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LISTERIA SPECIES AND LISTERIOSIS
Listeriosis is a disease which has been recognised for over 60
years, it is now known to affect a wide variety of animals as well as humans.
There are eight species in the Listeria genus: Listeria monocytogenes
the major species causing disease in humans and animals1.
Listeria ivanovii associated with animal and a small number of human infections2.
Listeria innocua, Listeria seeligeri very rarely associated with infection.
Listeria murrayii, Listeria welshimeri, Listeria denitrificans not associated with naturally
occurring infections.
L. monocytogenes is commonly found in the environment and is therefore
common in food and human faeces. It can grow slowly at refrigerator temperatures
and has been isolated from a wide variety of foods. Fresh and frozen poultry
(60%), cooked-chill foods (24%), salami and continental sausages (24%), soft
cheese (10%), pre-packed salads (7%)3. Although relatively few cases
of listeriosis have been epidemiologically associated with food stuffs in the
UK3,4 a number of outbreaks associated with food have been reported
in other countries5,6,7,8. Caution should be taken in assessing food-borne
infections because the extent and route of such infections are unknown.
The annual number of cases of listeriosis reported to the Central Public Health
Laboratory has risen from 50 in 1967 to 259 in 1987. Reported deaths are 38
in 1983 rising to 59 in 19873. In human listeriosis there is a high
rate of mortality (23%) and the disease is largely confined to pregnant women,
neonates and patients with underlying immuno-suppression2.
The use of efficient, selective media will undoubtedly improve the rate of isolation
and may reveal the chain of infection which is still very obscure.
Table 1 Differentiation of the species of the genus Listeria |
L.
monocytogenes |
L.
ivanovii |
L.
innocua |
L.
welshimeri |
L.
seeligeri |
L.
grayi |
L.
murrayi |
L.
denitrificans |
|
Bet haemolysis on blood agar |
+
|
++
|
-
|
-
|
(+)
|
-
|
-
|
- |
Nitrates reduced to nitrites |
-
|
-
|
-
|
-
|
-
|
-
|
+
|
+
|
CAMP test with Staphylococcus aureus |
+
|
-
|
-
|
-
|
(+)
|
-
|
-
|
-
|
CAMP test with Rhodococcus equi |
- |
+
|
-
|
-
|
-
|
-
|
-
|
-
|
|
|
|
|
|
|
|
|
|
Acid produced from: |
|
|
|
|
|
|
|
|
D-mannitol |
-
|
-
|
-
|
-
|
-
|
+
|
+
|
-
|
L-rhamnose |
+
|
-
|
V
|
V
|
-
|
-
|
V
|
-
|
D-xylose |
-
|
+
|
-
|
+
|
+
|
-
|
-
|
-
|
a -methyl D-mannoside |
+
|
-
|
+
|
+
|
V
|
NS
|
NS
|
NS
|
V = Variable reaction; NS = Not stated; ( ) = Weak reaction |
FIGURE 1 Dichotomous key for the identification of the species of Listeria
IDENTIFICATION OF LISTERIA SPECIES
(adapted with permission from reference 1.)
Criteria of identification for Listeriae
Gram positive rod, VP +, urease -, catalase +, oxidase - aesculin hydrolised,
acid no gas from glucose and salicin, tumbling motility below 30°
C.
Colony form - non pigmented on nutrient agar with characteristic
caramel/sour butter smell, 1-2 mm diameter with a ground glass appearance
and emulsifies in
saline to a smooth suspension.
Motility test - heavily inoculate brain-heart infusion broth or nutrient
broth and hold at room temperature for 4-6 hours. Examine by hanging drop technique.
If negative re-test after 18 hours at room temperature.
Catalase test - most strains are strongly positive but some may be weak
or negative.
Haemolysis - Listeria monocytogenes produces
a narrow zone of b-haemolysis (1-2 mm) on horse
blood agar.
CAMP test - use 5% v/v sheep blood in nutrient agar and pour
a thin layer over nutrient agar base. Streak Staphylococcus aureus NCTC
1803 and Rhodococcus
equi NCTC 1621 across the plates. The test strains of listeriae are streaked
at right angles to the Staphylococcus aureus or Rhodococcus equi inoculum.
Incubate at 35°C overnight. Positive results
are when an enhanced zone of haemolysis occurs between two cultures.
VP test - some strains may require yeast extract added to the VP broth
to give sufficient growth for this test.
Carbohydrate fermentation - use API 50 CH system. Incubate up to 7 days
at 35° C.
Nitrate reduction - Use the rapid method of Blazeric D. N. & Ederer
G. M. `Biochemical Tests in Diagnostic Bacteriology' J. Wiley & Son Inc.
New York. 1975.
Acknowledgement is gratefully made to the Central Public Health Laboratory for permission to use this information and for Table 1 with Figure 1.
References
1 McLauchlin J. (1988) Listeria Workshop DMRQ. Central Public
Health Service. 4/5th May.
2 McLauchlin J. (1987) J. Appl. Bact. 63. 1-11.
3 Hall S. M. et al. (1988) Lancet. ii.
4 Kerr K. G., Dealler S. F. and Lacey R. W. (1988) Lancet. ii. 1133.
5 Bille J. and Glauser M. P. (1988) Bull. Bund. fÏr Gesund. 3. 28-29.
6 Fleming D. W. et al. (1985) N. Engl. J. Med. 312. 404-407.
7 Linnan M. J. et al. (1988) N. Engl. J. Med. 319. 823-828.
8 Schlech W. F. et al. (1983) N. Engl. J. Med. 308. 203-206.