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Domain: Bacteria Phylum: Actinobacteria Class: Actinobacteria Order: Actinomycetales Family: Genus: Microthrix Species:


   Microthrix bacteria are one of the many bacterial genera found within activated sludge 

in wastewater treatment plants (WWTT's). In fact, certain species Microthrix parvicella have only be detected in the activated sludge of WWTPs. Some other species of Microthrix are found in marine environments and sediments. These bacteria are primarily responsible for some of the difficulty in separating the liquid and solid phases during activated sludge circulation. Their filamentous morphology causes bulk increases and material foaming that makes solid-liquid separation a bit difficult.

   Microthrix are very long, thin cells with diameters not not exceeding .8 micrometers. They 

are easily identified under a light microscope because of their characteristic morphology and coiled appearance. They are gram-positive and are able to be grown in isolated culture.

   They were first identified in the 1970's (Pasveer) but the first isolated cultures were not 

produced until almost twenty years after that. One set of conditions that they were isolated under was the following: Carbon source, glucose; Nitrogen source, ammonium sulfate; Sulphur source, sulfate; Added growth factor, vitamin B12 and thiamine; Isolation technique, dilution and plating; Maintenance, slant cultures at 4C.

    It is studied primarily because of the issues it causes in WWTPs. The bulking and foaming 

that Microtrix cause is a real issue because if the solid and liquid materials cannot be separated effectively, then the desired activated sludge bacteria will not perform as well when recycled back into the reactor. Because of the issue that Microtrix presents, methods are under development to limit its growth in activated sludge. These methods, however, must be delicate and specific enough to target only Microtrix and to avoid removing or reducing other crucial bacterial species from the activated sludge. One method that is being used at some WWTPs is the reduction of the oxygen partial pressures in the reactors. Microtrix are facultative aerobes, so reducing the concentration of oxygen within the reactors reduces their metabolic rate and prevents them from proliferating and causing bulking and foaming issues.

  Very little diversity has been documented among the Microthrix bacteria. While an important organism, it occupies a fairly specific niche role and thus is not heavily studied for its phylogenetic

relationships. Currently, its diversity lies in the different strains that have been obtained and experimented with. Different strains isolated by different scientists using WWTP sources from all over the world have indicated that Microthrix is diverse in its carbon and energy source. Though there are some differences in the metabolics of the different strains, there is no significant observed differences in the morphologies of the strains.

Image: http://macenvironmentalmicro.weebly.com/uploads/5/1/5/7/51578127/5101784_orig.jpg

Ecology and Significance

   As covered earlier, Microthrix are found in and studied for the purpose of WWTP's. Other 

than creating bulking and foaming issues in the activated sludge of the WWTP's, the bacteria have a capacity to effect the sludge by manipulating available substrates based on their environmental conditions. For example, when isolated from anaerobic environments, the Microthrix were found to contain very large storage granules full of lipids. In experiments adding 14C oleic acid to anaerobic activated sludge, the added lipid was undetectable in the solution after twenty minutes. After performing MAR-FISH on the sample, it was clear that the culprit for the massive lipid uptake was the Microthrix bacteria.

   This rapid and significant uptake of free lipids in activated sludge has implications on 

the performance of the activated sludge as a whole. Many important bacterial species in activated sludge use fats as primary electrons donors or carbon sources. Thus, the excessive and rapid uptake of fats by Microthrix would limit these species' ability to grow and successfully fill their role in the activated sludge process.

Genome Structure

    While no complete genome for Microthrix has been assembled as of yet, a draft genome 

was created by researchers in 2012. Here are some of the key statistics of the Microthrix draft genome:

Size: 4,202,850bp GC content: 66.4% Scaffolds/Contig of assembly: 13/16

    The genome assembled contained many sequences that confirmed the observed functions of 

the Microthrix bacteria. For example, there is an identified, intact pentose phosphate pathway as well as a TCA cycle. Also, confirming its ability to store large amounts of lipids, the Microthrix bacteria genome contains almost thirty homologs of long-chain fatty acid acetyl-CoA ligase.


     Primarily, Microthrix is an aerobic chemoorganotroph. It is also found to be a denitrifier

but, under aerobic conditions, cannot reduce nitrate past nitrite. As stated previously, Microthrix is facultative in the sense that it can survive and undergo its metabolism in anoxic conditions but strongly prefers oxygen as an electron acceptor. In fact, for optimal growth, Microthrix prefer low dissolved oxygen levels, around .4mg/L.

    There is a fair amount of controversy surrounding the preferred carbon and energy sources

of the Microthrix bacteria, mostly because of the differences observed in its metabolism in pure culture and in situ. In pure culture, optimal growth has been achieved using substrates such as glucose, succinate, and lactate. In in situ studies, fatty acids such as oleic acid, palmitic acid, and trioleic acid have been used as a carbon and energy source. Though these differences exist, it is accepted that while Microthrix have the ability to grow on an extensive range of carbon sources in isolation, their functional metabolism is that of oxidizing fatty acids like those observed in in situ experiments.


[1] Golyshin, Peter N. “Genome Sequence Completed of Alcanivorax borkumensis, a Hydrocarbon-degrading Bacterium That Plays a Global Role in Oil Removal from Marine Systems.” 3 (2003): 215-20. Print.

[2] Rossetti, S., Tomei, Maria C., Nielsen, Per H., Tandoi, Valter. ""Microthrix parvicella", a filamentous bacterium causing bulking and foaming in activated sludge systems: a review of current knowledge." FEMS Microbiology Reviews. 1 (2005): 49-64. Online Journal.

[3] Muller, Emilie EL., Pinel, Nicolas., Gillece, John D., et al. "Genome Sequence of ""Microthrix parvicella" Bio17-1, a Long-Chain-Fatty-Acid-Accumulation Filamentous Actinobacterium from a Biological Wastewater Treatment Plant". Journal of Bacteriology. 12 (2012): 6670-6671. Online Journal.

[4] Marx, Chris., Schmidt, Matt., Flanagan, Joe., et al., "Introduction of Activated Sludge Study Guide." Wisconsin Department of Natural Resources, Wastewater Operator Certification. 12 (2010). http://dnr.wi.gov/regulations/opcert/documents/wwsgactsludgeintro.pdf. Online



Page authored by _Patrick Barney____, student of Prof. Katherine Mcmahon at University of Wisconsin - Madison.