Abstract
Growth of the food-borne human pathogen
Listeria monocytogenes to large numbers in ready-to-eat food products greatly increases the risk of disease for susceptible consumers. A better knowledge of the population structure of
L. monocytogenes present in retailed food could allow better prevention strategies to be developed. We present the analysis of 450
L. monocytogenes isolates, 179 responsible for sporadic human cases of listeriosis and 271 isolated from foods collected from retailers. All isolates were investigated by multiplex PCR (food isolates), allowing serovar predictions, or serotyped (human isolates), and DNA macrorestriction patterns were determined. Isolates from different sources were significantly differently distributed into PCR groups. PCR group IIa, corresponding to serovars 1/2a and 3a, was predominant in food isolates (58%; OR
=
3.19;
P
<
1
×
10
−
7
). A larger proportion of human isolates belonged to PCR group IVb, corresponding to serovars 4b, 4d and 4e (44%; OR
=
5.69;
P
<
1
×
10
−
7
). DNA macrorestriction pattern analysis of PCR group IIa isolates showed that isolates from pork products had a very low diversity (ID
=
0.905) whereas isolates from humans were more diverse (ID
=
0.976). Furthermore, 78% of the pork product isolates belonging to PCR group IIa exhibited only two
AscI profiles,
a
1 and
a
2, which were very similar (94%). DNA array analysis of representative isolates showed that isolates with
a
1 and
a
2 profiles constitute a homogeneous population, whereas isolates exhibiting non
a
1–
a
2 profiles are more diverse. Six of the isolates with
a
1 and
a
2 profiles were selected and investigated for their gene content using a DNA array. With respect to 295 strains present in our data collection, a specific pattern of the presence and absence of 15 genes was identified. Five are predicted to encode internalins and cell surface proteins, and eight of the genes were missing in this group. They code for cell surface proteins, transcriptional regulators, an acylase, a sugar phosphorylase and proteins of unknown functions. The ability of strains to multiply in different niches may be determined by the presence or absence of these genes.