With regard to selectivity, much greater selectivity against background microflora was observed on BYPTA than on MYPA plates, especially in food samples with high background microflora, including Saengsik, red pepper powder, and soybean paste ( Fig. 1). In these food samples, differentiation and picking of suspicious colonies for the confirmation of B. cereus on MYPA was significantly hindered by dense, competing microflora occupying all parts of the plates ( Fig. 1). However, differentiation was much easier on BYPTA than on MYPA by the elimination of competing microflora (Fig. 1). Moreover, suspicious colonies on BYPTA appeared as white colonies, which are easily visible against the red background (Fig. 1). Fricker et al. (2008) reported that no characteristic pink color formation could be observed on MYPA because of the high acid formation by the background flora. In this study, the colonies of B. cereus on MYPA also appeared yellow, obscuring their differentiation. Background microflora in foods is one of the most common inhibiting factors for the recovery of pathogens on selective media ( Chon et al., 2011). Fricker et al. (2008) reported the difficulty in differentiating B. cereus colonies in dried vegetable samples by using conventional selective media, such as MYPA or PEMBA, because of numerous competing microflora.
Blood agar and sodium pyruvate were used to improve the recoverability and trimethoprim was used as an additional supplement in this study to exclude high background microflora in foods. It appears that trimethoprim effectively excluded some competing microflora, which was resistant to high concentrations of polymyxin B in Korean traditional spices or RTE vegetables, rendering the differentiation of suspicious colonies easier.
In the present study, a selective medium for B. cereus was developed and evaluated using pure culture and food samples. Our results demonstrate that this newly developed selective medium may be a useful alternative to MYPA for its selectivity particularly for foods with high background flora.