In this study, we describe a competitive polymerase chain reaction (PCR) for the quantification of the sequences of dsrAB in sulfatereducing microorganisms. We used the dsr1F/dsr4R set of primers, previously designed by Wagner et al. (1998), and a competitor sequence was constructed from the dsrAB genes of Desulfovibrio vulgaris. The detection limit of competitive PCR corresponded to 45 copies of the dsrAB genes per ng of extracted DNA, and most of the dsrAB sequences amplified and cloned from DNA extracted from Seine estuary sediments were amplified with a similar efficiency. Competitive PCR was then used to assess the abundance of dsrAB genes in the total DNA extracted from the sediment of the Seine estuary mudflats. We observed that the abundance of dsrAB coincided with the variation in the sulfate reduction rate with the depth of the sample, confirming the importance of 'dsrAB' sulfate-reducing microorganisms in sulfidogenesis in anoxic environments. We obtained values ranging from 0.045U10 3 to 6.63U10 3 copies of dsrAB per ng of extracted DNA, and values of the sulfate reduction rate ranging from 35 to 158 nmol cm 33 day 31 . These results are similar to those obtained in other studies using molecular biology techniques.

Water and sediments of the Seine estuary are contaminated by chemicals, especially cadmium, which favors survival and growth of cadmium-resistant bacteria. We investigated the diversity of the cadA gene, which encodes a Cd(2+)/ATPase protein transporter, in the microbial community of the Seine estuary. The cadA gene first isolated from S. aureus (pI258) was prevalent, with a conservation better than 98% identity, despite its presence in host bacteria of diverse species and genera. We report for the first time, eleven distinct Staphylococcus species, and also bacteria belonging to the Micrococcus and Halobacillus genera carrying the cadA gene. This cadA determinant was mostly plasmid-borne in the Staphylococcus genus, and IS257 sequences, which are known to participate in antibiotic resistance gene dissemination in S. aureus, were found to be located near to the cadA gene in 16/31 cadmium-resistant Staphylococcus strains and one Micrococcus strain. This suggests that IS257 has also contributed to the dissemination of the cadA resistance gene among staphylococci. These findings also emphasize on the existence of Staphylococcal bacteria in contaminated natural niches outside hospital environments.