CHARACTERIZATION OF THE NICHE OF THE LIBERIBACTER SPECIES

Banafsheh   Molki

This work aims to study the niche of Liberibacter species to assist the future attempt in isolating “Candidatus Liberibacter asiaticus” (“Ca. L. asiaticus”), the associated agent with citrus greening disease. To date, the pure culture of “Ca. L. asiaticus” has not been available which has delayed the availability of Koch's postulates tests and pathogenicity studies. To study the niche of Liberibacter species, microelectrodes were used to quantify the physicochemical conditions inside Asian citrus psyllids. Asian citrus psyllids are the carriers of “Ca. L. asiaticus” and “Ca. L. asiaticus” grow inside the body of these psyllids. I showed that the average oxygen tension in the hemolymph of infected Asian citrus psyllids was significantly lower than that of “Ca. L. asiaticus”-free Asian citrus psyllids. Moreover, a significant linear relationship between pH and infection level (“Ca. L. asiaticus” genome equivalents) was observed. These results illustrated that oxygen tension is an indicator of the infection status and pH is an indicator of infection level. These results suggest that “Ca. L. asiaticus” replicates in pH neutral and microerobic environments. The same measurements were performed for potato psyllid, the carrier of “Ca. Liberibacter solanacearum”, the associated agent with zebra chip disease in potatoes. It was observed that a significant relationship between oxygen tension and infection status and a significant linear relationship between pH and infection level exist in potato psyllids too. Subsequently, I developed a host -free “Ca. L. asiaticus” growth system inoculated with infected-Asian citrus psyllids. It was observed that the “Ca. L. asiaticus” genome equivalents increased almost 6 times after 7 days of incubation. I used antibiotics to alter the microbial structure of mixed cultures to understand the effect of the microbial structure on “Ca. L. asiaticus” growth. Exposure to vancomycin, streptomycin, and polymixin B increased “Ca. L. asiaticus” growth. Importantly, the abundance of Bacillus cereus was associated with the elimination of “Ca. L. asiaticus” in vitro, suggest that Bacillus cereus is an inhibitor to “Ca. L. asiaticus” growth and enrichment of Bacillaceae family might be suitable for a para-transgenic tool to control citrus greening disease.

CHARACTERIZATION OF THE NICHE OF THE LIBERIBACTER SPECIES

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