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 (
C
Las),” 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
C
Las. 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
C
Las. We tested commercially available synthetic amicoumacins A and B, along with a
B. safensis
-derived amicoumacin mixture, against
L. crescens
and
C
Las. 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
C
Las 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.
IMPORTANCE
For two decades, the citrus industry has been severely impacted by Huanglongbing (HLB), a devastating disease caused by “
Candidatus
Liberibacter asiaticus (
C
Las)” 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
C
Las 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.