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VIDAS LMX – Summary 2012 v01
2.3 Inclusivity / exclusivity
The inclusivity and the exclusivity of the alternative method are defined by analysis, respectively, of 50 positive
strains and 30 negative strains.
2.3.1 Protocols
Protocol for inclusivity
A culture of each
Listeria monocytogenes
strain was prepared in a nutritive broth culture. Then, a supplemented
LMX broth was inoculated with around 10
Listeria monocytogenes
per 225 ml and incubated for 26 hours at 37°C
before realization of VIDAS LMX test.
Protocol for exclusivity
The different negative strains were cultivated 24 h at 37°C and diluted in nutritive broth. Non select ive broths were
inoculated to obtain levels of around 10
5
cells in 225 ml broth before realization of VIDAS LMX test.
In case of result being discordant with regard to that waited, a new assay must be realized with, in parallel, the reference method and the
complete method VIDAS LMX.
2.3.2 Results and conclusion
The results are presented in appendix C.
Sixty strains of
Listeria monocytogenes
were detected out of the 60 tested by VIDAS LMX test.
The study of 31 strains not belonging to the genus
Listeria
or non
Listeria monocytogenes
showed cross reactions
with one strain of
Bacillus sphaericus
and two strains of
Staphylococcus
(
S.intermedius
and
S.aureus
) after
enrichment in the non selective enrichment broth.
For these 3 non target strains, the values of test (VT) were just above the threshold and inoculation levels were
high. Negative results were obtained after enrichment in the selective broth of the method.
3 Interlaboratory study
The aim of the interlaboratory study is to determine the variability of the results obtained in different laboratories
using identical samples and to compare these results with those obtained in the methods comparison study.
3.1 Study organization
Number of participating laboratories: 16 laboratories received samples. The laboratories list is presented in
appendix D.
Matrix: pasteurized milk (25 ml)
Strain:
Listeria monocytogenes
(origin “dairy product”).
Number of samples per laboratory: 24 samples per method were prepared to represent 3 levels of
contamination, with 8 samples per level for each method (48 samples per parcel).