Interactions in dual species biofilms between Listeria monocytogenes EGD-e and several strains of Staphylococcus aureus.

Six environmental isolates of Staphylococcus aureus and one collection strain were investigated for their ability to form monospecies biofilms and dual species biofilms with Listeria monocytogenes EGD-e on stainless steel coupons. All isolates were able to grow as biofilms but their ability to form monospecies biofilms differed. The population of L. monocytogenes EGD-e in dual species biofilms was not affected by the presence of S. aureus isolates except for strain CIP 53.156. The effect of L. monocytogenes EGD-e on the population of S. aureus was strain dependent: S. aureus population either increased or decreased or was not affected in the presence of L. monocytogenes EGD-e in dual species biofilms. Dual species biofilms were grown with L. monocytogenes EGD-e and the strain CIP 53.156 of S. aureus on stainless steel coupons under batch and dynamic conditions. Higher sessile populations of L. monocytogenes EGD-e were observed in the presence of S. aureus CIP 53.156. Microscope observations by scanning electron microscopy (SEM) revealed an intimate association of L. monocytogenes EGD-e and S. aureus CIP 53.156 in dual species biofilms. An increase of the number of L. monocytogenes EGD-e cells was observed in the presence of S. aureus CIP 53.156 cell-free supernatant. This activity was retained after ultrafiltation (<3 kDa), was heat stable but was lost after proteinase K treatment.