The effect of longterm exposure to mercury on the bacterial community in marine sediment

Effect of restoration of children's teeth with mercury amalgam on the prevalence of mercury- and antibiotic-resistant oral bacteria

The purpose of this study was to determine whether placement of mercury amalgam restorations in children's teeth induces an increase in oral bacteria resistant to mercury, penicillin, ampicillin, erythromycin, or tetracycline. Dental plaque and saliva samples from 16 children without mercury amalgam restorations were screened for bacteria resistant to mercury or to one of the antibiotics prior to, and 1 month after, placement of the amalgam restoration. Following amalgam placement, there was no significant increase in the number of children harboring bacteria resistant to mercury, penicillin, ampicillin, erythromycin, or tetracycline; neither was there an increase in the proportions of such organisms. This study has shown that the presence of mercury restorations in children's teeth has little effect on the prevalence of mercury- or antibiotic-resistant oral bacteria.

Estimation of the abundance of the cadmium resistance gene cadA in microbial communities in polluted estuary water

We describe herein a molecular method for estimating the abundance of the cadA gene, which encodes a Cd2+/ATPase protein transporter, in bacterial DNA extracted from samples of environmental water. Competitive polymerase chain reaction (cPCR) may be the most appropriate technique for assessing the prevalence of the cadA gene in microbial communities in highly heterogeneous and polluted environments, such as the Seine estuary (France). We describe the development of this method: (i) the choice of two specific primers, based on the sequences encoding the cadmium binding site and the ion channel domains; (ii) the construction of a competitor sequence and assessment of its amplification efficiency; and (iii) the estimation of the copy number of the cadA gene. The cadA content in the bacterial community is expressed as the number of gene copies per ng of total DNA extracted, which is independent of the DNA extraction yield. This molecular procedure was improved to analyze cadA levels in bacterial DNA extracted from estuary water accidentally contaminated with cadmium. Results revealed a subsequent increase in the copy number of the cadA gene in the microbial community.

Molecular ecological analysis of dietary and antibiotic-induced alterations of the mouse intestinal microbiota

A cultivation-independent approach, polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE), was used to characterize changes in fecal bacterial populations resulting from consumption of a low residue diet or oral administration of a broad-spectrum antibiotic. C57BL/6NHsd mice were weaned to either a standard nonpurified diet (LC-diet) or a low residue diet (LR-diet) and at 17 wk of age were randomly assigned to receive drinking water with or without 25 ppm cefoxitin for 14 d. On d 1, 2, 7 and 14, microbial DNA was extracted from feces, and the V3 region of the 16S rDNA gene was amplified by PCR and analyzed by DGGE. The diversity of fecal microbial populations, assessed using Shannon's index (H'), which incorporates species richness (number of species, or in this case, PCR-DGGE bands) and evenness (the relative distribution of species), was not affected by cefoxitin. However, use of Sorenson's pairwise similarity coefficient (C(s)), an index that measures the species in common between different habitats, indicated that the species composition of fecal bacterial communities was altered by cefoxitin in mice fed either diet. Dietary effects on fecal microbial communities were more pronounced, with greater H' values (P < 0.05) in mice fed the LR-diet (1.9 +/- 0.1) compared with the LC-diet (1.6 +/- 0.1). The C(s) values were also greater (P < 0.05) in fecal bacterial populations from mice fed the LR-diet (C(s) = 69.8 +/- 2.0%) compared with mice fed the LC-diet (C(s) = 50.1 +/- 3.8%), indicating greater homogeneity of fecal bacterial communities in mice fed the LR-diet. These results demonstrate the utility of cultivation-independent PCR-DGGE analysis combined with measurements of ecological diversity for monitoring diet- and antibiotic-induced alterations of the complex intestinal microbial ecosystem.

Effects of mercury contamination on the culturable heterotrophic, functional and genetic diversity of the bacterial community in soil

This study investigates the effect of mercury contamination on the culturable heterotrophic, functional and genetic diversity of the bacterial community in soil. The changes in diversity were monitored in soil microcosms, enriched with 25 &mgr;g Hg(II) g(-1) soil, over a period of 3 months. The culturable heterotrophic diversity was investigated by colony morphology and colony appearance on solid LB medium. Functional diversity was analysed as sole carbon utilisation patterns in ECOplates. Genetic diversity was measured as bands on denaturing gradient gel electrophoresis (DGGE) gels obtained by purification of total soil DNA and amplification of bacterial 16S rDNA fragments by polymerase chain reaction. Concentrations of bioavailable and total mercury were measured throughout the experiment. The effect on the culturable heterotrophic and genetic diversity was very similar, showing an immediate decrease after mercury addition but then slowly increasing throughout the entire experimental period. Pre-exposure levels were not reached within the time span of this investigation. The DGGE band pattern indicated that a shift in the community structure was responsible for recovered diversity. When analysed by Shannon-Weaver indices, functional diversity was found to increase almost immediately after mercury addition and to remain at a level higher than the control soil for the rest of the experiment. The fraction of culturable heterotrophic bacteria increased from 1% to 10% of the total bacterial number as a result of mercury addition, and the mercury-resistant population increased to represent the entire heterotrophic population.

Characterization of mercury resistance mechanisms in marine sediment microbial communities

While estuarine sediments are often severely polluted with mercury, few studies have focused on the mechanisms of adaptation to mercury contamination in marine sediment microbial communities. In this study, we report a high frequency of Gram-negative bacterial isolates that are resistant to the heavy metal mercury obtained from the aerobic culturable marine microbial community. We detected a low frequency of genes homologous to mer(Tn21) in isolates from three out of four different estuarine environments. Other mercury resistant culturable bacterial isolates lacking homology to the known mer genes were able to reduce Hg(II) to its volatile Hg(0) form, indicating the presence of divergent mer genes. In addition, a number of mercury resistant isolates, obtained from three of the four marine sites investigated, exhibited decreased resistance to mercury in the presence of the protonophore carbonyl cyanide m-chlorophenylhydrazone. Representative mercury resistant bacterial isolates were identified by phylogenetic analysis as belonging to the alpha and gamma subclasses of the class Proteobacteria.