The detection of thermophilic Campylobacter spp., as represented by Campylobacter jejuni, by the polymerase chain reaction (PCR) was investigated and compared with the selective agar isolation (SAI) method. Stationary-phase cultures of C. jejuni were inoculated into either blood-free enrichment broth (BFEB) or BFEB that contained 10% broccoli, crabmeat, mushroom, raw milk, and raw oyster rinses. Following a 48-h enrichment period, aliquots of food test portions were removed for simultaneous analysis by PCR and SAI. It was determined that the presence of charcoal and iron in the enrichment broth interfered with the PCR assay. Therefore, three DNA extraction techniques were developed and evaluated using a 16S rRNA primer pair in the PCR assay. The 50% end point (DL50) values (determined upon six initial C. jejuni spiking levels) were used to assess the frequency of isolation utilizing PCR versus SAI for the detection of this organism in the enrichment matrices. There were virtually no differences in detection of C. jejuni among enriched samples analyzed by PCR and SAI. Mean DL50 values (n = 3) for plain BFEB, broccoli, crabmeat, mushroom, raw milk, and raw oyster were, respectively, 0.02 (PCR) versus 0.01 (SAI), 0.01 versus 0.06, 0.07 versus 0.04, 0.03 versus 0.08, 0.01 versus 0.01, and 0.01 versus 0.01 CFU/5 g food. Significant variability in the detection limit of C. jejuni by PCR in the food enrichments was observed among DNA extraction techniques. Using 48-h enrichment cultures followed by PCR analysis could save 1 day of the time required for the presumptive identification of C. jejuni in suspected foods.
Determining the optimal method for DNA isolation from fruit jams
