Salmonella DNA persistence in natural seawaters using PCR analysis

The risks of false-positive responses were examined when using the polymerase chain reaction (PCR) method for the detection of Salmonella in the marine environment (water and shellfish). The degradation rates of DNA, both free and from dead Salmonella, were evaluated in natural seawaters maintained at 10 degrees and 20 degrees C, using PCR with Vir and invA primers. The DNA of dead Salmonella was detected up to 55 d in seawater collected in winter and stored at 10 degrees C. But in summer, the persistence was shorter: 10 d or even 2 d for a smaller inoculum (3 x 10(3) Salmonella ml-1). The role of the planktonic organisms present in spring and summer was pinpointed. For free DNA, the persistence times were shorter: from 2 to 4 d at 20 degrees C, and from 3 to 8 d at 10 degrees C showing that the nuclease activity of marine organisms is higher at warm temperatures. These data led us to recommend careful interpretations of direct PCR results, especially during cold periods and for samples collected close to terrestrial discharges of high concentrations of live, dead or lysed Salmonella. PCR is a rapid, specific and sensitive method, but should be applied with care to marine samples, in order to avoid false-positive responses.