64 Chapter 3
encoded) and virF (plasmid encoded) genes,
which are capable of distinguishing the
pathogenic (pYV^ +^ ) strains of serotypes 0 : 3,
0 : 8, and 0 : 9 from the less pathogenic (pYV - )
serotypes. In addition, a 163 - bp amplicon
from the ail chromosomically encoded
attachment and invasion gene can be used for
detection (Lliev and Najdenski, 2008 ).
The organism is killed by proper pasteuri-
zation. Nevertheless, its ability to survive
for protracted periods on surfaces such as
refrigerated milk cartons (Stanfi eld et al.,
1985 ) has resulted in a number of outbreaks
of yersiniosis caused by pasteurized milk
with the organism gaining access as a post -
pasteurization contaminant either directly or
indirectly from surfaces contaminated with
raw milk (Auliso et al., 1982 ; Greenwood
et al., 1990 ). For more information, see
Nesbakken (2006).Raw Milk Spoilage
In some countries, lactococci and coliform
growth is favored if milk is stored in unre-
frigerated cans and transported to the dairy
plant or collection center on the same day,
resulting in souring of the milk. This is espe-
cially the case for small producers. In many
countries milk is refrigerated to 7 ° C (or 5 ° C
or less if possible) within two hours of
milking, held at the farm, transported to the
creamery, and held at a temperature under
7 ° C at the site prior to pasteurization. Holding
the milk under these conditions exerts a
selective pressure favoring the growth of the
psychrotrophic microfl ora.
Psychrotrophs differ from psychrophiles;
the former have an ambient (greater than
20 ° C) optimum growth temperature but still
are able to grow well at refrigeration tem-
peratures (under 10 ° C). Although the psy-
chrotrophic microfl ora initially may constitute
a small proportion of the microfl ora (less
than 10%), they can predominate after two to
three days at refrigeration temperatures
(Shah, 1994 ). This does not induce spoilagecontaminate raw milk, which can still exhibit
microbiological quality parameters within an
acceptable range. Food poisoning arises from
ingestion of the viable organism rather than
the preformed toxin and results in vomiting,
abdominal pain, and diarrhea, and only rarely
becomes septacemic. Culture enrichment in
buffered peptone water may be required prior
to real - time PCR using primers that target a
262 - bp fragment of the Salmonella - specifi c
invA gene (Andrews and Baumler, 2005 ). It
has been detected in raw bulk tank milk at a
frequency of up to 9%. It should be recog-
nized that a high percentage of human salmo-
nellosis cases arise from raw milk or raw
milk products (CDC, 2002, 2003 ).
Staphylococcus aureus. Staphylococcus
aureus is a Gram - positive coccus that is gen-
erally a poor competitor but can predominate
under low - water - activity conditions such as
brine solutions. Food poisoning is usually
through ingestion of pre - formed heat - stable
enterotoxins of which a wide range can
potentially be produced. Symptoms include
vomiting and diarrhea. Staphylococcus
aureus can cause bovine mastitis, in which
case the organism is excreted directly in the
milk. Some mastitis strains have been shown
to be methicillin resistant (MRSA). PCR
assays target the fmhA, femA, or catalase
genes. Perhaps the largest dairy - related
staphylococcal food poisoning outbreak
occurred in Japan in 2000 when approxi-
mately 13,400 cases were attributed to the
consumption of powdered skim milk that had
been inadvertently contaminated with raw
milk (Ryser, 2001 ).
Yersinia enterocolitica. Yersinia entero-
colitica is a small ovoid Gram - negative
bacillus that is slightly acid fast. Human
symptoms include mesenteric lymphadentis
simulating acute or sub - acute appendicitis
that affects mainly male school children aged
5 to 15 years. Infection is usually as a result
of contact with animals and birds or their
products, including milk and milk products.
PCR assays target the ail (chromosomally