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ture step. This should be considered when designing a matrix
validation or verification study based upon a stakeholder's
insight into the respective formula, source, environment, etc.,
but it is unclear whether it would affect strain selection.
Strain genomics
The abundance of a strain in a host or ecosystem might
correlate with how well that organism can infect and spread.
Salmonella strains that predominated in a beef slaughterhouse
carried plasmidborne type IV secretion systems, which have
been linked to persistent infections in numerous pathogens
(17). This study showed that identifiable genetic elements
could be linked to persistence of particular strains in a
defined environment. However, this study did not explore
the effect of these genetic elements on outgrowth of the
strains in culture media. Hence, we can draw no conclusions
about their significance for matrix validation and verification.
CLINICAL CONSIDERATIONS AND OUTBREAK
DATA
An analysis of FoodNet data from 1996 to 2006 differentiating
Salmonella serotypes by clinical severity found that the
serotypes Salmonella Enteritidis and Salmonella Heidelberg
had significantly higher case fatality, hospitalization, and incidence
rates, with a higher proportion of invasive disease than
Salmonella Typhimurium. Conversely, Salmonella Newport
had a significantly lower proportion of invasive disease than
Salmonella Typhimurium, as well as a lower case fatality rate
(47). In a study of spatial and temporal patterns of Salmonella
infections in Florida, the most prevalent Salmonella
serotypes were Enteritidis, Newport, Javiana, Sandiego, and
Braenderup (54). Risks would seem to rise when the prevalent
serovars are those associated with more severe clinical
outcomes. We would clearly want to be able to detect strains
with the greatest potential to cause illness. This is addressed
by inclusivity testing during method validation, for which
purpose the foodborne relatedness measure of Luvsansharav
et al. (56) could help with serotype selection.
Although all strains of Salmonella are considered infectious,
some are more virulent than others. For example,
multi-drug-resistant Salmonella strains cause increased severity
and more prolonged disease in infected patients (24) and
are becoming more prevalent in outbreaks, including from
chicken in 2014 (Salmonella Heidelberg), cucumbers in 2015
(Salmonella Poona), papayas in 2017 (Salmonella Urbana),
poultry in 2018 (Salmonella Infantis and Salmonella Reading),
and pig ear pet treats in 2019 (19, 75). However, these
strains are no harder to detect with rapid methods. Hence, it
may be appropriate to use less virulent nonoutbreak strains
for matrix validations.
L. monocytogenes serotypes 1/2a, 1/2b, and 4b are responsible
for 98% of documented human listeriosis cases
(55). Serotype 4b strains are isolated mostly from epidemic
outbreaks of listeriosis, whereas serotypes 1/2a and 1/2b
are linked to sporadic L. monocytogenes infection (46, 92).
The serotypes most frequently involved in human listeriosis
(1/2a, 1/2b, and 4b) should be part of the inoculum for
Listeria challenge studies in foods (70) and among the inclusivity
strains for validation of Listeria detection methods. In
an outbreak investigation associated with a known strain and
unvalidated matrix, it would be valuable to extend the matrix
validation using the outbreak strain, if available. For most
other purposes, outbreak strains probably have less relevance
for matrix validation, which is convenient because they can
be challenging to obtain and only 42% of outbreak strains can
be traced back to a specific source (18).
Availability: isolate sourcing and considerations
Sourcing and maintenance of isolates can be both
logistically challenging and costly, often involving shipping
permits and/or material transfer agreements and possibly
restrictions on, or additional fees for, commercial use. The
risks associated with maintaining a culture collection must
be managed, e.g., through access restriction and appropriate
measures for disposal. In addition, isolates must be properly
stored, maintained, and monitored to ensure viability and
purity over time.
Many microbial strains and variants can be secured
through organizations that specialize in culture collection
maintenance and distribution, such as the American Type
Culture Collection (ATCC) and Agricultural Research
Service (ARS) Culture Collection at the Northern Regional
Research Laboratory (NRRL). These bodies have diverse
banks of foodborne pathogen isolates from agriculture, food,
environmental, and human origins (2, 77). An example of the
regional presence of different Salmonella serovars available
from ATCC is shown in Table 3.
There are also many reference collections for specific
pathogens, generally in government or academic institutions.
These may be ideal sources of genus, species, serovar, and
strain diversity. As an example, Dr. Martin Wiedmann and
colleagues in the College of Agriculture and Life Sciences at
Cornell University were selected by the technical committee
of the former International Life Sciences Institute of
North America, now the Institute for the Advancement of
Food and Nutrition Sciences, to house their Salmonella,
Listeria, and Cronobacter collections (29, 40, 45, 91). These
are maintained with additional pathogen and spoilage
organism collections, and many of these collections have
been described in the literature (37, 74). Wiedmann and
his colleagues have also worked to create tools such as the
Food Microbe Tracker (https://www.foodmicrobetracker.
net) to collaboratively capture information on biodiversity in
foodborne pathogens (84).
Federal and state agencies, particularly those with a
research arm, may serve as sources of isolates. Such centers
include the U.S. Department of Agriculture (USDA)
Meat Animal Research Center, the U.S. Food and Drug
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