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Type of publication: Journal Article
Type of document: Full Paper

Year: 2009

Authors: Reichert-Schwillinsky, F; Pin, C; Dzieciol, M; Wagner, M; Hein, I

Title: Stress- and growth rate-related differences between plate count and real-time PCR data during growth of Listeria monocytogenes.

Source: Appl Environ Microbiol. 2009; 75(7):2132-2138

Authors Vetmeduni Vienna:

Dzieciol Monika
Hein Ingeborg
Wagner Martin

Vetmed Research Units
Institute of Food Safety, Food Technology and Veterinary Public Health, Unit of Food Microbiology

To assess the overestimation of bacterial cell counts in real-time PCR in relation to stress and growth phase, four different strains of L. monocytogenes were exposed to combinations of osmotic stress (0.5 to 8% [vol/vol] NaCl) and acid stress (pH 5 to 7) in a culture model at a growth temperature of 10 degrees C or were grown under optimal conditions. Growth curves obtained from real-time PCR, optical density, and viable count data were compared. As expected, optical density data revealed entirely different growth curves. Good to moderate growth conditions yielded good correlation of real-time PCR data and plate count data (r(2) = 0.96 and 0.99) with similar cell counts. When growth conditions became worse, the numbers of CFU decreased during the stationary phase, whereas real-time-PCR-derived bacterial cell equivalents differed in this regard; the correlation worsened (r(2) = 0.84). However, fitted growth curves revealed that maximum growth rates calculated from real-time PCR data were not significantly different from those derived from plate count data. The overestimation of bacterial cell counts by real-time PCR observed in the stationary phase under higher-stress conditions might be explained by the accumulation of viable but nonculturable bacteria or dead bacteria and extracellular DNA. Considering these results, real-time PCR data collected from naturally contaminated samples should be viewed with caution.

Keywords Pubmed: Acids/pharmacology
Anti-Bacterial Agents/pharmacology
Colony Count, Microbial/methods*
Listeria monocytogenes/drug effects
Listeria monocytogenes/growth & development*
Microbial Viability
Osmotic Pressure
Polymerase Chain Reaction/methods*
Statistics as Topic
Stress, Physiological

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