7.9.2 The Organism and its Characteristics
L. monocytogenesis a Gram-positive, facultatively anaerobic, catalase-
positive, oxidase-negative, non-sporeformer. The coccoid to rod shaped
cells (0.4–0.5mm0.5–2.0mm) cultured at 20–25 1 C possess peritrichous
flagella and exhibit a characterisitic tumbling motility. Colonies on
tryptose agar viewed under oblique illumination have a characteristic
blue–green sheen.
L. monocytogeneselaborates a 58 kDab-haemolysin, listerolysin O,
which acts synergistically with the haemolysin produced by Staphylo-
coccus aureusto give enhanced haemolysis on blood agar. This reaction
forms the basis of a useful diagnostic test to distinguishL. monocyto-
genesfromL. innocua, and is known as the CAMP test after Christie,
Atkins, and Munch-Peterson who first described the phenomenon with
group B streptococci.
L. monocytogeneswill grow over a wide range of temperature from
0–42 1 C with an optimum between 30 and 35 1 C. Below about 5 1 C
growth is extremely slow with lag times of 1 to 33 days and generation
times from 13 to more than 130 h being recorded. The thermal survival
characteristics ofL. monocytogeneshave received considerable attention
following an outbreak in the United States associated with pasteurized
milk and the suggestion that the organism could survive commercial
pasteurization conditions. Despite some conflicting data in the literature,
it appears that the heat resistance ofL. monocytogenesis similar to that
of other non-sporeforming Gram-positives with a typical D 60 of a few
minutes and a D 70 of a few seconds. It was proposed thatL. monocyto-
genes cells in contaminated milk are protected from heat by their
intracellular location within milk leucocytes but subsequent studies have
failed to demonstrate any significant effect.
Models of the thermal inactivation ofL. monocytogenesin milk have
indicated that conventional HTST pasteurization achieves a reduction of
5.2 log cycles in the number of survivors; an acceptable safety margin
assuming low numbers of the organism present on the incoming milk.
Growth of all strains is inhibited at pH values below 5.5 but the
minimum growth pH is dependent on both strain and acidulant and has
been variously reported as between 5.6 and 4.4.L. monocytogenesis also
quite salt tolerant being able to grow in 10% sodium chloride and survive
for a year in 16% NaCl at pH 6.0.
The organism is ubiquitous in the environment. It has been isolated
from fresh and salt water, soil, sewage sludge, decaying vegetation, and
silage. Its prolonged survival in the environment has been demonstrated
in one study where the level of L. monocytogenesin sewage sludge
sprayed on to agricultural land remained unchanged for more than
8 weeks. Asymptomatic human and animal carriage is also common with
226 Bacterial Agents of Foodborne Illness