Graciela Lorca studies genetic systems to find positive and negative microbial interactions that lead to disease. She talks about her discovery of chemical inhibitors for the citrus greening disease bacterium, Liberibacter asiaticus,and how a specific strain of Lactobacillus johnsoniimodulates the immune system and may help prevent development of diabetes in people.

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Julie's Biggest Takeaways

Citrus greening disease, or huanglongbing, is a disease of citrus trees causing a major epidemic among citrus farmers around the world. The disease causes trees to sicken and eventually die, and is best diagnosed by PCR amplification of the bacterial DNA from the bacterium that causes the disease, Liberibacter asiaticus. Because the disease spreads through the tree at different rates, it's important that many samples be tested for accurate diagnosis.

Quarantining the disease has proved difficult, as undiagnosed roots can transmit the disease if they are used to hybridize with canopy plants. The disease becomes even harder to contain under bad weather conditions: the high winds of recent hurricanes can scatter the insect vector, the Asian citrus psyllid, leading to infection of new orchards.

Although L. asiaticuscan't be cultured, Graciela performed a screen on L. asiaticustranscription factors that were produced by E. coli. These were tested for inhibition by a chemical library, and discovered that a common treatment for gout, benzbromarone, inhibited protein activity. This discovery was confirmed using in vivoinfected plants and by expressing the gene in related bacterial species, Graciela and her team predict the protein plays a role in responding to osmotic stress. The protein target of the chemical differs widely between citrus greening disease and gout, but the protein-chemical interaction is similar enough to allow protein inhibition.

Is there a link between the microbiome and diabetes? 10 years ago, Lactobacillus johnsoniican rescue animals that are predisposed to diabetes. L. johnsoniiinactivates a host enzyme, IDO, which regulates proinflammatory responses. Activated immune cells can travel to the pancreas and attack beta cells, leading to diabetes. Regulating the proinflammatory response by administering L. johnsoniias probiotics offers the opportunity to control development of diabetes in predisposed people.

Links for This Episode

• MTM Listener Survey, only takes 3 minutes. Thanks!
• Graciela Lorca's lab website
• Pagliai F.A. et al. The Transcriptional Activator LdtR from 'Candidatus Liberibacter asiaticus' Mediates Osmotic Stress Tolerance. PLoS Pathogens. April 2014.
• Lai K.K., Lorca G.L. and Gonzalez C.F. Biochemical Properties of Two Cinnamoyl Esterases Purified from a Lactobacillus johnsonii Strain Isolated from Stool Samples of Diabetes-Resistant Rats. Applied and Environmental Microbiology. August 2009.
• Marcial G.E. et al. Lactobacillus johnsonii N6.2 Modulates the Host Immune Response: A Double-Blind, Randomized Trial in Healthy Adults. Frontiers in Immunology. June 2017.
• HOM Tidbit: Hartmann A., Rothballer M., and Schmid M. Lorenz Hiltner, a Pioneer in Rhisophere Microbial Ecology and Soil Bacteriology Research. Plant and Soil November 2008.