Evaluation of centrifugal ultrafilters for size fractionation of total mercury and methylmercury in freshwaters

Environmental context Although mercury associated with colloids is an important part of the aquatic Hg cycle, there is currently no fast and reliable method to separate complexes smaller than traditional filter pore sizes. We test commercially available centrifugal ultrafilters for their applicability to size fractionation of total Hg and methylmercury in freshwaters. Sorption of Hg onto the filters precludes their use for fractionation of inorganic Hg, the approach proved to be very suitable for methylmercury fractionation regardless of sample organic matter content. Abstract Amicon Ultra-15 centrifugal filters with nominal molecular weight cut-offs of 100, 30 and 3kDa, were tested for separating Hg complexes in freshwaters. Experiments used Hg-contaminated water from East Fork Poplar Creek (EFPC) and laboratory-prepared Hg solutions containing Suwannee River natural organic matter (SR-NOM). Investigations focussed on Hg and dissolved organic carbon blank levels, Hg sorption and leaching, Hg mass balance closure and spike recoveries of inorganic and methylmercury (MeHg). Hg spike recoveries for EFPC samples were low (57±16%, n=30) due to sorption. MeHg recovery averaged 87±9% (n=15) suggesting it was less affected by sorptive losses. SR-NOM samples yielded similar dissolved organic matter (DOM) and MeHg size fractionation patterns with ~20% of the MeHg found in the less than 3-kDa fraction. Overall, the distribution of MeHg followed a pattern similar to the DOM, indicating the importance of both sample DOM quantity and quality for MeHg partitioning in aquatic systems. Although the use of these ultrafilters for inorganic Hg in freshwater samples is not recommended, they were successfully used for MeHg in EFPC where the majority of MeHg was found to be either dissolved or associated with phases smaller than 3kDa.

Repressive effects of yeast extract on photoreactivation of Escherichia coli

Photoreactivation of Escherichia coli K12 (IFO 3301), in the presence or absence of yeast extract (YE), was investigated after inactivation by low-pressure UV lamp. An endonuclease sensitive site (ESS) assay was used to determine the UV-induced pyrimidine dimers in the genome of E. coli, while a colony-forming ability (CFA) test was also used to examine the survival ratio of E. coli. The YE solution reduced the CFA recovery at a final concentration of 125 mg/L. A dialysis of the YE solution indicated that the YE fraction (with nominal molecular weight >1,000 and <3,500) was effective at repressing the CFA recovery. Interestingly, the repair of ESS was equivalent regardless of the presence of the YE dialysate, while the CFA recovery was significantly repressed in the presence of YE. It was, therefore, suggested that YE components, probably with molecular weights of 1,000-3,500, were effective at repressing the CFA recovery of E. coli without affecting the ESS repair during photoreactivation.

Nutritional factors affecting the synthesis of an antiphagocytic factor by group E streptococci

Effects of components of the cultural medium on formation of a group E streptococcal antiphagocytic factor (APF), as detected by an indirect bactericidal test, were studied. Bovine serum albumin (BSA) replaced serum in promoting synthesis of APF in a chemically defined medium (CDM), Todd–Hewitt broth (THB), or tryptose phosphate broth (TPB). Stimulatory factors in BSA were not retained by diafiltration membranes of nominal molecular weight cutoff limits of 10 000 or greater. Citrate, lactate, pyruvate, or oleate, often found in BSA, could not replace BSA in stimulating synthesis of APF. Cells cultured in THB were more resistant to phagocytosis by porcine leukocytes than those grown in CDM or TPB, and the addition of varying amounts of THB to CDM stimulated a measurable response in synthesis of APF. Specific substrates caused different rates of APF synthesis; in decreasing order of effectiveness were glucose or mannose, sucrose, fructose, and trehalose. Proteolytic activity, which might cause the production of phagocyte-sensitive cells by destroying APF activity during culture, was not detectable in significant amounts in subcultures of the age employed in bactericidal tests.