Abstract
BackgroundHuanglongbing (HLB) is a worldwide devastating disease of citrus. There are no effective control measures for this newly emerging but century-old disease. Previously, we reported a combination of Penicillin G and Streptomycin was effective in eliminating or suppressing the associated bacterium, ‘Candidatus Liberibacter asiaticus’ (Las).ResultsHere we report the bacterial composition and community structure in HLB-affected citrus plants during a growing season and while being treated with antibiotic combinations PS (Penicillin G and Streptomycin) and KO (Kasugamycin and Oxytetracycline) using the Phylochip™ G3 array. Both antibiotic treatments resulted in significantly lower Las bacterial titers (Pr<0.05) and hybridization scores. Of the 50,000+ available operational taxonomic units (OTUs) on PhyloChip™ G3, 7,028 known OTUs were present in citrus leaf midribs. These OTUs were from 58 phyla, of which five contained 100 or more OTUs, Proteobacteria (44.1%), Firmicutes (23.5%), Actinobacteria (12.4%), Bacteroidetes (6.6%) and Cyanobacteria (3.2%). In the antibiotic treated samples, the number of OTUs decreased to a total of 5,599. The over-all bacterial diversity decreased with the antibiotic treatments, as did the abundance of 11 OTUs within Proteobacteria, Firmicutes, Bacteroidetes and Planctomycetes. Within the Proteobacteria, ten OTUs representing the class γ-proteobacteria increased in abundance after four months of treatment, when the Las bacterium was at its lowest level in the HLB-affected citrus field plants.ConclusionsOur data revealed that Proteobacteria was constantly the dominant bacterial phylum recovered from citrus leaf midribs, with the α-proteobacterial and the γ-proteobacterial classes vying for prevalence. In addition, the level of bacterial diversity found in the leaf midribs of field citrus was greater than previously described. Bacterial cells in close proximity may be able to modify their microenvironment, making the composition of the microbial community an important factor in the ability of Las to cause HLB progression. A low Las level was seen as an annual fluctuation, part of the bacterial population dynamics, and as a response to the antibiotic treatments.
Highlights
Huanglongbing (HLB) is a worldwide devastating disease of citrus
Specific operational taxonomic units (OTUs) associated with the antibiotic treatments and sampling time points Principal coordinate analysis (PCoA) based on the weighted Unifrac distances between samples was performed with PhyloChip community data sets, and the results suggested that there were significant differences among the treatments and the sampling time points
PS and KO, resulted in decreases in the number of OTUs in the dominant phyla, except Cyanobacteria, and the over-all diversity of bacteria decreased from 7,028 OTUs to 5,599 OTUs by April 2011
Summary
Huanglongbing (HLB) is a worldwide devastating disease of citrus. We reported a combination of Penicillin G and Streptomycin was effective in eliminating or suppressing the associated bacterium, ‘Candidatus Liberibacter asiaticus’ (Las). Huanglongbing (HLB) is one of the most serious diseases of citrus and causes great losses in the citrus industry worldwide. HLB is associated with three species of fastidious and phloem-limited α-proteobacteria in the genus ‘Candidatus Liberibacter’: ‘Ca. Liberibacter asiaticus’ (Las), ‘Ca. Liberibacter africanus’, and ‘Ca. Liberibacter americanus’ [2], of which Las is the only species in the USA. HLB resistant citrus varieties are being developed to combat the disease, it will likely take over 10 years to produce and evaluate these resistant varieties in Florida [3]. Since Florida citrus trees are already infected, it is essential to develop an efficient treatment to combat HLB in the interim.
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