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Applied and Environmental Microbiology, April 2000, p. 1453-1459, Vol. 66, No. 4
Department of Food Science, Cornell
University, Ithaca, New York 14853
Received 14 June 1999/Accepted 6 January 2000
A fluorogenic probe-based PCR assay was developed and evaluated for
its utility in detecting Bacillus cereus in nonfat dry milk. Regions of the hemolysin and cereolysin AB genes from an initial
group of two B. cereus isolates and two Bacillus
thuringiensis isolates were cloned and sequenced. Three
single-base differences in two B. cereus strains were
identified in the cereolysin AB gene at nucleotides 866, 875, and 1287, while there were no species-consistent differences found in the
hemolysin gene. A fluorogenic probe-based PCR assay was developed which
utilizes the 5'-to-3' exonuclease of Taq polymerase, and
two fluorogenic probes were evaluated. One fluorogenic probe (cerTAQ-1)
was designed to be specific for the nucleotide differences at bases 866 and 875 found in B. cereus. A total of 51 out of 72 B. cereus strains tested positive with the cerTAQ-1 probe,
while only 1 out of 5 B. thuringiensis strains tested
positive. Sequence analysis of the negative B. cereus
strains revealed additional polymorphism found in the cereolysin probe target. A second probe (cerTAQ-2) was designed to account for additional polymorphic sequences found in the cerTAQ-1-negative B. cereus strains. A total of 35 out of 39 B. cereus strains tested positive (including 10 of 14 previously
negative strains) with cerTAQ-2, although the assay readout was
uniformly lower with this probe than with cerTAQ-1. A PCR assay using
cerTAQ-1 was able to detect approximately 58 B. cereus CFU
in 1 g of artificially contaminated nonfat dry milk. Forty-three
nonfat dry milk samples were tested for the presence of B. cereus with the most-probable-number technique and the
fluorogenic PCR assay. Twelve of the 43 samples were contaminated with
B. cereus at levels greater than or equal to 43 CFU/g, and
all 12 of these samples tested positive with the fluorogenic PCR assay.
Of the remaining 31 samples, 12 were B. cereus negative and
19 were contaminated with B. cereus at levels ranging from
3 to 9 CFU/g. All 31 of these samples were negative in the fluorogenic
PCR assay. Although not totally inclusive, the PCR-based assay with
cerTAQ-1 is able to specifically detect B. cereus in nonfat
dry milk.
0099-2240/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Development of a Fluorogenic Probe-Based PCR Assay
for Detection of Bacillus cereus in Nonfat Dry
Milk
*
Corresponding author. Mailing address: 311 Stocking
Hall, Cornell University, Ithaca, NY 14853. Phone: (607) 255-2896. Fax: (607) 255-8741. E-mail: cab10{at}cornell.edu.
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