Higher Microbial Population Research Articles

Microbial flora of some soils of Mawson Base and the Vestfold Hills, Antarctica

A microbiological survey of 40 soils from contaminated and pristine localities of Mawson Base or the Vestfold Hills (Davis Base) is described. High microbial populations (from 106 to in excess of 109 cells g-1 dry soil) were associated with soil contaminated by man or animals, soil from beneath a moss bed, or soil from beneath translucent quartz pebbles and containing green algae. Lower microbial populations (generally between 104–106 cells g-1 soil) were associated with pristine soils lacking an identifiable primary producer. No correlation was evident between population density and soil particle size, moisture content, pH or salinity as determined by the electrical conductivity (E.C.) of soil suspensions, although relationships were evident between the components of the microflora and E.C. Microbial groups associated with low E.C. were chitinolytic bacteria, fungi and green algae; favoured by high E.C. were halotolerant bacteria. The microflora of uncontaminated soils appeared to be dominated by Gram-negative aerobic rods (68% of the total flora) with Moraxella, Acinetobacter and Pseudomonas being the most frequently isolated genera. The remaining microflora comprised Grampositive pleomorphic bacteria (mainly coryneforms), 13%; filamentous Gram-negative aerobic rods (morphologically akin to Bacillus species), 12%; and yeasts, 7%. Streptomyces was identified among the dominant microflora of only one soil. Nitrogen-fixing bacteria were never isolated, although their presence was indicated in 40% of the soils tested by acetylene-reduction assay. Anaerobes were not detected in any soil using dilutionplate techniques, but were occasionally detected by enrichment in semi-solid media. All of 242 isolates obtained at 10° incubation were able to grow at 15°, 3.3% failed to grow at 18° and a further 9.5% failed to grow at 25°. Of 304 isolates obtained on one-tenth strength Trypticase-Soy agar (0.05% nutrient concentration), 10% failed to grow on the full-strength medium. Soils from the vicinity of Mawson or Davis bases contained a variety of fungi (Cladosporium, Chrysosporium, Acremonium, Phoma, Aureobasidium and Trichoderma species being isolated) in contrast to pristine soils, from which only Verticillium was occasionally isolated.

The effect of high microbial populations on the physical properties of kaolin slurries

Large populations of microorganisms were shown to have little or no effect on the physical properties of kaolin slurries. The physical properties that were measured were brightness, soluble salts, pH, and viscosity. This was found to be the case when autoclaved samples of kaolin slurries were inoculated with strains of Pseudomonas putida, an organism frequently encountered in kaolin slurries, and incubated for five weeks; or, when uninoculated samples of kaolin slurries, and incubated for five weeks; or, when uninoculated samples of kaolin slurries were incubated for as long as three months to allow the naturally occurring contaminating organisms to grow. In each case, from 10 5 to over 10 7 colony forming units ml −1 developed. In several instances, however, the viscosity increased slightly but within acceptable limits. When kaolin slurries were inoculated with massive numbers of microorganisms in their own spent media, the viscosity increased significantly over a period of five weeks in some, but not all, cases. Brightness, pH and soluble salts content, however, did not change. It was probable that microbial slime material in the spent media plus additional slime material formed owing to the introduction of nutrients from the spent media into the kaolin slurries contributed to the increase in viscosity. The increase in viscosity took place sufficiently slowly, however, that under normal, practical working circumstances the kaolin slurries would have been used before the increase in viscosity became apparent. Moreover, the numbers of microorganisms used in these experiments were more than an order of magnitude greater than that which would develop naturally in kaolin slurries not treated with preservatives. It was concluded from these experiments that under normal conditions high numbers of microorganisms would have little or no effect on the physical properties of kaolin.

’n Mikrobiologiese studie van ‘kaal kolle’ in die Giribesvlakte van Kaokoland, S.W.A.-Namibië

A microbiological study of ‘bare patches’ in the Giribes plains of Kaokoland, S.W.A.-Namibia. A study was made of the density of the bacteria and fungus populations in, on the edges of, and between ‘bare patches’ in the Giribes plain, Kaokoland. The density of the microbial populations showed, with the exception of anaerobic bacteria, a close relation to the density of the higher plants of these soils. The soil samples of the bare patches had the lowest microbial population level while the edge of the patches had the highest microbial population. Sixty three species of fungi belonging to 37 genera in total were isolated from these three soil types. The nature of the fungus population closely resembles those of other dry areas. Aspergillus and Penicillium species were generally the most numerous while many yeasts and dark coloured Dematiaceae also occured quite commonly. S. Afr. J. Bot . 1982, 1: 69-74

Investigations into rhizosphere microflora

Effect of light and dark treatments on rhizosphere and rhizoplane microflora of two crops,viz Triticum aestivum. Linn. andHordeum vulgare Linn. has been investigated. Higher microbial population was recorded from the rhizosphere of plants exposed to continuous light while continuous dark set harboured lesser microflora. The microflora in 12 hours alternate light and dark set exhibited an intermediate condition. Amino acids (both qualitatively and quantitatively) in root extracts were the least in continuous dark and the maximum in continuous light set. The variation in root leakage, physiological and morphological condition of the plants was thought to be the cause for change in microbial activity.

Alcohol content as an index of quality for prepeeled potatoes

Samples of French-Fry sulphited potato cuts were collected from a commercial prepeeler and analyzed for alcohol content and microbial population. Samples prepared in a laboratory were inoculated with a high and a medium level of microorganisms produced by mixing cultures of forty different organisms isolated from spoiled prepeeled potatoes. The potato samples with high microbial populations showed high alcohol contents indicating that the microorganisms contributed both directly by forming alcohol as a metabolic by-product and indirectly by lowering the oxygen level within the packages thereby promoting anaerobic respiration in the potato tissue.

Excretion of Thiamin and Biotin by the Roots of Higher Plants

THE fact that soil in the immediate vicinity of plant roots supports a much higher microbial population than that existing outside the plant's zone of influence has been recognized for many years. Although it has been suggested that ‘root excretions’ account for this effect, these hypothetical exudates have never been identified, nor has their mode of action been satisfactorily explained. In studying the relation of plants to soil-borne diseases, it was considered desirable, therefore to examine the possibility that excretion of minute amounts of bacterial growth factors by the growing plant might be a fundamental factor in determining the character of the rhizosphere microflora.