Single cell variability of L. monocytogenes grown on liver pâté and cooked ham at 7 degrees C: comparing challenge test data to predictive simulations.
OBJECTIVE
The variability in growth between individual Listeria monocytogenes cells was investigated on liver pâté and cooked ham. These results were compared to Monte Carlo simulations based on data collected previously in broths (Francois et al., submitted for publication).


RESULTS
Single cells were isolated by a dilution protocol and inoculated on 15 g samples of liver pâté and cooked ham, pasteurized in the packaging. Of each product, 250 samples were inoculated, of which 50 samples were analysed for L. monocytogenes on each analysis day. Results were compared to simulations, based on distributions that describe the variability of the individual cell lag phases and generation times of L. monocytogenes cultivated in broths. Based on the same simulation techniques, the variability effect was investigated for different inoculum levels (10, 100, 10,00 and 10,000 cells). It was demonstrated that the expected variability of the outgrowth of L. monocytogenes in a challenge test is very high for low inoculum levels.


CONCLUSIONS
The variability in growth characteristics observed between different single L. monocytogenes cells on foods is very large. The simulations based on the previously collected optical density data in broths, could be confirmed by foods inoculated with single L. monocytogenes cells.


CONCLUSIONS
The large variability between different individual L. monocytogenes cells has serious consequences for the experimental design of a challenge test. One thousand cells per portion are necessary in order to reduce the variability to acceptable levels and quantify the behaviour of the pathogen consistently with a reasonable number of challenge tests.
Francois
K
K
Laboratory of Food Microbiology and Food Preservation, Department of Food Safety and Food Quality, Ghent University, Ghent, Belgium.
Devlieghere
F
F
Uyttendaele
M
M
Standaert
A R
AR
Geeraerd
A H
AH
Nadal
P
P
Van Impe
J F
JF
Debevere
J
J
D000818:Animals; D015169:Colony Count, Microbial / Q000379:methods; D003470:Culture Media; D005506:Food Contamination; D005516:Food Microbiology; D008089:Listeria monocytogenes / Q000254:growth & development*; D008461:Meat Products / Q000382:microbiology*; D013997:Time Factors
D003470:Culture Media