to microbiological heat resistance, resulting in the inability to
accurately predict the microbicidal efficacy of many thermal
processes. This is illustrated at right
with Salmonella and with E. faecium
NRRL B-2354, wherein more lethality
occurred in a wheat-based product
at lower temperatures and higher aw
than at much higher temperatures
and lower aw. This information was
presented at the 2006 International
Association for Food Protection
annual meeting and is referenced in
a forthcoming 2012 publication (see
Figures 1 and 2). 6, 7
Furthermore, it is not correct to
extrapolate thermal heat resistance
between different food matrices even
when they are at the same water
activities. 5 Different approaches to
modeling microbial heat resistance
exist, but these present many
potential limitations. Hence, there is
a need for an appropriate surrogate
microbe to validate actual in-factory
conditions. Some researchers believe
that E. faecium NRRL B-2354 is the
only appropriate surrogate for use in
thermal processing of dry foods in
place of Salmonella. 8
I have9, 10 described surrogates as harmless microbes with
correlated survival and growth parameters to specific pathogens
that are frequently used to validate the efficacy of critical limits
for Critical Control Points. They can be naturally occurring
or artificially added and should be nonpathogenic (emphasis
mine). Both the U.S. Department of Agriculture (USDA)
and Food and Drug Administration have acknowledged the
use of surrogate microorganisms as a valid approach where
appropriate. An advantage of using surrogates is that they
can be used under actual factory or pilot plant conditions to
validate the efficacy of a process without the need to attempt to
simulate such processes in a laboratory. 10
the identification of this strain by that means should not be
considered conclusive, especially as the differences between
the likely species referenced above
via rDNA analysis were on the
order of one to two base pairs. 13 The
determining factor was a biochemical
analysis with a Vitek unit, which is
a biochemically based phenotypic
approach that is dependent upon
the database of patterns stored
within each unit. Results are not
reproducible in a laboratory apart
from use of another Vitek unit
with the same database. Hence, the
identification described by Ma et al.
could be viewed as indeterminate.
The consequences of deciding species
identity of this organism are more
than a purely academic exercise, since
the species durans and hirae have
rarely, if ever, been associated with
human illness. Nevertheless, the work
of Ma et al. appears to have been
used to call the strain E. faecium by
one or more of the culture collections
and is referenced as a definitive
confirmation by the Almond Board
of California. 14
It is noteworthy that ATCC does
not guarantee the accuracy of the strain identity. A note at the
bottom of the link on the ATTC website wherein ATCC 8459
is described reads, “While ATCC uses reasonable efforts to
include accurate and up-to-date information on this site, ATCC
makes no warranties or representations as to its accuracy.
Citations from scientific literature and patents are provided for
informational purposes only. ATCC does not warrant that such
information has been confirmed to be accurate.”1 The Almond
Board of California specifically recommended the use of the
NRRL B-2354 strain as it was provided without cost and had
been used in a surrogate study. 14
Figure 2: Comparison of Inactivation of Surrogate and
a Three-strain Salmonella Cocktail in an Oven-treated
Low aw Product at 200 °F
Figure 1: Heat Resistance of Enterococcus
(Pediococcus) sp. NRRL B-2354 in a Cooked Wheat
High aw Product
Identity of NRRL B-2354
Data in a study by Ma et al. 11 indicated that rDNA
sequencing (500 base-pair region) performed on ATCC 8459
indicated it to be in the genus Enterococcus. However, further
speciation by this technique was not possible. It appeared
closely related to E. faecium, or E. hirae or E. durans. 11 The
authors referred to it in their paper as an E. faecium based upon
further phenotypic (e.g., Vitek) analysis. However, Franz et
al. 12 stated that “The classical taxonomy of the enterococci
is vague because there are no phenotypic characteristics
that unequivocally distinguish them from other Gram-positive, catalase negative, coccus-shaped bacteria.” Hence,
History of Use in the Food Industry
Other Enterococcus strains: The good, the bad and the ugly
Historically, tests for Enterococcus spp. in foods or food
processing environments or surfaces have been used as
indicators of hygiene, fecal contamination, quality indicators
or sanitation efficacy. A review of the beneficial uses of
Enterococcus spp. (e.g., bacteriocins, probiotics, cheese
manufacture) and their implication in food spoilage and as
fecal hygienic indicators and in disease was provided by Franz
et al. 12 The authors demonstrated that contamination of cheeses
and ready-to-eat meats is fairly common. However, foodborne
infections with Enterococci are unproven. Food microbiologists
typically regard Enterococcus spp. to be food quality indicators,
