The risks of false-positive responses were examined when using the pol
ymerase chain reaction (PCR) method for the detection of Salmonella in
the marine environment (water and shellfish). The degradation rates o
f DNA, both free and from dead Salmonella, were evaluated in natural s
eawaters 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 sum
mer, the persistence was shorter: 10 d or even 2 d for a smaller inocu
lum (3 x 10(3) Salmonella ml(-1)). The role of the planktonic organism
s present in spring and summer was pinpointed. For free DNA, the persi
stence times were shorter: from 2 to 4 d at 20 degrees C, and from 3 t
o 8 d at 10 degrees C showing that the nuclease activity of marine org
anisms 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 h
igh concentrations of live, dead or lysed Salmonella. PCR is a rapid,
specific and sensitive method, but should be applied with care to mari
ne samples, in order to avoid false-positive responses. .