Helping us Pass the Sniff Test for Composting – the Amazing Actinobacter

The Sniff Test helps us produce great compost with minimal negative effects for our communities. If we get the composting process right, we allow the Actinobacter to flourish. Actinobacter produce geosmin, a compound that we can smell in the parts per trillion which gives the compost a pleasant earthy odour (Gerber and Lechevalier 1965).

“Freshly plowed soil has a typical odor which was undoubtedly detected even by primeval men and extolled in all tongues by bucolic poets” (Gerber and Lechevalier 1965)

The flourishing of Actinobacter during composting has been termed as a sign of composting success (Arnold 2011). Who are the Actinobacter and why should we be amazed by them? Actinobacter are a group of filamentous fungi like bacteria that used to be called Actinomycetes.

“Actinobacter are Gram-positive bacteria that constitute one of the largest bacterial phyla, and they are ubiquitously distributed in both aquatic and terrestrial ecosystems. Many Actinobacteria have a mycelial lifestyle. They have an extensive secondary metabolism and produce about two-thirds of all naturally derived antibiotics in current clinical use, as well as many anticancer, anthelmintic and antifungal compounds. Consequently, these bacteria are of major importance for biotechnology, medicine and agriculture” (Barka et al. 2016.)

Some Actinobacter thrive at thermophilic composting temperatures in the 55-65 C range and are important for decomposition of lignin and celluloses, as well as killing potential pathogens.

“Thermophilic actinobacteria thrive at relatively high temperatures ranging from 40-80 C. These are of two types: strictly thermophilic and moderately thermophilic actinobacteria. The former can grow in the temperature range between 37 and 65 C, but optimum proliferation takes place at 55-60 C. While moderately thermophilic actinobacteria thrive at 28-60 C and require 45-55 C for optimum growth, another group known as thermotolerant actinobacteria can survive at temperatures up to 50 C.” (Shivlata and Satyanarayana 2015).

Many of us, including myself, were aware and taught that Actinobacter were important in the composting process, but they were not very active until later in the active composting sate and during curing (Environment Canada 2013). I stand corrected, and now join others who had already figured out that Actinobacter are very important, even in the primary thermophilic phase of the composting process. I learned by experience in our small scale composter, where the white Actinobacter were obvious even after one week of composting food waste at high temperatures! I also learned that when the composting process is going well, we can smell that earthy smell even after one week.

Actinobacter growth in 50% food waste compost after one week of composting - adequate aeration and temperature management is the key.

Actinobacter growth in 50% food waste compost after one week of composting – adequate aeration and temperature management is the key.

Actinobacter were identified as important primary decomposers during the thermophilic composting of sewage sludge within the first few days of composting (Nakasaki et al. 1985). They observed that the Actinobacter did not grow at temperatures above 70 C.

During composting of municipal organics in Sweden, Actinobacter comprised less than 10% of the microbial population at a full-scale composting plant, whereas earlier observations in a pilot study indicated that Actinobacter constituted 50% of the microbial population during composting (Steger et al. 2007). In further work in Finland, it was noted that the presence of Actinobacter in the thermophilic stage indicated a fast, well-aerated composting process, whereas Clostridium spp (producers of bad odor) indicated an oxygen limiting environment even at high temperature and high pH (Partenan et al. 2010).

We can conclude that there is at least a double benefit to encouraging Actinobacter to flourish at our composting facilities:

  1. Actinobacter produce geosmin, which has a more positive and earthy odour, rather than the disagreeable odours that may of us are familiar with at compost facilities and
  2. Actinobacter produce compounds known to discourage pathogens, not only in the composting process, but also plant pathogens when the compost is used for crops.

Its all about the Sniff Test! With good design of our compost facilities and good management of the composting process, we can do it!


Arnold, P. 2011. Actinomycetes: The Sign of Composting Success. Compost Council of Canada. Atlantic Regional Workshop, Halifax , NS. March 15, 2011

Barka, E.A., P. Vatsa, L. Sanchez, N. Gaveau-Vaillant, C. Jacquard, H-P. Klenk, C. Clement, Y. Ouhdouch and G.P. van Wezel. 2016.  Taxonomy, physiology, and natural products of Actinobacteria. Microbiol. Mol. Biol. Rev 80: 1-43 doi:10.1128/MMBR00019-15

Environment Canada. 2013. Technical Document on Municipal Organics Processing. ISBN: 978-1-100-21707-9

Gerber, N.N. and H.A. Lechevalier. 1965. Geosmin, an Earthy-Smelling Substance Isolated from Actinomycetes. Applied Microbiology: 13: 935-938.

Nakasaki, K. M. Sasaki, M. Shoda and H. Kubota. 1985. Effect of temperature on composting of sewage sludge. Applied and Environ. Microbiol: 50: 1526-1530.

Partanen, P. J. Hultman, L. Paulin, P Auvinen and M. Romantschuk. 2010. Bacterial diversity at different stages of the composting process. BMC Microbiology 2010. 10:94

Shivlata, L. and T. Satyanarayana. 2015. Thermophilic and alkaliphilic Actinobacteria: biology and potential applications. Frontiers in Microbiology doi:10.3389/fmicb.2015.01014

Steger, K., A.M. Sjogren, A Jarvis, J.K. Jansson and I. Sundh. 2007. Development of compost maturity and Actinobacteria populations during full-scale composting of organic household waste. J. Applied Microbiol: ISSN 1364-5072 doi:10.1111/j.1365-2672.2006.03271.x

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