R. Lindqvist, PREPARATION OF PCR SAMPLES FROM FOOD BY A RAPID AND SIMPLE CENTRIFUGATION TECHNIQUE EVALUATED BY DETECTION OF ESCHERICHIA-COLI O157-H7, International journal of food microbiology, 37(1), 1997, pp. 73-82
A sample treatment method based on buoyant density centrifugation whic
h separates bacteria from food, concentrates bacteria and removes PCR
inhibitors is described. The method involves a one minute centrifugati
on of food homogenate layered over a gradient medium (Percoll(R) or Ba
cXtractor(TM)) in Eppendorf tubes, followed by a single wash step. The
small scale of this treatment makes it possible to process many sampl
es in a short time. To evaluate the method beef and minced beef sample
s, spiked with strains of Escherichia coli O157:H7, were treated and t
hen analysed by PCR aimed at verocytotoxin- (VT1 and VT2) and eae-gene
s. The detection limits in 1:10 (w/v) beef and minced beef homogenates
were 125-250 cfu ml(-1) (1250-2500 cfu g(-1)) and 1000 cfu ml(-1) (1x
10(4) cfu g(-1), respectively. The enrichment of spiked samples in buf
fered peptone water at 37 degrees C for 6 hours before buoyant density
centrifugation and PCR, allowed 0.5 cfu g(-1) beef and 5 cfu g(-1) mi
nced beef to be detected. This combination of enrichment and buoyant d
ensity centrifugation was also used for analysis of 43 beef samples fr
om a consignment in which E. coli O157:H7 had been detected, and detec
ted VT-genes in all 43 samples. E. coli O157:H7 was also separated and
detected in spiked samples of milk, lettuce, shrimps, and blue cheese
at arbitrary concentrations of 3000 cfu ml(-1). The present sample pr
eparation method has the potential to be applicable to many other comb
inations of bacteria and food, and in connection with other detection
methods than PCR as well. (C) 1997 Elsevier Science B.V.