Isolation of Indole-3-Aldehyde As a Growth Inhibitor of Legionella pneumophila from Diaion HP-20 Resins Used to Culture the Bacteria

The addition of Diaion HP-20 resin to a medium can enhance the growth of the Legionella species. We recently reported that elemental sulfur was isolated as a growth-inhibiting substance produced endogenously by Legionella pneumophila from the methanol extracts of the resins used to culture the bacteria. In this study, we attempted to isolate other growthinhibiting substances involved in the extracts, and identified indole-3-aldehyde as one of the inhibitors. The compound was involved in the medium as a constituent and it weakly inhibited the growth of L. pneumophila, indicating that the compound partly contributes to inhibit the bacterial growth when cultured without HP-20 resins.