Purpose: To evaluate the effectiveness of enrichment media and detection methods in detecting pathogens in SIW samples following short enrichments.
Methods: Eighteen test portions of 100 ml SIW were artificially inoculated with low levels of Salmonella and another 18 test portions were inoculated with E. coli O157:H7 to achieve contamination level of 3 to 45 CFU/100 ml. Samples were left at room temperature for 1 h, mixed with 300 ml of BPW or mEHEC media for 1:4 sample:media ratio and incubated at 41°C for 8 or 18 h. Salmonella and E. coli O157:H7 levels were quantified after 8 h on XLT4 and Sorbitol MacConkey agar, respectively. This served as the culture confirmation to ensure sufficient levels. After enrichment, Salmonella levels in BPW and mEHEC was 1.2 x 107 and 1.7 x 107 CFU/ml, while E. coli O157:H7 levels was 1.1 x 107 and 1.2 x 107 CFU/ml, respectively. Samples were tested for Salmonella using both methods, while method 1 was used for detection of E. coli O157:H7. All tests were repeated three times with three samples tested each time.
Results: Method 1 showed 100% sensitivity for both organisms regardless of enrichment media and times. Method 2 showed some inconsistency with the 8 h BPW Salmonella samples, but was able to detect consistently using the mEHEC after 8 and 18 h. As a result, Method 1 was chosen for short enrichment tests, whereas both systems were used for longer enrichment times.
Significance: Low levels of Salmonella and E. coli O157:H7 can be accurately detected in SIW samples after short incubation times using molecular methods. It is important to internally validate the methods to ensure fitness for the purpose.