Compost Magazine

Composting tips, advice and science.

Digital art image of thermophilic bacteria.

The Mind-Boggling Role of Bacteria In Compost

Have you ever wondered what does the heavy lifting in your compost heap?

The answer is a single-celled microscopic organism that comes in a variety of shapes and sizes and can be found living in virtually every ecosystem on Earth – bacteria.

These microscopic microorganisms are truly tiny (about 500 million times smaller than the volume of a grain of sand) – but they make up for the small size in abundance. In fact, just a single teaspoon of compost can contain one billion bacteria.

Different types of bacteria have different functions – some produce antibiotics, some are beneficial for human health, while others can cause disease when we’re exposed to too much of them. 

And, of course, they can turn waste organic material into compost. 

What is bacteria’s role in compost?

Cartoon image of bacteria in compost.

The simple answer

Bacteria break down organic material into compost. Their biggest role takes place at the start of the process when sugars and carbohydrates are easily available.

As these get consumed, fungus and secondary decomposers start to play a larger role. Bacteria are still important, though, and different types of bacteria, which can break down tougher material, get involved. 

The more complex answer

The process through which bacteria (along with other microorganisms) turn organic material into compost can be broken down in several areas. 

Decomposition: Bacteria (helped by) fungi break down complex proteins, carbohydrates, and fats into simpler molecules like carbon dioxide, water, and minerals. It does this by releasing a number of different enzymes with different functions. For example, one enzyme has the ability to modify lignin

Mineralization: As bacteria consume organic materials, they convert organic compounds into inorganic compounds. These include nitrogen, phosphorus and potassium, all of which are essential for plant growth. 

Humification: Bacteria (along with other microorganisms) also play a role in creating humus in compost. (Humus is a dark matter found in compost that is thought to improve soil structure. However, neither humus nor the process with which it is created is well understood yet.) 

Different types of bacteria play different roles in this process. For example, nitrifying bacteria are responsible for converting ammonia into a form of nitrate that is available to plants. 

Not every process is beneficial for plants! Bacteria, along with fungi, also convert nitrates into gases such as nitrogen oxide or nitrous oxide, which means they are unavailable to microorganisms in the soil and the plants those microorganisms feed. 

Bacteria also play other roles that are helpful to gardeners. For example, thermophilic bacteria help generate heat which kills weeds seeds and pathogens. As we shall see, some bacteria also create antibiotics that can attack pathogens.

Aerobic (with air) bacteria also help to reduce the smell associated with compost piles by breaking down organic materials into simpler compounds that do not produce unpleasant smells. However, when oxygen levels are low, anaerobic (without air) bacteria take over and cause more smells to be released.

Bacteria at different stages of composting

Bacteria are broken down into different types based on the temperature they are most active at. Do note this is a broad-stroke approach, as different bacteria within these categories will be active at different temperatures. 

Mini-infographic shows the different temperatures at which phsychrophilic, mesophilic and thermophilic bacteria thrive. Licenced for re-use under licensed under a Creative Commons Attribution 4.0 International License.

Psychrophilic bacteria

Psychrophilic bacteria can be found in cold environments such as polar regions, permanently frozen tundra, glaciers, and oceans. 

These cold-loving microorganisms can survive at incredibly low temperatures – with Margesin and Vita stating that viable bacteria have been found at temperatures as low as -100 celsius. That’s thanks to some unique properties, which include antifreeze proteins and an ability to produce enzymes at lower temperatures. 

There’s been limited research into the composting potential of the microbes. Hou et al found psychrophiles can play a key role in composting, but other research emphasizes the greater potential of thermophiles. That matches the experience of most gardeners, with the composting process generally slowing down in winter. 

They’re still highly useful for the composter, as it means some composting can take place either in cooler conditions and in less well-insulated piles of compost. 

Mesophilic bacteria

The first stage of composting usually involves Mesophilic bacteria. These organisms do best in temperatures ranging from 20 to 40 degrees Celsius (68 to 104 F).

Mesophiles can be divided into two categories: aerobic (which requires oxygen) and anaerobic (which doesn’t need oxygen). In an ideal composting environment, anaerobic bacteria thrive. However, as they oxidize elements (especially carbon) they generate heat. In a large or insulated pile, the heat they generate is trapped, which may kill many or lead them to become dormant – and create ideal conditions for our next class of bacteria.

Some mesophiles do reappear as the compost cools down where, as we will see, they are joined by other decomposers. 

Thermophilic bacteria

Thermophiles were of great concern to scientists in the 1930s, as they were worried they weren’t killed by heat in processes like pasteurization. There’s a good reason for this – thermophiles love heat.

Some thermophiles can be found in hot springs, geothermal soils and hydrothermal vents. We don’t know exactly the hottest conditions they can survive in, but scientists have speculated it could be as high as 200°C. 

However, thermophilic bacteria can be found in cooler conditions too. They’ve been found in the sea, seven thousand feet underground and in mountains twelve thousand feet above sea level. 

Thermophiles Ability to Survive

Ernest Shackleton standing in snow with two fellow explorers in front of a sledge.

Thermophiles are extremely tough microorganisms. In fact, some thermophiles are also extremophiles – an organism that is able to survive under extreme conditions. 

This ability to survive is illustrated by an analysis of powdered milk found at a hut used by Ernest Shackleton on his expedition to Antarctica in 1907. More than 90 years after it had been left there in the bitterest of conditions, two strains of thermophilic bacteria still survived. 

That sounds impressive – but is nothing compared to bacteria which, having been trapped for millennia, are now being released from melting permafrost

Image credit: Wiki Commons

Thermophiles, or at least certain varieties, are able to consume sulphur and hydrogen and fix carbon from air. 

In a compost heap, thermophiles continue to digest compost material, with a specific focus on proteins and fats. Specific types of thermophiles will work on more resistant materials, such as cellulose and pectin. 

The thermophile stage provides the highest demand for oxygen. Most thermophilic bacteria will eventually die as oxygen is eliminated – unless sufficient oxygen is provided via turning, forced aeration or other methods. If there is sufficient air, thermophilic bacteria will continue to generate heat until readily available sources of nutrition have been exhausted, 


As thermophilic bacteria die or become dormant, the composting process is continued by fungi – and another form of bacteria called Actinomycetes. 

Although a form of Mesophilic bacteria, Actinomycetes resemble, and to some extent act like fungi. Taking up to 5% of the compost, they send out long filaments which can resemble spider webs and help to break down tougher materials (such as lignin) that have been resistant to other bacteria. In the process, they release carbon, nitrogen and ammonia, making them available for plants.

These bacteria are not just useful for soil – more than half the antibiotics we use come from Actinomycetes!  (The antibiotics they release are another reason other bacteria start to die off in the maturing process.)

How can you encourage (the right) compost bacteria?

Woman turns compost in front of a compost pile.

Whatever you do, there will be bacteria in your compost heap (and everywhere else, for that matter!) However, to maximize the numbers of the bacteria that provide the fastest decomposition, it’s with taking the following steps: 

  • Break compost materials up to maximize surface area
  • Ensure the compost heap is neither too dry nor too wet
  • Add a mixture of brown and green degradable materials
  • Ensure there is sufficient air in the compost heap
  • Ensure the pile is big enough or insulated enough to provide heat
  • Ensure the pH level is between 6 and 8 (although this is not usually a problem the amateur composter needs to worry about)

After your compost heap has cooled down, you can help ​​actinomycetes do their job by ceasing to turn the compost. 

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Compost Bacteria Mini Infographic by Compost Magazine is licensed under CC BY-SA 4.0