Summary: The effect of spatial separation on interactions between subsurface bacterial colonies was tested using a model system: the inhibition of by nisin-producing and nisin-non-producing subsp. Separation distance was controlled by altering the number of inoculum organisms within the agar. Mean separation distance was calculated by determining the mean volume available to each cell at the start of the experiment. Inhibition was assessed by comparing the growth of in pure culture with its growth in the presence of subsp. Increasing the distance between colonies resulted in an exponential decrease in inhibition. When and subsp. colonies were within 100 μm of each other, the increase in cell numbers per 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 culture control. A separation distance of 1000 μm resulted in a colony growth increment of 2.5 × 10 c.f.u. per colony, a log reduction of 3.0 compared to the control. Increasing the separation distance to 3000 μm resulted in a colony growth increment of 1.3 × 10 c.f.u. per colony, a log reduction of 0.9 compared to the control. The effects of nisin and acidity were investigated by using a nisin-non-producing strain of subsp. and by buffering the medium. Data were obtained for the effect of separation on inhibition, as well as competition between colonies of the same species. The inhibition was mathematically described in terms of a simplified ‘territory’ model of immobilized bacterial growth. There was a strong qualitative agreement between the mathematical model and the experimental data. It was concluded that the phenomenon of propinquity is of important consideration when modelling and predicting microbial growth within solid food systems.


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