Spatial distribution of bacterial colonies in a model cheese

Area of Science:

• Food microbiology
• Dairy science
• Microbial ecology

Background:

• Bacterial colonies drive cheese ripening, but their spatial distribution within the cheese matrix is poorly understood.
• Metabolite diffusion, crucial for ripening, is influenced by bacterial colony proximity.

Purpose of the Study:

• To model and experimentally measure the spatial distribution of bacterial colonies in a model cheese.
• To investigate the impact of initial inoculation levels on colony distribution, size, and spacing.

Main Methods:

• Utilized confocal microscopy of fluorescent Lactococcus lactis (producing green fluorescent protein) in an ultrafiltration (UF) model cheese.
• Compared theoretical random distribution (Poisson model) with experimental measurements.
• Quantified bacterial cell counts, colony distances, and diameters at various inoculation levels.

Main Results:

• Final bacterial cell counts were independent of the initial inoculation level (10^4 to 10^7 CFU/g).
• Bacterial colonies exhibited random distribution, consistent with Poisson's model.
• Lower inoculation levels led to increased mean distances between colonies (25–250 μm) and larger colony diameters.
• A 50-fold increase in inoculation level increased the interfacial area by 7-fold for the same cell biomass.

Conclusions:

• Initial inoculation level is a critical factor determining bacterial colony distribution and spacing in cheese.
• The spatial arrangement and interfacial area of colonies significantly influence the cheese ripening process at a microscopic level.
• Future discussions on cell numbers in cheese should consider inoculation levels to account for distribution effects.