Applied and Environmental Microbiology, July 2002, p. 3432-3441, Vol. 68, No. 7
Modeling the Interactions of Lactobacillus curvatus Colonies in Solid Medium: Consequences for Food Quality and Safety
P. K. Malakar, D. E. Martens, W. van Breukelen, R. M. Boom, M. H. Zwietering, and K. van 't Riet
The big issue:
Bacteria can be grown in a number of different kinds of test media, in the laboratory. Some of these are surfaces, some are solid matrixes, and many are liquids (stirred and unstirred). When bacteria which spoil food are tested for their ability to produce toxins or to cause other trouble for people, their growth dynamics and so forth are measured in the test media. Now, the best test media at a certain level is going to be the food item itself. But real laboratory media can be standardized, defined chemically, and used much more extensively.
So it is important to understand how the growth characteristics of spoilage organisms change among the different kinds of media, and the different kinds of foods, and how to relate the two.
This paper approaches the problem by constructing a mathematical model that looks at how growth and acid production by certain bacteria vary with the type of media, type of food, and density of initial population (inoculation).
The organism they model is Lactobacillus, which produces lactic acid in food. The organism can produce more lactic acid then even it can live in, and sours dairy products.
They create models based on differential equations that track the growth of cells and the production of lactic acid within a colony. Diffusion of the lactic acid will eventually acidify the environment around the colony, making it inhospitable even to the cells which would normally produce it. Thus, colony growth is self-limiting, even when nutrients are incompletely utilized.
The authors proceed to create real cultures in liquid and semi-solid media, demonstrating the accuracy of their mathematical model. Despite some deviation at 20+ hours, the model predicted the behavior of the cultures accurately.
The most important result of this is the finding that because of the mass action diffusion of lactic acid, solid media can be modeled by liquid media only when the inoculum size is high, and the colonies themselves begin interacting relatively quickly, creating a somewhat homogenous environment. When heterogeneity is high in the media, that is, when the inoculum is low and therefore spread unevenly on the scale of colonies, the colonies don't interact until they are already self-limited; that is, the heterogeneity becomes accentuated over time and a solid media is needed to model the growth.
This is particularly important if the cells produce, say, some toxin just during one phase of their life cycle.
The results seem rather intuitive, which makes them pleasing though rather unexciting. The models are a bit complex, but easily generalized and constructed in a common sense fashion.
All in all, good stuff!
Modeling the Interactions of Lactobacillus curvatus Colonies in Solid Medium: Consequences for Food Quality and Safety
P. K. Malakar, D. E. Martens, W. van Breukelen, R. M. Boom, M. H. Zwietering, and K. van 't Riet
The big issue:
Bacteria can be grown in a number of different kinds of test media, in the laboratory. Some of these are surfaces, some are solid matrixes, and many are liquids (stirred and unstirred). When bacteria which spoil food are tested for their ability to produce toxins or to cause other trouble for people, their growth dynamics and so forth are measured in the test media. Now, the best test media at a certain level is going to be the food item itself. But real laboratory media can be standardized, defined chemically, and used much more extensively.
So it is important to understand how the growth characteristics of spoilage organisms change among the different kinds of media, and the different kinds of foods, and how to relate the two.
This paper approaches the problem by constructing a mathematical model that looks at how growth and acid production by certain bacteria vary with the type of media, type of food, and density of initial population (inoculation).
The organism they model is Lactobacillus, which produces lactic acid in food. The organism can produce more lactic acid then even it can live in, and sours dairy products.
They create models based on differential equations that track the growth of cells and the production of lactic acid within a colony. Diffusion of the lactic acid will eventually acidify the environment around the colony, making it inhospitable even to the cells which would normally produce it. Thus, colony growth is self-limiting, even when nutrients are incompletely utilized.
The authors proceed to create real cultures in liquid and semi-solid media, demonstrating the accuracy of their mathematical model. Despite some deviation at 20+ hours, the model predicted the behavior of the cultures accurately.
The most important result of this is the finding that because of the mass action diffusion of lactic acid, solid media can be modeled by liquid media only when the inoculum size is high, and the colonies themselves begin interacting relatively quickly, creating a somewhat homogenous environment. When heterogeneity is high in the media, that is, when the inoculum is low and therefore spread unevenly on the scale of colonies, the colonies don't interact until they are already self-limited; that is, the heterogeneity becomes accentuated over time and a solid media is needed to model the growth.
This is particularly important if the cells produce, say, some toxin just during one phase of their life cycle.
The results seem rather intuitive, which makes them pleasing though rather unexciting. The models are a bit complex, but easily generalized and constructed in a common sense fashion.
All in all, good stuff!
posted by BenK at 4:29 PM
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