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School of Pure and Applied Biology, University of Wales Cardiff, Cardiff CF1 3TL, UK
Institute of Food Research, Norwich Research Park, Colney, Norwich NR4 7UA, UK
1 Author for correspondence: Linda V. Thomas. Tel: + 44 1222 874000 ext. 6806. Fax: +44 1222 874305. e-mail: sablvt@cf.ac.uk
ABSTRACT
Summary: The effect of spatial separation on interactions between subsurface bacterial colonies was tested using a model system: the inhibition of Listeria monocytogenes by nisin-producing and nisin-non-producing Lactococcus lactis subsp. lactis. 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 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. When L. monocytogenes and Lac. lactis subsp. lactis colonies were within 100 µm of each other, the increase in cell numbers per L. monocytogenes 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 L. monocytogenes colony growth increment of 2.5 x 102 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 L. monocytogenes colony growth increment of 1.3 x 106 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 Lac. lactis subsp. lactis 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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