Discovery and Characterization of Low-Molecular Weight Inhibitors of Erwinia tracheiphila

Abstract

Plant pathogenic bacteria in the genus Erwinia cause economically important diseases, including bacterial wilt of cucurbits caused by Erwinia tracheiphila. Conventional bactericides are insufficient to control this disease. Using high-throughput screening, 464 small molecules (SMs) with either cidal or static activity at 100 µM against a cucumber strain of E. tracheiphila were identified. Among them, 20 SMs (SM1 to SM20), composed of nine distinct chemical moiety structures, were cidal to multiple E. tracheiphila strains at 100 µM. These lead SMs had low toxicity to human cells and honey bees at 100 µM. No phytotoxicity was observed on melon plants at 100 µM, except when SM12 was either mixed with Silwet L-77 and foliar sprayed or when delivered through the roots. Lead SMs did not inhibit the growth of beneficial Pseudomonas and Enterobacter species but inhibited the growth of Bacillus species. Nineteen SMs were cidal to Xanthomonas cucurbitae and showed >50% growth inhibition against Pseudomonas syringae pv. lachrymans. In addition, 19 SMs were cidal or static against Erwinia amylovora in vitro. Five SMs demonstrated potential to suppress E. tracheiphila when foliar sprayed on melon plants at 2× the minimum bactericidal concentration. Thirteen SMs reduced Et load in melon plants when delivered via roots. Temperature and light did not affect the activity of SMs. In vitro cidal activity was observed after 3 to 10 h of exposure to these five SMs. Here, we report 19 SMs that provide chemical scaffolds for future development of bactericides against plant pathogenic bacterial species.

Compound Prioritization through Meta-Analysis Enhances the Discovery of Antimicrobial Hits against Bacterial Pathogens

“…For in silico analyses, we used the in vitro or in vivo data obtained from our earlier published studies using high throughput screening of 4182 compounds on nine different pathogens [ 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ]. This library was screened at the specified concentration (between 10 and 200 µM) against each pathogen ( Table 1 ).…”

“…Further, the lead compounds were separated from the hit compounds to determine whether lead compounds possess a different PCP profile compared to hit compounds. Additional details about the experiments performed to select and validate the antimicrobial potency of these compounds (i.e., dose-response assays, activity spectrum assays on beneficial and other pathogenic bacteria, antimicrobial efficacy in planta and in chickens, microbiome studies) are described in our published studies [ 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ]. By analyzing the previous eleven primary screening data, 2073 SM (49.6%) were identified as hits (SM that completely inhibited the growth or attenuated at least 75% virulence of the designated pathogen(s); Supplementary File 1 ).…”

“…A total of 18 lead compounds were identified from our previously published drug discovery studies ( Table 3 and Supplemental File 1 ) [ 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ]. These compounds have been shown to significantly reduce the load of designated pathogens and severity of the disease in vivo, with minimal impact on host, its microbiota, commensal bacteria, and probiotics/biocontrols.…”

“…These compounds have been shown to significantly reduce the load of the designated pathogens in vitro, with minimal impact on commensal bacteria and probiotics/biocontrols, but with limited antimicrobial efficacy in vivo. Additional details about the experiments performed to select and validate the antimicrobial potency of these compounds ( n = 18 + 73) are described in our published studies [ 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ]. These 91 compounds were used ( Supplemental File 1 ) to determine whether lead compounds possess different PCP profile compared to hit compounds.…”

“…The pre-selected library showed an increased rate in the identification of SM with growth inhibition properties in vitro (up to 16-fold) against several model organisms (e.g., Escherichia coli , Candida albicans , Caenorhabditis elegans , and Bacillus subtilis ) compared to the original library. From this subset, in our earlier studies, we screened a library of 4182 compounds to identify compounds effective against several plant ( Xanthomonas , Erwinia , Clavibacter , and Acidovorax ), animal ( Mycoplasma and Escherichia ), and foodborne-human ( Salmonella and Campylobacter ) pathogens that are of public health and economic importance [ 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ]. The objective of this study was to characterize and associate the antimicrobial properties of the pre-selected library based on their PCP.…”