Purpose: Our objective was to compare culture- and PCR-based assays for E. coli O157 detection in the feces of naturally infected feedlot cattle.
Methods: Fecal samples were collected from pens of feedlot cattle enrolled in an E. coli O157 intervention study. Samples considered high shedders (≥ 104 CFU/g feces) by a semi-quantitative culture method, culture positive after enrichment (immunomagnetic separation and plating on selective medium), or culture negative for E. coli O157 were used. One hundred fecal samples from each category (high shedder, enrichment positive, culture negative) were randomly selected for evaluation by real-time PCR. DNA was extracted pre- and post-enrichment from each sample and subjected to a previously-published E. coli O157 real-time assay, based on three genes, rfbE, stx1, and stx2. All real-time assays were run in triplicate, and a sample was considered positive when both the rfbE gene and either stx1 or stx2 had an average CT value ≤ 36.
Results: For fecal samples identified as high shedders, 37% (37 of 100) were positive for E. coli O157 by PCR prior to enrichment, and 85% (85 of 100) were positive by PCR post-enrichment. For enrichment-positive cultures, 4% were detected by PCR prior to enrichment and 43% were PCR positive post-enrichment. Seven samples were culture negative and PCR positive prior to enrichment, and 40 were PCR positive but culture negative after enrichment.
Significance: Our data suggest a discrepancy between culture- and molecular-based detection methods for E. coli O157 in cattle feces; however, the agreement is best in samples containing high concentrations of E. coli O157.