Decay micromorphology was studied systematically for diversely preservative treated Pinus radiata and Fagus sylvatica 20 x 20 x 500 mm stakes across 13 in-ground field test sites, during a 6.5 year exposure. Sites were selected to maximise occurrence of a diverse range of decay types. Micromorphology that suggested orientation of tunnelling bacteria with the cellulose microfibrils of the S2 and S1 cell wall layers was common, an observation that is not consistent with the bulk of the information contained in the literature. This suggested that contrary to current theory, tunnelling bacteria are profoundly affected by the fine ultrastructure of wood (e.g. microfibrils, lamellae and layer infaces). Departures from profound wood cell wall ultrastructure effects on micromorphology, such as apparently random tunnelling, are probably a response to an exogenous stimulus i.e. other than degradative biochemistry and wood ultrastructure. For tunnelling bacteria such a third stimulus might include the putative avoidance mechanism reported by other workers in the field. In addition, it is to be expected that micromorphological patterns produced by a unicellular microorganism would be affected by regular cell division, this being absent for filamentous fungi. Therefore, the often reported micromorphology patterns for tunnelling bacteria might primarily be due to regular cell division coupled with avoidance, two events that would cause a departure from the patterns caused by cell wall ultrastructure. A putative bacterial penetration mechanism purported that a bacterium undergoes rhythmic apical elongation and subsequent distal shortening. Apical extension, elongation and narrowing of the bacterium occurs until the finite volume of the bacterium causes the distal region to break away from its temporarily fixed location, leaving behind a cross wall. This mechanism seems more likely than the previously reported formation of cross walls following complete immobility of the whole bacterium, a behaviour that does not fit with classical understanding of vegetative bacteria. The new hypothesis presented also fits with the enormous plasticity exhibited by tunnelling bacteria during cell wall penetration as shown in TEM micrographs i.e. it is likely that this ability would be further exploited during ongoing formation of tunnels, along paths of minium resistance where possible.

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