Royal jelly (RJ) is known to contain 10-hydroxydecanoic acid (10HDAA), which has been shown to have immune activation properties, including the promotion of M cell differentiation. However, the natural concentration of 10HDAA in RJ is relatively low. To enhance the functional use of RJ as an immunostimulatory food ingredient, this study aimed to increase its 10HDAA content using bacteria capable of converting 10-hydroxy-2-decenoic acid (10H2DA) to 10HDAA in RJ. A lactic acid bacterium,