Related Experiment Videos
Inducible gene expression by nonculturable bacteria in milk after pasteurization
Thusitha S Gunasekera1, Anders Sørensen, Paul V Attfield
1Centre for Fluorimetric Applications in Biotechnology, Department of Biological Sciences, Macquarie University, Sydney, New South Wales 2109, Australia. tgunasek@rna.bio.mq.edu.au
Applied and Environmental Microbiology
|March 28, 2002
Summary
Heat treatment kills culturable bacteria in milk, but many nonculturable bacteria remain metabolically active. These viable but nonculturable (VBNC) cells can still express genes, indicating ongoing cellular activity despite loss of culturability.
Area of Science:
- Food Microbiology
- Bacteriology
- Molecular Biology
Background:
- Heat treatments like pasteurization aim to reduce bacterial load in milk.
- Assessing bacterial viability in milk is crucial for food safety and quality.
- Traditional methods like CFU counts may underestimate bacterial viability.
Purpose of the Study:
- To evaluate bacterial viability in milk after heat treatment using multiple indicators.
- To compare traditional CFU counts with reporter gene expression and membrane integrity assays.
- To investigate the metabolic activity of heat-stressed bacteria in milk.
Main Methods:
- Compared Colony Forming Units (CFU) on agar plates with de novo GFP reporter gene expression.
- Utilized propidium iodide exclusion to assess membrane integrity.
- Applied heat treatment (63.5°C for 30 min) to milk inoculated with marked bacteria.
Main Results:
- Direct viable counts (dye exclusion) were significantly higher than CFU counts in pasteurized milk.
- Heat treatment caused a >4-log reduction in culturable E. coli and P. putida (CFU).
- However, GFP expression decreased by <2.5 log units, indicating metabolic activity in nonculturable cells.
Conclusions:
- A significant subpopulation of bacteria in pasteurized milk are viable but nonculturable (VBNC).
- Heat treatment renders bacteria nonculturable but does not eliminate their metabolic activity or gene expression.
- Metabolically active VBNC bacteria pose potential challenges for microbial assessment in heat-processed foods.