Summary auto-generated
This article describes "Candidatus Microthrix parvicella," a filamentous bacterium found in activated sludge sewage treatment plants worldwide, where it causes operational problems such as foaming and bulking. The organism is gram-positive with a distinctive winding filamentous morphology, containing polyphosphate inclusions and testing positive for catalase. Phylogenetically, it is a deep-branching member of the actinomycetes subphylum. Despite 12 years of intensive cultivation attempts involving over 700 micromanipulated filaments and numerous media formulations, the bacterium grows with extreme difficulty in pure culture. Only two successful isolates were obtained: DAN1-3 (growing on modified NTM) and Ben43 (growing on freshly prepared R2A). Both isolates yielded nearly identical 16S rRNA gene sequences, confirming they represent the same organism present across multiple treatment plants. Electron microscopy confirmed gram-positive cell wall ultrastructure. Because insufficient phenotypic data exist for formal taxonomic description, the authors propose the "Candidatus" status—a provisional designation for incompletely characterized organisms. This naming preserves the familiar "Microthrix" designation essential for industry communication while acknowledging the organism's incompletely described status.
Key findings
- "Candidatus Microthrix parvicella" is a gram-positive, filamentous bacterium that is difficult to culture, with only two successful pure culture isolates obtained after 12 years of attempts
- The organism is phylogenetically classified as a deep-branching member of the actinomycetes subphylum within gram-positive bacteria, confirmed by 16S rRNA gene sequencing
- Electron microscopy confirmed distinctly gram-positive cell wall ultrastructure with cells measuring 0.5-0.9 μm by 0.4-0.5 μm
- The bacterium possesses polyphosphate inclusions, is catalase-positive, and displays characteristic irregular, winding coiled filaments hundreds of micrometers long
- Only freshly prepared R2A and modified NTM media supported growth, though inconsistently, and the organism grows extremely slowly requiring more than four weeks to produce macroscopic colonies
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Abstract
Candidatus Microthrix parvicella is a filamentous bacterium that grows with great difficulty in cultures from the mixed liquor of activated sludge sewage treatment plants. It is gram positive, and the ultrastructure of its cell walls has been determined to be of the gram- positive type by electron microscopical examination. Phylogenetically, it is a deep-branching member of the subphylum actinomycetes within the gram-positive phylum of the domain Bacteria. As for phenotypic features, it is known that the organism contains a polyphosphate inclusions and that it is catalase positive. In mixed cultures in activated sludge plants and in pure culture in the laboratory, it has a characteristic and distinctive winding filamentous morphology, with filaments hundreds of micrometers long.