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Bacteria Survive Pasteurization, Resist Culturing

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Pasteurization of milk may not kill as many bacteria as previously thought, say researchers from Macquarie University in
Australia and the University of Copenhagen, Denmark, in the April 2002 issue of the journal Applied and Environmental
Microbiology. While these organisms, which can cause human disease or spoilage, were not able to be cultured,
molecular tests suggest that they are still metabolically active.

"Culture-based techniques are most commonly used to determine viable counts in dairy products but they have the
limitation that they are unable t enumerate viable but non-culturable (VBNC) organisms," say the researchers "VBNC
organisms may potentially be capable of causing infections and may contribute to milk spoilage.

The term VBNC is often used to describe bacteria that while unable to be detected through normal cultures, are by other
measurements still alive. According to the researchers the VBNC state may be a survival method adopted by bacteria
exposed to extreme conditions or could simply represent injured bacteria that have lost the ability to grow.

"The results demonstrate that a substantial portion of cells rendered incapable of forming colonies by heat treatment
are nevertheless metabolically active," say the researchers. "This observation is important because it highlights a
potential problem for milk quality and safety."

(T.S. Gunasekera, A. Sorensen, P.V. Attfield, S.J. Sorensen and D.A. Veal. 2002. Inducible gene expression by
nonculturable bacteria in milk after pasteurization. Applied and Environmental Microbiology, 68: 1988-1993.)