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Nestle Toll House Cookie Dough E. coli Outbreak,
PFGE, MLVA = CSI Source of Article: http://www.marlerblog.com/ There are a lot of things I
love about my job, but one is the science behind linking people in dozens of
states to food that they consumed that poisoned them. Once that link is
scientifically determined, and the outbreak stopped, then the time comes to
learn from the mistakes so the next outbreak can be avoided. Frankly, we all
need to appreciate (can you say increase funding) those public servants who
toil in local, state and federal health agencies tracking food borne
diseases. Without the work of dedicated people interviewing victims, testing
stool and analyzing data, most outbreaks would never be determined, most
victims would never be fairly treated, and most outbreaks would repeated
without learning from past manufacturing mistakes. The science behind all the
hard work is fascinating - especially, the "CSI" part. The process of obtaining
the DNA fingerprint is called PFGE (Pulse Field Gel Electrophoresis). This
technique is used to separate the DNA of the bacterial isolate into its
component parts. It operates by causing alternating electric fields to run
the DNA through a flat gel matrix of agarose, a polysaccharide obtained from
agar. The pattern of bands of the DNA fragments — or “fingerprints” — in the
gel after exposure to the electrical current is unique for each strain and
sub-type of bacteria. By performing this procedure, scientists can identify
hundreds of strains of E. coli O157:H7 as well as strains of Listeria and
campylobacter, and other pathogenic bacteria. The PFGE pattern of the
bacteria can then be compared and matched up to the PFGE pattern of the
strain of infected persons who consumed the contaminated product. MLVA (Multiple Loci VNTR
Analysis) is a method employed for the genetic analysis of particular
microorganisms, such as pathogenic bacteria, that takes advantage of the
polymorphism of tandemly repeated DNA sequences. "VNTR" means
"Variable Number of Tandem Repeats". This method is well known in
forensic science since it is the basis of DNA fingerprinting in humans. When
applied to bacteria, it contributes to forensic microbiology through which
the source of a particular strain might eventually be traced back. In a
typical MLVA assay, a number of well-selected and characterized (in terms of
mutation rate and diversity) loci are amplified by polymerase chain reaction
(PCR) so that the size of each locus can be measured. From this size, the
number of repeat units at each locus can be deduced. The resulting
information is a code that can be easily compared to reference databases. When PFGE and MLVA patterns
match, they, along with solid epidemiological work (e.g., was the person
exposed to the suspect food item), are proof that the contaminated product
was the source of a person's illness. In the Nestle Toll House
Cookie Dough E. coli O157:H7 outbreak the science and epidemiological work
produces the below partial “line list.” This list (as of June 25, 2009) lists
76 persons (that number is now 80) who are linked together by a common PFGE
of E. coli O157:H7. Most have MLVA patterns reported - some were still
pending. The bottom line is that this list, along with solid epidemiological
work, show the “CSI” link between these 76 people and the Cookie Dough they
consumed. |
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