Uncultured bacteria linked to wastewater treatment plant sludge problems

Inside wastewater treatment plants, a problem as old as the technology itself keeps operators awake: sludge bulking. The sludge swells, thickens, refuses to settle properly, and suddenly the plant's efficiency collapses. For decades, operators have known the culprit is often filamentous bacteria—microbes that grow in long threads instead of compact clumps—but identifying which ones and why they flourish has remained frustratingly difficult. A new study from researchers examining an enhanced biological nutrient removal plant in Japan has identified a family of bacteria previously unknown to science that appears to be a major driver of this problem.

The bacteria belong to a group called MiDAS 4 f37-13, members of the larger Bacteroidota family. What makes them significant is that they are, in the language of microbiology, uncultured—meaning scientists have never successfully grown them in a laboratory. They exist in the wild, in the churning tanks of wastewater plants, but remain largely mysterious. The research team used fluorescence in situ hybridization, a technique that tags specific bacterial DNA with fluorescent markers, to examine samples from the Japanese plant. Under the microscope, the bacteria revealed a filamentous morphology, the same thread-like structure that causes sludge to bulk and settle poorly. Remarkably, they resembled a morphotype first described in 1863 by Eikelboom, a shape previously associated with a different bacterial group entirely—the Gamma-proteobacteria Acinetobacter.

Within the MiDAS 4 f37-13 family, two genera stood out: midas_g_535 and midas_g_543. These two were far more abundant than their relatives in the same family, and when the researchers analyzed their abundance against the plant's sludge volume index—a standard measure of how well sludge settles—they found a statistically significant correlation. In plain terms, when these bacteria became more numerous, sludge bulking worsened. The implication is direct: these two genera are likely responsible for the settling problems plaguing this particular treatment plant.

But the story does not end with identification of a nuisance. Metagenome analysis—a technique that sequences all the genetic material in a sample without needing to culture individual organisms—revealed that members of the MiDAS 4 f37-13 family carry genes associated with denitrification, the process by which nitrogen is removed from wastewater. They also possess genes for polyphosphate accumulation and polyhydroxyalkanoate synthesis, both processes central to nutrient recovery. This suggests that while these bacteria cause operational headaches through sludge bulking, they may simultaneously be performing valuable work in removing nitrogen and phosphorus from the treated water.

The discovery opens a paradox that operators and researchers will now have to navigate. These filamentous bacteria are simultaneously a problem and potentially a solution. Understanding their ecology—how they grow, what conditions favor them, what metabolic roles they play—could lead to strategies that either suppress them when bulking becomes severe or harness their nutrient-removal capabilities while controlling their tendency to form problematic filaments. For now, the bacteria remain uncultured and largely inscrutable, but their fingerprints are clear in the sludge of Japanese wastewater plants, and the question of what to do about them is just beginning.