Influence of soft rot bacteria on growth of Listeria monocytogenes on potato tuber slices

Growth of Listeria monocytogenes on potato tuber slices and its interaction with four representative species of soft rot bacteria (Pseudomonas fluores cens, P. viridiflava, Erwinia carotovora subsp. carotovora, and Xanthomonas campestris) were investigated. When potato tuber slices were inoculated wi th one of two L. monocytogenes strains (Scott A and ATCC 15313), an increas e in numbers of 3 to 4 logs per gram of tissue was observed with samples th at were stored at 20 degrees C for 6 days. However, an increase of about 2 logs was observed with samples that were stored at 8 degrees C for 12 days. When potato slices were simultaneously inoculated with L. monocytogenes an d one of the four soft rot bacteria, the growth of L. monocytogenes was inh ibited in the presence of P. fluorescens or P. viridiflava but was not sign ificantly affected in the presence of E. carotovora or X. campestris. The a ntagonism of the two pseudomonads to L. monocytogenes was also observed in potato tuber extract and in culture media. Formation of inhibition zones wa s observed only in iron-deficient media but not in the medium supplemented with FeCl3. In addition, production of fluorescent siderophore (pyoverdin) by these two pseudomonads was demonstrated. L monocytogenes was unable to c olonize macerated plant tissue induced by soft-rotting bacteria 2 days befo re inoculation of the pathogen. These results indicate that growth of L. mo nocytogenes on potato tuber slices is differentially affected by soft rot b acteria and that antagonism of fluorescent pseudomonads to L. monocytogenes is possibly caused by the production of iron-chelating siderophore by thes e pseudomonads.