microorganisms.