CONICET | Buscador de Institutos y Recursos Humanos

Xanthomonas citri subsp. citri (Xcc) is the causal agent of citrus canker, a disease that affects different species of citrus. Xcc, as well as other Xanthomonas spp., produces xanthan, a polyanionic exopolysaccharide (EPS), composed by a cellulose backbone with a side chain containing a glucuronic acid residue linked to a mannose that connects to the backbone and a second terminal mannose. Mannose residues can be substituted by nonglycosidic groups (acetyl or pyruvate). Xanthan is an important virulence factor that affects bacterial growth in planta and biofilm formation. We previously demonstrated an association between xanthan structure/composition and its role as virulence factor. Considering this background, we aimed to analyze whether xanthan structure/composition changed in presence of citrus leaf extract. For this work, we cultured Xcc in grapefruit leaves aqueous extract (GLE). First, we observed that Xcc grew better in GLE than in peptone-yeast extract-malt extract medium (PYM). Xanthan synthetized by Xcc growing in GLE (XanGLE) showed different macroscopic qualities compared to xanthan produced by Xcc growing in PYM (Xan-PYM). Using scanning electronic microscopy, we observed that Xan-GLE presented a more heterogenous and disordered structure than Xan-PYM. The atomic force microscopy analysis revealed Xan-PYM molecules as long strands or double strands, while Xan-GLE molecules appeared to be shorter than the Xan-PYM ones and with a different arrangement (looking like spiders). On the other hand, analytical determinations reveled that Xan-GLE has higher content of pyruvate groups than Xan-PYM. These results could explain the behavior of the xanthan molecules in presence of GLE. A greater number of pyruvate groups causes an increase in intramolecular electrostatic repulsions between negative charges of xanthan molecules, affecting the ability of this EPS to hydrate and to interact with other molecules in the media.