Detection of Salmonella by Indicator Agar Media and PCR As Affected by Alfalfa Seed Homogenates and Native Bacteria

Detection of Salmonella by Indicator Agar Media and PCR as Affected by Alfalfa Seed Homogenates and Native Bacteria
March 2003
Letters in Applied Microbiology, Volume 36 Issue 3 Page 152
C.-H. Liao and L.M. Shollenberger

Abstract
Aims: To investigate and prevent the undesirable effect of native bacteria and alfalfa seed homogenates on detection of Salmonella in alfalfa seeds by indicator agar media and polymerase chain reaction (PCR).

Methods and Results: The relative sensitivity of five indicator agar media, including modified semisolid RV (MSRV), xylose-lysine-Tergitol 4 (XLT4), Hektoen enteric agar (HEA), brilliant green agar (BGA) and bismuth sulphite agar (BSA), for detection of Salmonella in the presence of a large number of native bacteria from alfalfa seeds was examined. The detection limit as measured by the ratio between the numbers of native bacteria and Salmonella was estimated to be 106 to 1 for MSRV and 103 to 1 for XLT4, HEA, BGA or BSA. Presence of alfalfa seed homogenates markedly reduced the sensitivity of Salmonella detection by PCR.
The minimal number of Salmonella detectable by PCR was determined to be 1-10 and 100-1000 CFU in the absence and presence of seed homogenate, respectively. Application of anti-Salmonella immunomagnetic beads permitted detection of 2-5 CFU of heat-injured cells in 25 g of seeds within 24 h by PCR.

Conclusions: The MSRV medium is more sensitive than other indicator agars for detecting a small number of motile Salmonella in samples containing a large number of native bacteria. Application of immunomagnetic beads eliminates the PCR-inhibitory activity of seed homogenates and improves the detection of Salmonella in inoculated seeds.

Significance and Impact: The results generated from this study will aid the seed distributors, sprout growers and public health officials to identify and recall the Salmonella-contaminated seed lots to be used for sprout production.