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
Lv. Thomas et al., 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, Microbiology, 143, 1997, pp. 2575-2582
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.