SPATIAL INTERACTIONS BETWEEN SUBSURFACE BACTERIAL COLONIES IN A MODELSYSTEM - A TERRITORY MODEL DESCRIBING THE INHIBITION OF LISTERIA-MONOCYTOGENES BY A NISIN-PRODUCING LACTIC-ACID BACTERIUM

The effect of spatial separation on interactions between subsurface ba cterial colonies was tested using a model system:the inhibition of Lis teria monocytogenes by nisin-producing and nisin-non-producing Lactoco ccus lactis subsp. lactis. Separation distance was controlled by alter ing the number of inoculum organisms within the agar. Mean separation distance was calculated by determining the mean Volume available to ea ch cell at the start of the experiment. Inhibition was assessed by com paring the growth of L. monocytogenes in pure culture with its growth in the presence of Lac. lactis subsp. lactis. Increasing the distance between colonies resulted in an exponential decrease in inhibition. Wh en L. monocytogenes and Lac. lactis subsp, lactis colonies were within 100 mu m of each other, the increase in cell numbers per L. monocytog enes colony was only 0.6 c.f.u. (which indicated some cells had become non-viable). This was a log reduction of 3.5 compared to the pure cul ture control. A separation distance of 1000 mu m resulted in a L. mono cytogenes colony growth increment of 2.5 x 10(2) c.f.u. per colony, a log reduction of 3.0 compared to the control. Increasing the separatio n distance to 3000 mu m resulted in a L. monocytogenes colony growth i ncrement of 1.3 x 10(6) c.f.u. per colony, a log reduction of 0.9 comp ared to the control. The effects of nisin and acidity were investigate d by using a nisin-non-producing strain of Lac. lactis subsp. lactis a nd by buffering the medium. Data were obtained for the effect of separ ation on inhibition, as well as competition between colonies of the sa me species. The inhibition was mathematically described in terms of a simplified 'territory' model of immobilized bacterial growth. There wa s a strong qualitative agreement between the mathematical model and th e experimental data. It was concluded that the phenomenon of propinqui ty is of important consideration when modelling and predicting microbi al growth within solid food systems.