Thiothrix is a filamentous bacteria morphotype as well as a genus with seven recognized species. When referring to Thiothrix genus, we are referring to the actual genetic properties. Within the Thiothrix genus, Thiothrix unzii, Thiothrix lacustris, Thiothrix eikelboomii, and four currently unnamed species are recognized.

The Thiothrix family tree is:

• Kingdom Bacteria
• Phylum Proteobacteria
• Class Gammaproteobacteria
• Order Thiotrichales
• Family Thiotrichaceae
• Genus Thiothrix

Thiothrix filaments may possess the Thiothrix or the type 021N morphotype. The type 021N morphotype is recognized by its diameter of 1.6-2.5 µm, variable cell shape (rectangular, oval, barrel) and indentations of the septa. (Jenkins, 2004). 

Typically the type 021N filament type is a long filament, which at high abundance often bridges the flocs together, slowing down the settling properties of the sludge (increasing the SVI-sludge volume index).

The 3rd Edition Manual (Jenkins, Richard, Daigger) of 2004 references Thiothrix I and Thiothrix II morphotypes. These descriptions are similar (sheath, rectangular cell shape, typically extend from the floc structure) with the most notable difference being the filament diameter size (if it’s greater than 1.6 µm it’s Thiothrix I, and if less than 1.6 µm diameter Thiothrix II).

Following suit of many academic publications, and for simplicity of microscopy recognition, we shortened the morphotype descriptions in our reports to simply “Thiothrix,” as have several other recognized authorities in the field.

Thiothrix are aerobic heterotrophs (use organic carbonaceous materials in the presence of free dissolved oxygen), but may also exhibit mixotrophic (use various sources rather than only one source of carbon) and chemolithoautotrophic (use inorganic compounds such as iron, nitrogen and sulfur) activity under aerobic conditions. (Nielson, 2000).

Role in bulking episodes

Thiothrix filaments can oxidize sulfide and thiosulfate producing intracellular sulfur granules observed within the filaments. Thiothrix is commonly responsible for bulking episodes in municipal and industrial plants and the recognized growth causes for this filament in situ are elevated concentrations of low molecular weight organic acids and sulfide (Richard, 2008).

Organic acids may be naturally occurring in many industrial wastes and septage or formed in areas of septicity/fermentation (common places of organic acid formation include collections systems, lift stations, primary clarifiers and sludge-handling return side-streams).

Control strategies

Common control strategies for Thiothrix bulking include chlorination to selectively destroy filamentous bacteria extending from the floc, process configuration changes (such as step feeding of influent to various locations within the aeration basin), or locating the source and preventing the formation of organic acids by maintaining positive redox values in the problem area.

In addition to the seven recognized species within the Thiothrix genus, there are several other genus that are candidates or confirmed genus that are able to possess Thiothrix morpholoigcal traits. Leptothrix and Leucothrix genus (Sevior labs, 2020) have been confirmed for these capabilities, however it is important to note that Leptothrix may also grow in non-filamentous form or can grow with Sphaerotilus (and likely type 1701) morphological traits.

Based on ongoing studies we are conducting linking filamentous morphotypes to suspected genetic characteristics, Dokdonella and Thialkalispira are also candidates to possess Thiothrix morphology.

Lastly, note that there is high genetic diversity associated with the type 021N morphotype, which also includes the species Thiothrix eikelboomii.

As we continue to learn more about filamentous bacteria from an academic level, the good news is that thanks to the early pioneers in this field, we have a solid track record based on 40 years of experience in correlating the morphological traits of filaments to respective troubleshooting strategies that have proven successful in operations. 

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About the author: Ryan Hennessy is the microbiology and operations specialist at Midwest Contract Operations Inc. He was trained and mentored by Dr. Michael Richard for over 10 years in wastewater microbiology, and serves as a microbiology services consultant. Hennessy is a licensed wastewater treatment and municipal waterworks operator in the state of Wisconsin and fills in as needed for operations at several facilities. He can be reached at rhennessy@mco-us.com.