Tweaking a bacterial gene builds better wastewater granules

Municipal and industrial wastewater systems increasingly rely on microbial granules—compact, fast-settling clumps of bacteria—to clean water efficiently. These granular communities, including aerobic granular sludge (AGS), are prized for their ability to remove pollutants while occupying less space than traditional systems. Yet scientists still do not fully understand the biological switches that tell microbes to stick together and form stable granules. In a study led by corresponding author Yasu S. Morita, researchers turned to Mycobacterium smegmatis, a nonpathogenic model for Mycobacterium tuberculosis that naturally forms granules in the lab. M. smegmatis carries just one known gene that directly alters levels of the signaling molecule cyclic-di-GMP (c-di-GMP): dcpA. That gene encodes a single enzyme with two opposing activities—diguanylate cyclase (DGC), which builds c-di-GMP, and phosphodiesterase (PDE), which breaks it down—making it a clean system to test how c-di-GMP affects aggregation. To probe this biological switch, the team made engineered strains that overexpress either the normal dcpA or a version called dcpA ΔEAL that is specifically defective in its PDE activity, and then compared those strains to the wildtype.

The scientists examined how these engineered strains behaved in a variety of growth tests linked to granulation. They looked at different forms of biofilm growth, cell morphology, plastic surface adhesion, granulation itself, and how well aggregates settled. Two main readouts—sludge volume index and microscopy—helped distinguish dense, spherical aggregates called granules from loose flocs. Those measurements showed that the M. smegmatis aggregates were indeed granules rather than flocs. Settleability, a practical measure for wastewater applications, was especially robust when cells were grown in a carbon rich medium known to promote granulation. To test granule stability, the team applied low concentration Tween-80 to induce dispersion; under that stress the engineered strains maintained stable granulation more effectively than the wildtype. Taken together, the results indicate that overproduction of DcpA, and the resulting change in intracellular cyclic-di-GMP, enhances granulation and helps granules persist.

Although this work focuses on a laboratory model organism, it points to a biologically simple way to influence a complex engineering outcome. By showing that manipulation of a single c-di-GMP modulating gene, dcpA (and its PDE-defective variant dcpA ΔEAL), can alter granule formation and persistence, the study highlights cyclic-di-GMP as a promising lever to promote granular behavior. The finding that engineered overexpression of DcpA helps granules resist dispersion by a surfactant like Tween-80 suggests routes to make granules more robust in real-world conditions where shear forces or chemical stresses can cause breakdown. Importantly, the work reinforces Mycobacterium smegmatis as a useful model for teasing apart the biological mechanisms behind granulation. Those insights could be translated into strategies to improve granular technologies such as aerobic granular sludge systems, ultimately helping wastewater treatment processes become more efficient and reliable.