Purpose: The purpose of this study was to develop a method using improved enrichment medium and immunomagnetic separation (IMS) with antibody-coated Dynabeads® protein G for detection of S. aureus in lettuce and sprouts.
Methods: The brain heart infusion (BHI) was used as basal medium and three supplement concentrations of manganese sulfate, sodium pyruvate and yeast extract was optimized with response surface methodology. The recovery of acid (pH 4.0), cold (4°C for 5 h) and heat (55°C15S) damaged S. aureus was also compared with the modified BHI (MBHI) and BHI. The artificially inoculated samples lettuce and sprout with low (0.54 log CFU/g) and high (1.35 log CFU/g) levels were enriched in MBHI and BHI followed with immunomagnetic separation.
Results: The best experimental parameter for the MBHI were manganese sulfate 0.023%, sodium pyruvate 0.762% and mannitol 1.986%. When incubated in MBHI for 6 h with the initial cell number 1.01 log CFU/ml, the mean of colony number could get to 5.43 ± 0.18 log CFU/ml. The recovery of acid, cold and heat-damaged S. aureus incubated in MBHI at 35°C for 4 h and 6 h was significantly higher than in BHIL (P < 0.05). After 4 h enrichment in MBHI with the low inoculum level, it was able to be detected by the immunomagnetic separation method.
Significance: This study suggests that with the low (0.54 log CFU/g) level spiked sample lettuce and sprouts could be detected using the immunomagnetic separation method after 4 h enrichment in the developed enrichment medium MBHI.