Salmonella Rapid
Culture Method
Salmonella is a Gram-negative, rod-shaped, motile bacterium with a widespread occurrence in animals, especially in poultry and swine. Environmental sources of the organism include raw meat, raw
poultry and raw seafood, to name only a few. Detection is critical as
is the most frequently reported cause of foodborne illness ( 40,000–
50,000 cases reported annually), and the infectious dose can be as low
as 1– 10 cells.
Current Methods
Current testing methods (FDA/BAM)
involve a pre-enrichment step followed by a second selective enrichment step, followed by plating (which can
include secondary plating). This process takes 72–96
hours ( 3–4 days), depending on the food sample being
analyzed, and must be followed by biochemical/serolog-ical confirmation of Salmonella contamination, increasing the time to results to up to 5 days.
As indicated above, two enrichment steps are
typically needed to recover Salmonella since the
bacteria are usually present in very low numbers and
may be stressed or damaged from food processing
methods. These enrichment steps are general enrichments and are
designed to cultivate any microorganisms that may be present in the food
sample, potentially masking the presence of very low numbers of true
pathogens.
In addition, the secondary enrichment media, modified Rappaport
Vassiliadis broth, must be prepared from its individual raw materials to
ensure sufficient recovery of any contaminating microorganisms.
appropriate temperatures, according to
manufacturers’ or guideline instructions.
Raw ground beef, ground chicken, lettuce, shrimp and egg shells were obtained
from local suppliers. Food samples were
inoculated with low [~1 colony forming
unit (CFU)/25g] or high (~1.1 CFU/g) microorganism amounts. The final inoculated
concentrations were confirmed by MPN
(most probable number). Side-by-side
samples (both inoculated with microorganisms and uninoculated negative controls) were compared using the Rapid
Salmonella method and by the appropriate
USDA/FSIS and FDA-BAM reference
methods for each matrix.
A total of 102 Salmonella serovars/
strains were obtained and tested as
indicated above. Almost all (98 of 102) of
these strains produced a positive purple-colored colony. In addition, a total of 30
species not belonging to the genus
Salmonella were analyzed in the same
manner. All these strains showed atypical
(non-purple colonies) or no growth.
www.remel.com/salmonella
Rapid Confirmation
To address the need for a rapid, accurate test for confirmation of
Salmonella contamination in food products, a new method, the Salmonella
Rapid Culture Method, has been developed. This method involves a single
enrichment in ONE Broth-Salmonella, followed by plating on Brilliance™
Salmonella chromogenic medium, taking - less than 2 days to generate an
initial result. Thermo Scientific recently received AOAC-RI certification for
this method (License no. 120802).
ONE Broth-Salmonella is a highly nutritious enrichment broth containing
a specific growth promoter to ensure excellent recovery of stressed and
damaged Salmonella cells, while inhibiting the growth of competing
microorganisms. This effective medium allows enrichment to be performed
in a single 18–24 hour incubation, eliminating the need for a secondary
enrichment.
Following enrichment, the sample is plated onto chromogenic culture
media. Chromogens within the medium enable differentiation of Salmonella
colonies (bright purple) from any remaining organisms that are able to grow,
such as Klebsiella (blue colonies) and Enterobacter (clear colonies to no
growth), thus reducing the number of false positives that require
confirmation.
Testing Results
Fresh Salmonella cultures were prepared from frozen stocks using
standard enrichment media [tryptic soy broth (TSB), brain heart infusion
(BHI)] or ONE Broth-Salmonella) and incubated for 18–24 hours at the
Summary
This study demonstrated that the rapid
method described above detected almost
all serovars of Salmonella when spiked at
levels from low to high in food sources in
which Salmonella contamination has been
observed. Since 98/102 serovars
produced the “typical” purple colonies, it
is highly unlikely that a specific serovar
would go undetected and lead to an
outbreak of salmonellosis (the serovars
not detected are not typically found in
food). By screening such large numbers of
serovars/species, Thermo Scientific is
helping food companies to feel secure
knowing that food is free from Salmonella
contamination when tested using this
methodology.
In addition, this study demonstrated
that it is possible to discriminate between
Salmonella species and other selected
non-Salmonella organisms, demonstrating
the method’s selectivity. It also ensures
that samples contaminated with non-
Salmonella organisms do not demonstrate
a false-positive color development,
allowing the proper identification of food
pathogens present in the food being
tested. This will ensure a safe food supply
free of Salmonella while preventing the
waste from discarding any false-positive
food lots.
This robust method makes rapid
identification of pathogens in food products
faster and easier, reducing time to market
and eliminating the risk of food recalls.
