The abundance and viability of microorganisms in groundwater were studied using a scientific borehole in the Tono uranium deposit area, central Japan. Groundwater samples were collected from scientific borehole TH-6 at four depths; 104, 132 and 153 m in sedimentary rocks and 177 m in granite rock using autoclaved geochemical water samplers. The total cell count using epifluorescence microscopy was of the order of 10⁵ to 10⁶ cells ml^-1, showing non-systematic change in microbial parameters with depth. Viability estimated from cell membrane integrity ranged from 22.3% to almost 100%, and showed a tendency to increase with depth. In contrast, viability determined using both activities of the electron transport system (ETS) and esterase showed inconsistent depth-profiles. The ETS-active cell count corresponded to 0.57-24.7% of the total count; the minimum ETS-count was found at 153 m, just above the sediment-granite unconformity where the minimum redox potential was estimated. The esterase-active cell count corresponded to 0.4-10.3% of the total, with a maximum at 132 m, the lignite-derived organic-rich layer, and a minimum was found at 153 m, as seen for the ETS-active counts. These inconsistent profiles suggest a difference in microbial viability and activity between each depth. In addition, depth-specific microflorae may develop in response to the availability of various electron acceptors and donors in the subsurface.

View full abstract

Pseudomonas aeruginosa, Pseudomonas fluorescens, and Burkholderia cepacia were able to grow in a phosphorus-poor (0.01 mM of phosphate) synthetic medium in chemostat cultures. In contrast to bacteria grown in a phosphorus-rich (10 mM of phosphate) chemostat, they underwent an extensive changes in lipid composition, with phospholipids being replaced with ornithine-containing lipids. Nevertheless, their susceptibility to twelve antibiotics, and seven germicides changed little. No special tendencies in drug susceptibility related to the replacement of membrane lipids were recognized. Thus, though growing in a phosphorus-poor environment, these bacteria did not exhibit any special resistance to antibacterial agents. With regard to susceptibility to low pH, P. aeruginosa grown under phosphorus-poor conditions demonstrated greater resistance.

View full abstract

We investigated the horizontal transfer of nodulation (nod) genes to a Bradyrhizobium elkanii strain, lacking common nod genes as a recipient, in soils and microcosms using selection systems of antibiotic resistance and legume nodulation. We observed the horizontal transfer of nod genes at 4C in Nakazawa soil where peculiar strains (HRS strains) of B. japonicum harboring high copy numbers of insertion sequences dominated. In microcosms containing HRS strains as donors, we detected a similar horizontal transfer from B. japonicum HRS strain NK5 to the B. elkanii recipient more efficiently at 4C, which was verified by examining hybridization, nodulation and Nod factor production. These traits were, however, gradually lost during successive cultures. Plasmid analysis indicated that this event was not due to the simple transfer of plasmid carrying common nod genes. These results suggest the potential for horizontal transfer of nod genes among bradyrhizobia and other bacterial populations in soil environments.

View full abstract

Effects of inorganic elements on zoospore release, zoospore germination, mycelial growth and sporangium formation of Pythium aphanidermatum were investigated. The stage of zoospore release was highly influenced by many kinds of inorganic elements. The effective concentration as evaluated from 50% inhibition of zoospore release was 0.16, 0.19, 0.36, 0.4, 0.5, 2.5, 30, 168, 171 and 470 mg/L for Cu, Ag, Cr, Zn, iodine, Ni, Fe, Ca, Mg and K, respectively. Silver prominently inhibited both zoospore release and germination. Also, iodine showed inhibitory effects at every growth stage examined. The results indicated that several inorganic elements have the potential to suppress Pythium root rot disease in hydroponics, although further investigation is required to evaluate plants and conditions suitable for the method.

View full abstract

Acidophilic chemoorganotrophic bacteria of the genus Acidocella, Acidiphilium, and Acidobacterium were tolerant to concentrations of monomeric aluminum ion (Al³⁺) of up to 100 mM in an acid medium of pH 3.5 or less. Furthermore, their growth was remarkably enhanced by the presence of aluminum. The maximum amount of Al absorbed was 2.02 mg·g dry cell wt^-1 for Acidocella facilis. Aluminum phosphate as well as aluminum sulfate among aluminum chemicals tested provided for the most luxuriant growth of this bacterium, whereas aluminum acetate and aluminum lactate had inhibitory effects on growth. The enhanced growth seemed to be caused by a chemical buffering action of the aluminum, but could not be explained merely by an alleviation of H⁺ stress or by the scavenging of growth-inhibitory organic acids excreted. The reasons why the acidophiles are genuinely aluminophilic as well as aluminotolerant still remain to be elucidated. Considering that monomeric Al³⁺ is dominant under strongly acid conditions, the possible cultivation of acidophiles at a pH below 4 affords a useful in vitro model system to probe the aluminum toxicity of plants as well as aluminum-microorganism interactions.

View full abstract

The microbial degradation of electronic insulation polyimides was monitored and evaluated using electrochemical impedance spectroscopy (EIS). The microbial inoculum, comprising Aspergillus versicolor, Chaetomium species, Cladosporium cladosporioides, and Tricoderma viride, was isolated previously from deteriorated polyimides. After inoculation, fungal growth on the polyimides resulted in distinctive EIS spectra indicative of polymer insulation failure, which directly related to polymer integrity. Degradation appeared to progress in a number of steps and two distinctive stages in the decline of film resistance were detected in the EIS cells within 24 and 72 days after inoculation. The early stage of the decrease in resistance may be related to the ingress of water molecules and ionic species into the polymeric materials, while the second stage probably resulted from partial degradation of the polymers by fungal growth on the polymer film. The relationship between changes of impedance spectra and microbial degradation of the polymer was further supported by scanning electron microscopy (SEM) of fungi growing on the surface of the inoculated polyimides. Our data indicate that the EIS technique can be used to detect the early degradation of resistant polymers and that polyimides are susceptible to biodegradation by fungi.

View full abstract