Amicoumacins produced by the native citrus microbiome isolate Bacillus safensis inhibit the Huanglongbing-associated bacterial pathogen "Candidatus Liberibacter asiaticus".

Abstract

Huanglongbing (HLB) is a devastating citrus disease associated with the gram-negative, phloem-limited, and unculturable bacterium "Candidatus Liberibacter asiaticus (CLas)," which is transmitted by the Asian citrus psyllid Diaphorina citri. Despite extensive research, effective, long-term, and sustainable solutions for managing HLB remain elusive. Oxytetracycline (OTC) is currently used as an emergency measure, but there is an urgent need for alternative compounds to complement or replace OTC. In this study, we identified amicoumacins, a class of antimicrobial compounds produced by the bacterium Bacillus safensis CB729 isolated from the citrus microbiome, and demonstrated their ability to suppress CLas. Genome mining of B. safensis CB729, combined with metabolomic analysis and bioassay-guided fractionation, revealed the presence of amicoumacins and related derivatives in fractions inhibitory to Liberibacter crescens, a culturable surrogate for CLas. We tested commercially available synthetic amicoumacins A and B, along with a B. safensis-derived amicoumacin mixture, against L. crescens and CLas. We determined the MICs of amicoumacin A (1.25 µg/mL) and amicoumacin B (10 µg/mL) against L. crescens. Furthermore, amicoumacin B and the amicoumacin mixture significantly reduced CLas populations in ex vivo citrus hairy root assays. This study highlights the potential of amicoumacins as a promising group of natural products for the management of HLB, offering valuable insights for the development of novel and sustainable disease control strategies.IMPORTANCEFor two decades, the citrus industry has been severely impacted by Huanglongbing (HLB), a devastating disease caused by "Candidatus Liberibacter asiaticus (CLas)" and transmitted by the Asian citrus psyllid (Diaphorina citri). Despite extensive research, effective, long-term, and sustainable solutions remain unavailable for growers. Currently, medically relevant antibiotics, such as oxytetracycline (OTC), are used as an emergency response to combat HLB in Florida, the most affected citrus-producing state in the U.S. This underscores the urgent need for alternative treatments that can be used in rotation or as replacements for OTC. Here, we present amicoumacins, a group of bioactive secondary metabolites with antibiotic properties. We identified amicoumacin B and its derivatives from the culture broth of a Bacillus safensis isolate, native to citrus, and demonstrated their ability to inhibit Liberibacter spp. and reduce CLas populations in citrus tissue. This study highlights how microbial discovery can lead to the identification of antimicrobial compounds with potential applications in plant disease management.