Australasian Science Magazine

Tunnelling bacteria caused the premature failure of pine posts in a kiwi fruit orchard even though the posts had been pretreated with a highly toxic wood preservative used to protect them against wood-degrading fungi.

By Adya Singh & Thomas Nilsson

Bacteria that have evolved a clever way to feed by tunnelling within cell walls can thrive in pretreated wood in humid conditions, and may even have damaged new houses in Auckland.

Our common perception of bacteria is that they are simple organisms capable of performing only small tasks for their survival and well-being. However, it is beginning to emerge that bacteria have evolved more sophisticated strategies and can ingeniously interact with their living hosts and effectively talk to their own kind within colonies.

This sophistication also extends to nutrition, with the discovery that some bacteria have evolved an ingenious way of obtaining nutrients from wood. By tunnelling within the cell walls of wood, these bacteria are also sheltered from predatory amoebae and nematodes.

We made this discovery while investigating the premature failure of radiata pine posts in a kiwi fruit orchard even though the posts had been pretreated with copper–chrome–arsenate, a highly toxic wood preservative used to protect them against wood-degrading fungi. We found that an unusual type of microbial activity was the main reason for the decay of the posts. The decay pattern was different from what is typically caused by fungi.

A detailed examination combining light microscopy with high resolution transmission electron microscopy showed that the unusual form of decay was produced by bacteria that had penetrated into the cell walls of the wood, producing spectacular features that had the appearance of a branched network of tunnels containing periodic bands.

We still do not know which known taxonomic groups these bacteria belong to, as it is difficult to obtain them in a pure culture for conventional identification. Our naming of these bacteria as “tunnelling bacteria” is primarily based on the cell wall degradation pattern they produced.

Each bacterium produces a single tunnel as it moves on a slimy material within the cell wall (Figs 1, 2). The slime is secreted from the entire bacterial surface, so the bacterium is encased in a slime tube within the tunnel. This facilitates bacterial movement, the release of cell wall-degrading enzymes from the bacterial surface, and sequestration of cell wall degradation products (nutrients for the bacteria). When they move, the bacteria do not cross their neighbour’s paths, which suggests that tunnelling bacteria are constantly talking to one another to ensure a coordinated action.

Why Tunnelling?

Why do these bacteria tunnel within cell walls to obtain nutrition rather than attacking the exposed faces of the cell wall? Our hypothesis is that tunnelling bacteria have evolved unique strategies to escape predation. Amoebae and nematodes are common in natural environments such as soils, and can enter into heavily degraded regions of wood that have lost their integrity and are greatly loosened. However, amoebae and nematodes cannot penetrate into intact cell wall regions even if they are being tunnelled, as they do not produce the enzymes necessary to degrade intact wood cell walls. Thus, tunnelling bacteria can live, reproduce and feed on wood cell wall substances in a risk-free environment.

Another reason may have to do with a highly efficient use of the cell wall-degrading enzymes they produce by minimising the loss of these vitally important elements, which can occur when the cell wall is degraded via erosion. Eroding cell wall faces in natural environments are exposed to a watery medium, and the extracellular enzymes produced by the bacteria can diffuse into the water and become diluted.

Tunnelling bacteria can degrade all wood cell wall regions, including the middle lamella, which functions like a glue between wood cells and is rich in lignin, a polymer that is difficult to degrade. This maximises use of the substrate as these bacteria can readily move between neighbouring cells.

Indeed tunnelling bacteria can degrade wood substrates that are resistant to more rapid wood degraders like white-rot and brown-rot fungi. These substrates include wood treated to extremely high copper–chrome–arsenic retentions, wood species with high lignin content, and tropical heartwoods that are rich in toxic extractives.

Environmental and Ecological Significance

Since our initial discovery, reports of the occurrence of tunnelling bacteria in aquatic and other terrestrial environments have come from many parts of the world, including our own subsequent work in New Zealand and Sweden.

Wherever white-rot and brown-rot fungi are ineffective wood degraders, tunnelling bacteria are likely to play a major role in carbon recycling. Although the use of preservatives that are toxic to humans is being increasingly restricted in Australia and New Zealand, there may still be huge quantities of discarded copper–chrome–arsenic-treated timbers in landfill sites, and tunnelling bacteria can play an important role in degrading these timbers.

If it is proved that tunnelling bacteria can also break down the components of copper–chrome–arsenate, and therefore reduce their toxicity, the residues leached into aquatic environments from landfill sites will not be as damaging to aquatic organisms. The potential also exists to deploy these bacteria to clean up environments polluted with other toxic substances.

Decontaminated wood can also serve as a source of nano­-cellulose for high-value composite products. In nature, the degraded wood material can also serve as a source of shelter and nutrients for a wide variety of organisms, such as millipedes.

Economic, Historical and Cultural Importance

Ancient wooden constructions recovered from archaeological expeditions can reveal the culture of the period in which the objects were created. They can also inform us of climate conditions of the past through growth rings engraved annually during wood formation. However, unless wooden constructions are found in extreme conditions that are unfavourable to wood-destroying microorganisms, such as dry sand and the extremely low temperatures of the Arctic and Antarctic regions, they are generally degraded by fungi and bacteria over time and can be in a state of fragmentation.

Understanding the nature and state of degradation is helpful for conserving and restoring ancient wooden objects – often from hundreds of fragments – as reconstruction requires specific technologies to bring the objects to their near-original forms. Some familiar examples include the ancient wooden ships Mary Rose and Cog, which sunk in ocean waters and became buried in sea floor sediments. Microscopic examination of recovered pieces prior to restoration work suggests that wood cell wall degradation was mainly caused by bacteria. The images obtained gave fine detail about the integrity of wood tissues, and this formed the basis for developing and employing effective restoration technologies.

Bacteria and fungi can cause significant economic losses by destroying timbers in buildings and other constructions. Recent examples are leaky houses in Auckland built just over a decade ago. These were designed to give a Mediterranean look and were built from untreated or inadequately treated timber frames that were constantly exposed to humid conditions. This left the timber frames open to microbial attack, and they reached the point of collapse within a few years of construction. The replacement costs have been estimated to be billions of dollars.

Diagnostic work only investigated the role of fungal species because of the perception that fungi cause greater damage to wood than bacteria, and there was a lack of expertise in recognising bacterial decay by investigating authorities. However, it is likely that bacterial tunnelling was also present, as tunnelling bacteria often co-exist with soft-rot fungi in soils and other humid environments.

Future Directions for Research

The discovery of wood destroyed by bacteria that tunnel through cell walls is based on microscopic examination of wood samples obtained from natural environments, as well as those exposed to isolated mixed bacterial cultures in the laboratory. However, it has not been possible to identify their taxonomy with any degree of certainty due to difficulties isolating these bacteria in a pure culture. This is an area of future work that can benefit from the application of modern molecular biology to clearly understand how tunnelling bacteria carry out cell wall degradation and conversion into nutrients.

All bacteria, including these single-celled tunnelling bacteria, multiply and form a colony. It is well-known that bacteria behave responsibly within a colony and communicate with their own kind as well as with other types in mixed colonies in order to efficiently utilise substrates and determine the colony size. Probing into the nature of communication among tunnelling bacteria will be a rewarding challenge since wood cell walls are a nanoporous substrate with pore sizes measuring only 2–4 nm.

Recent investigations have shown that bacterial communication relies on the production and release of small chemical substances. This would mean that the extracellular substances that tunnelling bacteria produce for communication within wood cell walls will have to be smaller than 2 nm. Obtaining substrates that are degraded exclusively by tunnelling bacteria and then characterising substances that facilitate communication will be a huge challenge. But the success will bring not only scientific but also application benefits, as bacterial control strategy can be based on targeting communication substances.