Studies on the microbiology of silicon.

A study was made of the interactions between the element silicon, mainly as
silicic acid, and various microbial processes.
The effect of silicon compounds on fungal growth was determined under both
oligotrophic and nutrient-rich (copiotrophic) conditions. Mycelium of Aspergillus
oryzae was grown from a spore inoculum added to ultra-pure water (upw) containing
silicon compounds, but not in upw alone. Growth of other fungi also only occurred in
upw when silicon compounds were added. Increased growth of fungi also followed
the addition of silicon compounds to Czapek Dox medium. Silicic acid also increased
the protein content of fungi grown under such nutrient-rich conditions. The fungi
solubilised the insoluble silicon compounds under both oligotrophic and copiotrophic
conditions. Silicon was not however, accumulated by fungi as electron-dense hyphal
bodies. Addition of silicic acid to nutrient rich media also increased the growth of
species of Streptomyces but decreased the chlorophyll content of the alga, Dunaliella
parva; the growth of two yeasts and the bacteria, E. colt and S. aureus also was not
affected by silicon addition; the observed stimulatory effect therefore appears to be
restricted to filamentous microorganisms.
The effect of silicon compounds on various microbial processes was also
investigated. Silicic acid stimulated the production of citric acid by Aspergillus niger,
but decreased nitrification and sulphur oxidation in this fungus. Silicic acid addition
also led to a reduction in antibiotic production by species of Streptomyces. Studies were initiated to study the possibility that fungi and bacteria can erode the
surface of both bulk and porous silicon wafers. While no such surface erosion was
evident, we observed that E. coif underwent extensive extreme pleomorphism when
growing under starvation conditions for up to 14 days. Such pleomorphism consisted
of the formation of bulbous protrusions from the normal rod, dumbbell-shaped cells
and long filaments, these were up to 50g in length (compared to the normal 1-3μ,
rods). Such filamentation was clearly caused by the inability of the bacterial cells
(rods) to separate on division. The observed bacterial pleomorphism was not
however, silicon-specific, as it was also found to occur on titanium and glass surfaces.
Such extreme pleomorphism may have important implications in relation to the
growth of E. coli in low nutrient environments and may influence the bacterium's
ability to affect pathogenesis.
While the microbiology of silicon has largely been neglected the results of this thesis
show that there is considerable interaction between this element and microbial growth.
Future studies should in particular be directed towards determining if silicon can be
used as an energy source by microorganisms. Additionally, the observed phenomenon
of extreme pleomorphism in E. coil is clearly worthy of further study.