Abstract
Listeria monocytogenes
is the main bacteria incriminated in contaminating cold storage food and ready-to-eat products. Survival of four
L. monocytogenes
food isolates was examined during freezing and cold storage. Slices of artisanal cheese were inoculated with
Listeria
strains and incubated at −20 °C. Their survival rate was recorded during the storage period. Then, transcript levels of four virulence genes (
hlyA
,
iap
,
fri
, and
flaA)
were evaluated using a semiquantitative reverse transcriptase PCR. Our results revealed that after 6 months of frozen storage, the number of
L. monocytogenes
cells had declined by 2.04 ± 0.1 log
10
colony-forming units (CFU) g
−1
, 2.52 ± 0.1 log
10
CFU g
−1
, and 2.58 ± 0.1 log
10
CFU g
−1
for the strains S2, S3, and S4, respectively. Our data revealed that all the studied genes (
hlyA
,
iap
,
fri
, and
flaA
) were expressed after 6 months of incubation in artisanal cheese at −20 °C, and the transcript level has been affected by the factor freezing, whether for the reference strain ATCC 19115 or for other isolates. The
iap
gene expression was decreased for the four strains after 6 months of storage, and
hlyA
expression rate was consistently slightly lower. Transcript levels of
fri
and
flaA
genes were consistently higher in
L. monocytogenes
cells before cold exposure than in stressed cells and were significantly affected by the freezing process.