Petrobactin protects against oxidative stress and enhances sporulation efficiency in Bacillus anthracis Sterne

Abstract

Bacillus anthracis is a gram-positive bacillus that under conditions of environmental stress, such as low nutrients, can convert from a vegetative bacillus to a highly durable spore that enables long-term survival. The sporulation process is regulated by a sequential cascade of dedicated transcription factors but requires key nutrients to complete, one of which is iron. Iron acquisition by the iron-scavenging siderophore petrobactin is the only such system known to be required for vegetative growth of B. anthracis in iron-depleted conditions, e.g., in the host. However, the extent to which petrobactin is involved in spore formation is unknown. This work shows that efficient in vitro sporulation of B. anthracis requires petrobactin, that the petrobactin biosynthesis operon (asbA-F) is induced prior to sporulation, and that petrobactin itself is associated with spores. Petrobactin is also required for both oxidative stress protection during late stage growth and wild-type levels of sporulation in sporulation medium. When considered with the petrobactin-dependent sporulation in bovine blood also described in this work, these effects on in vitro growth and sporulation suggest that petrobactin is required for B. anthracis transmission via the spore during natural infections in addition to its key functions during active anthrax infections.\n\nImportanceBacillus anthracis causes the disease anthrax, which is transmitted via its dormant, spore phase. However, converting from bacilli to spore is a complex, energetically costly process that requires many nutrients including iron. B. anthracis requires the siderophore petrobactin to scavenge iron from host environments. We show that in the Sterne strain, petrobactin is required also for efficient sporulation, even when ample iron is available. The petrobactin biosynthesis operon is expressed during sporulation, and petrobactin is biosynthesized during growth in high iron sporulation medium but instead of being exported, the petrobactin remains intracellular to protect against oxidative stress and improve sporulation. It is also required for full growth and sporulation in blood (bovine), an essential step for anthrax transmission between mammalian hosts.