Degradation of lignocellulosics by unique tunnel-forming bacteria

Abstract

Transmission electron microscopic observations on a wide range of decaying wood samples obtained from both field situations and laboratory exposure tests have confirmed bacteria to have a capacity to degrade intact highly lignified substrates including preservative-treated and naturally durable woody tissues. Studies have shown a number of bacterial forms to be involved and have provided morphological evidence for in situ lignin degradation confirming 14C-labelled experiments with synthetic and natural lignins. A unique type of bacterial attack (tunnelling) characterized by the development of tunnels containing peculiar cross-tunnel wall secretions has been recognized. Cytochemical studies have shown the extracellular tunnel secretions to contain negatively charged constituents, while transmission electron microscopic energy-dispersive X-ray spectroscopy has shown these tunnels to bind heavy metals during decay of timbers treated with metal-containing preservatives. The tunnelling bacteria are motile nonflagellated Gram-negative rods. These bacteria have highly plastic cell envelopes which can produce vesicles. Our studies provide evidence that tunnelling bacteria can remove total cell wall material, including lignin, and show a relationship between their motility and the unique tunnels they produce.