Assessment of unconventional antimicrobial compounds for the control of ‘Candidatus Liberibacter asiaticus’, the causative agent of citrus greening disease

Abstract

In this study, newly identified small molecules were examined for efficacy against 'Candidatus Liberibacter asiaticus' in commercial groves of sweet orange (Citrus sinensis) and white grapefruit (Citrus paradisi) trees. We used benzbromarone and/or tolfenamic acid delivered by trunk injection. We evaluated safety and efficacy parameters by performing RNAseq of the citrus host responses, 16S rRNA gene sequencing to characterize citrus-associated microbial communities during treatment, and qRT-PCR as an indirect determination of 'Ca. L. asiaticus' viability. Analyses of the C. sinensis transcriptome indicated that each treatment consistently induced genes associated with normal metabolism and growth, without compromising tree viability or negatively affecting the indigenous citrus-associated microbiota. it was found that treatment-associated reduction in 'Ca. L. asiaticus' was positively correlated with the proliferation of several core taxa related with citrus health. no symptoms of phytotoxicity were observed in any of the treated trees. trials were also performed in commercial groves to examine the effect of each treatment on fruit productivity, juice quality and efficacy against 'Ca. L. asiaticus'. Increased fruit production (15%) was observed in C. paradisi following twelve months of treatment with benzbromarone and tolfenamic acid. these results were positively correlated with decreased 'Ca. L. asiaticus' transcriptional activity in root samples. 'Candidatus Liberibacter asiaticus' (CLas) is a fastidious, phloem-limited bacterium and the main causative agent of Huanglongbing (HLB), also known as citrus greening disease 1. Since its emergence in China nearly 100 years ago, a multitude of management strategies have been implemented in an attempt to mitigate the extensive economic damage caused by this pathogen. In areas such as California, where the disease is not yet widespread, management strategies are focused on increased testing for HLB and removal of infected trees, however, the disease remains prevalent in the majority of citrus producing regions throughout the world 1-5. Recent treatment strategies have included a combination of insecticides, antimicrobials and thermotherapy, but an effective long-term treatment, that is specific for CLas, remains elusive. In December 2018, the U.S. Environmental Protection Agency approved the use of oxytetracycline on citrus 6 ; however, CDC and FDA officials have expressed concerns

Antibacterial activity of grapefruit peel extracts and green-synthesized silver nanoparticles

“…Interbacterial transmission of antibiotic resistance through horizontal gene transfer (conjugation, transduction, and transformation) has made the situation critical worldwide [3]. To overcome this problem, intensive research has been done in recent years, and the use of plant extracts and nanoparticle has emerged as promising alternatives [6][7][8][9][10][11][12][13]. Medicinal herbs and plant extracts have been successfully used in traditional medicine around the world for millennia [14], and herbal remedies have various advantages because of their availability, fewer reported side effects, cost, high tolerance toward patients, and lack of bacterial resistance [15].…”

Cocktail Therapy of Fosthiazate and Cupric-Ammoniun Complex for Citrus Huanglongbing

“…At present, the prevention and treatment of HLB mainly includes three areas: use of pathogen free nursery trees, control of psyllids to reduce the spread of HLB, and complete removal of HLB trees (Bassanezi et al, 2020 ). There are also various drug treatment methods, including the combination of benzbromarone and tolfenamic acid (Gardner et al, 2020 ), penicillin and streptomycin (PS) (Iftikhar et al, 2016 ), melatonin (Nehela and Killiny, 2020 ) and so on. There are also methods to reduce the contagion of HLB through management (Gottwald et al, 2007 ) and solar thermotherapy (Doud et al, 2017 ).…”

Osmotic stress induces long-term biofilm survival in Liberibacter crescens

“…asiaticus have not been found. Consequently, current treatment for this disease involves a combination of insecticides, antimicrobials, and thermotherapy [ 23 ]. The goal of this study was to evaluate global changes in gene expression in response to natural environmental stress or artificially induced stress conditions by agricultural practices.…”

“…Physiological responses to long-term heat stress were also examined to further understand the long-term effects of thermotherapy in agriculture management, and the potential development of tolerance and/or survival mechanisms that may become prevalent. Similarly, dimethyl sulfoxide (DMSO) has been used as a carrier for the delivery of new antimicrobial compounds, such as benzbromarone and tolfenamic acid [ 23 ]. Gene expression studied under such conditions have the capacity to advance Huanglongbing therapies and uncover the potential of new or combined treatments.…”