ABSTRACT The bacterium Erwinia amylovora is a gram-negative pathogen of rose family plants worldwide and is the causal agent of the devastating fire blight disease, causing substantial damage to apple and pear trees annually. Cellulose is an exopolysaccharide produced by Erwinia amylovora from host carbohydrates that improves the three-dimensional structure of biofilms in the xylem of apple host plants, contributing to the devastating fire blight disease. Cellulose biosynthesis is regulated by the second messenger c-di-GMP, which is produced and degraded by diguanylate cyclases (DGCs) and specific phosphodiesterases (PDEs), respectively. While several DGCs and PDEs have the potential to contribute to cellulose production, the specificity of these enzymes is poorly known. In this study, host carbohydrates were added to growth media in vitro, and knockout mutant strains were used to examine the specificity of PDEs and a DGC with respect to cellulose production under carbohydrate-rich conditions. Experimental results indicate that glucose, sucrose, and sorbitol each resulted in an increase in cellulose production, which caused bacteria to engage in autoaggregation behavior in liquid medium. The pdeC gene had the strongest negative impact on cellulose production out of the E. amylovora PDEs, while mutation of dgcB abolished cellulose production. A mutant strain unable to metabolize sorbitol was unable to produce cellulose on sorbitol-containing medium, and sugar fermentation occurred in all sugar-enriched media, indicating that fermentative metabolism may favor cellulose production in specific host environments such as the xylem.
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