274 Chapter 11
2.3 Erosion Bacteria
Erosion bacteria are considered the dominant microbial degraders of archaeo-
logical, waterlogged wood. It is now accepted that waterlogged wood main-
tained under conditions of minimal oxygen availability is decayed primarily
by erosion bacteria although some attack by soft rot fungi and tunnelling bac-
teria can be found in these timbers. It has been suggested that the very low
oxygen conditions are periodically alleviated for this to occur.
Decay by erosion bacteria has been reported in different wood species result-
ing in the utilisation of wood polysaccharides; the extent of lignin modification
is not known. This type of degradation is characterised by the formation of ero-
sion troughs or grooves on the lumen surface of wood cell walls (Figure 7) and
is most effectively visualised using scanning electron microscopy (SEM). The
eroded areas are associated with single-celled bacteria that are typically rod
shaped and decay is progressive from the lumen surface towards the compound
middle lamella of the cell wall. Initially, a population of erosion bacteria can be
observed on the lumen surface, sometimes orientated in a parallel fashion fol-
lowing the alignment of the cellulose microfibrils. Grooves excavated in the S 3
layer of the wall by individual bacterial cells are more or less the same size as
the cells, indicative of restricted enzyme activity around each bacterium.
Attachment to the lumen surface is through the formation of a glycocalyx
and as progressive breakdown occurs, the secondary cell wall is converted
Figure 7An illustration showing microbial attack of wood
(After Singh and Kim, 1997)