Laboratory-dependent bacterial ecology: a cautionary tale.

Although laboratory dependence is an acknowledged problem in microbiology, it is seldom intensively studied or discussed. We demonstrate that laboratory dependence is real and quantifiable even in the popular model Escherichia coli. Here laboratory effects alter the equilibrium composition of a simple community composed of two strains of E. coli. Our data rule out changes in the bacterial strains, chemical batches, and human handling but implicate differences in growth medium, especially the water component.

Desorption kinetics of Cd, Zn, and Ni measured in soils by DGT.

DGT (diffusive gradients in thin films) was used to measure the distribution and rates of exchange of Zn, Cd, and Ni between solid phase and solution in five different soils. Soil texture ranged from sandy loam to clay, pH ranged from 4.9 to 7.1, and organic carbon content ranged from 0.8% to 5.8%. DGT devices continuously remove metal to a Chelex gel layer after passage through a well-defined diffusion layer. The magnitude of the induced remobilization flux from the solid phase is related to the pool size of labile metal and the exchange kinetics between dissolved and sorbed metal. DGT devices were deployed over a series of times (4 h to 3 weeks), and the DIFS model (DGT induced fluxes in soils) was used to derive distribution coefficients for labile metal (Kdl) and the rate at which the soil system can supply metal from solid phase to solution, expressed as a response time. Response times for Zn and Cd were short generally (<8 min). They were so short in some soils (<1 min) that no distinction could be made between supply of metal being controlled by diffusion or the rate of release. Generally longer response times for Ni (5-20 min) were consistent with its slow desorption. The major factor influencing Kdl for Zn and Cd was pH, but association with humic substances in the solid phase also appeared to be important. The systematic decline, with increasing pH, in both the pool size of Ni available to the DGT device and the rate constant for its release is consistent with a part of the soil Ni pool being unavailable within a time scale of 1-20 min. This kinetic limitation is likely to limit the availability of Ni to plants.

Determination of chromium speciation in natural systems using DGT.

The techniques of diffusive gradients (DGT) and equilibration (DET) in thin-films have been combined in a single probe that can determine Cr(III) and Cr(VI) simultaneously in solution. The assembly has a layer of polyacrylamide hydrogel overlying a separate layer of resin embedded in gel. Cr(III) species accumulate exclusively and quantitatively in the resin layer, while Cr(VI) species equilibrate with both hydrogel and resin layers. The species are separated by peeling the two layers apart. Chromium is then eluted from each of the two layers. Cr(III) and Cr(VI) were determined quantitatively in standard, mixed solutions by in situ separation with DGT and detection by GF-AAS. With this method, Cr(III) is typically preconcentrated by a factor of 10 over a 24 h deployment, and limits of detection of 8 ng/L Cr(III) and 0.3 micro g/L Cr(VI) were achieved. Due to the inbuilt preconcentration of Cr(III), the technique is particularly good at measuring low concentrations of Cr(III) in the presence of an excess of Cr(VI). Measurements were performed in three soils with various levels of chromium contamination. A concentration of 3 micro g/L of labile Cr(III) was measured reproducibly in the presence of 290 micro g/L of unreactive Cr species and 0.2 micro g/L of labile Cr(III) was measured in the presence of 24 micro g/L of unreactive Cr. The unique feature of the method is that the separation of Cr(III) from Cr(VI) occurs in situ. The Cr species are then stable in the resin and gel prior to analysis, eliminating the artefacts associated with sampling and storage, which are particularly prevalent for redox-sensitive elements. Therefore, it has great potential for assessing Cr(III) and Cr(VI) concentrations in situ in environments near redox boundaries where possible dynamic changes in Cr(III) and Cr(VI) concentrations are occurring.

Experiments supporting the use of ambient aerosols for quantitative respirator fit testing.

Most methods for testing facial seal leakage on subjects undergoing respirator fit tests involve comparing the generated aerosol particulate concentration inside the subject's respirator to the concentration in a test chamber. These aerosols are produced by fogging substances such as corn oil, dioctyl sebacate, or dioctyl phthalate (DOP) into the test chamber. The health effects of these substances and of their aerosols on respiratory systems are uncertain. The proposed alternate method uses as a test medium ambient particles which exist in most room atmospheres. The proposed method eliminates the need for a test chamber and for an intentionally produced aerosol. The subject is tested for respirator inleakage by comparing the particulate count concentration inside the subject's respirator to that of the room atmosphere outside the respirator. This method is less expensive and simpler to administer than the use of oil or other deliberately produced aerosols because it uses an existing ambient test medium. Statistical analysis of the test data indicates favorable comparison with the conventional chamber-aerosol method.