Towards a Quantitative Modeling of the Synthesis of the Pectate Lyases, Essential Virulence Factors in Dickeya Dadantii

A dynamic mathematical model has been developed and validated to describe the synthesis of pectate lyases (Pels), the major virulence factors in Dickeya dadantii. This work focuses on the simultaneous modeling of the metabolic degradation of pectin by Pel enzymes and the genetic regulation of pel genes by 2-keto-3-deoxygluconate (KDG), a catabolite product of pectin which inactivates KdgR, one of the main repressors of pel genes. This modeling scheme takes into account the fact that the system is composed of two time-varying compartments: the extracellular medium, where Pel enzymes cleave pectin into oligomers, and the bacterial cytoplasm where, after internalization, oligomers are converted to KDG. Using the quasi-stationary state approximations, the model consists of some nonlinear differential equations for which most of the parameters could be estimated from the literature or from independent experiments. The few remaining unknown parameters were obtained by fitting the model equations against a set of Pel activity data. Model predictions were verified by measuring the time courses of bacterial growth, Pel production, pel mRNA accumulation and pectin consumption under various growth conditions. This work reveals that pectin is almost totally consumed before the burst of Pel production. This paradoxical behaviour can be interpreted as an evolutionary strategy to control the diffusion process so that as soon as a small amount of pectin is detected by the bacteria in its surroundings it anticipates more pectin to come. The model also predicts the possibility of bistable steady states in the presence of constant pectin compounds.

💡 Research Summary

The paper presents a dynamic mathematical model that simultaneously describes extracellular pectin degradation by pectate lyases (Pels) and intracellular genetic regulation of pel genes in the plant pathogen Dickeya dadantii. The system is divided into two time‑varying compartments: the extracellular medium, where Pel enzymes hydrolyze polymeric pectin into oligomers, and the cytoplasm, where internalized oligomers are converted into 2‑keto‑3‑deoxygluconate (KDG). KDG binds to and inactivates the transcriptional repressor KdgR, thereby derepressing pel transcription and promoting further Pel synthesis. By applying quasi‑steady‑state approximations, the authors reduce the complex metabolic‑regulatory network to a set of nonlinear ordinary differential equations. Most kinetic parameters are obtained from literature values or independent experiments (e.g., Michaelis‑Menten constants for Pel activity, growth rates, KDG conversion rates). The remaining unknown parameters are estimated by fitting the model to time‑course measurements of Pel activity under various pectin concentrations. Validation experiments include simultaneous monitoring of bacterial growth, pel mRNA accumulation, Pel enzymatic activity, and pectin consumption. The model accurately reproduces the observed “paradoxical” behavior: pectin is almost completely consumed before a sharp burst of Pel production. The authors interpret this as an evolutionary strategy—early detection of a small amount of pectin triggers anticipatory pel expression, ensuring rapid exploitation of incoming substrate. Additionally, the model predicts bistable steady states when pectin is supplied continuously, suggesting that D. dadantii can exist in low‑ or high‑Pel expression modes depending on initial conditions. This bistability could generate phenotypic heterogeneity within a bacterial population, enhancing adaptability to fluctuating environments. Overall, the work demonstrates that integrating extracellular catabolism with intracellular regulatory feedback yields a predictive framework for virulence factor production, offering insights for the design of anti‑virulence strategies and for broader applications of systems biology to pathogenic bacteria.