496 Chapter 28
The organism grows on simple laboratory
media in the pH range between 5 and 9. On
solid agar, the colonies are translucent, dew-
drop - like, and bluish when viewed by 45 °
incident transmitted light (Henry ’ s illumina-
tion step). Biochemically, this organism can
be confused with such organisms as
Lactobacillus, Brochothrix, Erysipelohrix ,
and Kurthia. A variety of biochemical tests
have been devised to separate L. monocyto-
genes from other Listeria species, such as L.
innocua , L. welshimeri , and L. murrayi.
Serotyping is also important in the identifi ca-
tion of this organism, the most important
ones being 1/2a, 1/2b, 1/2c, 3a, 3b, 3c, and
4b. Listeria is a psychrotroph capable of
growing at temperatures as low as 2.5 ° C and
as high as 44 ° C. Because dairy products have
been implicated in outbreaks of listeriosis,
much research has been directed toward
cheese and milk products. The organism has
been found to survive the processing of
cottage cheese, cheddar cheese, and Colby
cheese. A question of great concern is
whether L. monocytogenes can survive the
current pasteurization temperature of milk
(i.e., 63 ° C for 30 min or 72 ° C for 15 s). Data
on this issue are still inconclusive, and
research on this topic is still ongoing. It is
important to note that at present, the time and
temperature regulation for pasteurization of
milk has not been affected by the possible
heat resistance of L. monocytogenes.
The disease starts with infection of the
intestine, though the infective dose is cur-
rently unknown. Patients may develop transi-
tory fl u - like symptoms such as malaise,
diarrhea, and mild fever. In severe cases,
virulent strains are capable of multiplying in
macrophages and later producing septicemia.
When this occurs, the bacteria can affect the
central nervous system, the heart, the eyes,
and may invade the fetus of pregnant women
and result in abortion, stillbirth, or neonatal
sepsis.
Several well - documented cases of listerio-
sis have been reported in Nova Scotia (1981),psychrotroph. After ingestion of large
numbers of this organism, the susceptible
person can develop fever, abdominal pain,
and diarrhea, with nausea and vomiting
occurring less frequently. More serious intes-
tinal disorders include enteritis, terminal
ileitis, and mesenteric lymphadenitis. Extr-
aintestinal infections of Y. enterocolitica
have been reported, including septicemia,
arthritis, erythema nodosum, sarcoidosis,
skin infection, and eye infection.
Foods suspected of being a source of yer-
siniosis in the United States include choco-
late milk, milk powder, chow mein, tofu, and
pasteurized milk. Pork products have also
been suspected.
Isolation of this organism typically goes
through an enrichment step using nutrient
broth or Rappaport broth and then through a
plating medium using an enteric agar (SS,
XLD, DCL, etc). The CIN agar (cefsulodin -
irgasan - novobiocin agar) is commercially
available for the isolation of this pathogen;
however many other organisms, such as
Salmonella and Serratia, also grow on this
agar. In the author ’ s laboratory, a new agar
named KV202 has been developed that
allows Salmonella and Serratia colonies to
be separated from Yersinia by the develop-
ment of black colonies. Control of yersiniosis
depends on the proper handling of raw and
cooked food of all types, especially pork
products, as well as water for food process-
ing. There have been no reported outbreaks
of Yersinia enterocolitica between 1988 and
1992, but in 2002 there were 2 outbreaks, 27
cases and 1 death reported.
Listeria monocytogenes
Listeria monocytogenes has developed into a
very important food pathogen in the past 20
years from the standpoint of economic and
public health impact. The organism is a
small, short, Gram - positive non - spore -
forming rod. It is motile by a characteristic
tumbling motion or slightly rotating fashion.