Effects of environmental parameters on microbial rhamnolipid biosynthesis and bioreactor strategies for enhanced productivity

Microbial rhamnolipid biosynthesis is dependent on complex regulatory networks that are affected by feedback from the external environment. In the past decade, progress has been made toward strain development and process engineering control measures to by-pass complex metabolism, improve production, and mitigate foaming. The effects of medium composition on gene expression and product synthesis patterns are relatively well studied, albeit mostly with the best-known (pathogenic) rhamnolipid producer – Pseudomonas aeruginosa. The effects of other process control parameters (including dissolved oxygen, shear stress, temperature and pH) are less known. A more robust understanding of how the process responds to gradients in these parameters (i.e. simulating in larger-scale bioreactor environments with poor mass transfer characteristics) would help facilitate process development beyond the 10 L scale, which is rarely reported. Currently, a variety of bench-scale bioreactor processes have been employed including fed-batch, sequential, continuous, cell-immobilized, and solid state. Fed-batch and sequential processes have achieved the highest productivities and yields, but generally the current productivity benchmark remains insufficient for commercial application at well under 1 g L−1 h−1. This review examines the scientific literature on the response of rhamnolipid metabolism to environmental parameters as well as bioreactor strategies supporting process development.