into amorphous material mixed with bacterial cells and bacterial slime. It can
therefore be concluded that extracellular mucilage is important in bacterial
cell adhesion, motility and enzymatic degradation of the cell wall. In soft-
wood timbers, attack of the cell wall by erosion bacteria can also result in the
formation of angular cavities in the S 2 layer and this is probably caused by
interaction between the orientation of cellulose microfibrils and extracellular
bacterial enzymes. So far, identification of erosion bacteria from field speci-
mens has not been successfully accomplished. A greater understanding of
their physiological requirements in laboratory culture and the optimum con-
ditions for promoting adhesion and decay at cell wall surfaces is necessary
for this to be achieved.
2.4 Tunnelling Bacteria
Decay of wood cell walls by tunnelling bacteria is found in wood exposed to
the sea. This type of decay was also recorded in saturated wood exposed in
the warm water of industrial cooling towers. In both situations, oxygen is
limiting, but does not approach the near-anaerobic conditions of burial in silt
and sediments. The presence of tunnelling bacteria in wood in soil contact is
indicative of the ubiquitous nature of these organisms and their ability to
decay preservative-treated wood and durable heartwood is further evidence
of the aggressive capabilities of these wood-decay organisms.
Tunnelling bacteria remove the polysaccharide components of wood cell
walls and some of the lignin. The appearance of the decayed wood is soft and
darkened. Longitudinal sections of the decayed wood viewed with a transmis-
sion light microscope, display regions of granulation in softwood tracheids.
Fine tunnels are found at the margin of these regions and with appropriate stain-
ing and good microscope optics, single-celled bacteria can be seen at the ends
of the tunnel. The granulation zones develop from a loose, irregular network of
fine tunnels following bacterial penetration into the wood cell wall (Figure 8).
Under polarised light, the zones of granulation show loss of birefringence con-
firmingthe breakdown of the crystalline cellulose in these infected areas. But
not all examples of tunnelling bacterial decay show exactly the same pattern of
attack. In some cases the tunnels appear wider, are fewer in number and appear
to radiate more precisely from a central point producing an ‘ice-fern’ pattern.
Decay patterns associated with tunnelling bacteria are not uniform and dif-
ferences in their appearance may be influenced by environmental factors, wood
species or indeed by the bacteria themselves, since to date none have been
identified.
The penetration of these bacteria into wood cell walls is initiated by adhesion
to the lumen surface via an extracellular glycocalyx. The bacterium lyses the S 3
layer and is then able to tunnel in the S 2 and S 1 layers. The direction of tunnelling
Conservation of Ancient Timbers from the Sea 275