Reinterpreting a community outbreak of Salmonella enterica serotype Enteritidis in the light of molecular typing

Background: In November 2005, a large outbreak due to Salmonella enterica serotype Enteritidis (S. Enteritidis) was observed within children who had eaten their meals at 53 school cafeterias in Florence and the surrounding area. A total of 154 isolates of S. Enteritidis were recovered from human cases between November 2005 and January 2006. All strains were assigned phage type 8 (PT8) and a common XbaI pulsotype. This paper reports the findings of a molecular epidemiological investigation performed on 124 strains of S. Enteritidis isolated in the years 2005 and 2006 in Florence and the surrounding area, including the epidemic isolates. Methods: One hundred twenty-four human isolates of S. Enteritidis identified in the period January 2005 – December 2006 were submitted to molecular typing by single enzyme – amplified fragment length polymorphism (SE-AFLP). Results: Molecular subtyping by SE-AFLP yielded five different profiles. In the pre-epidemic phase, type A included 78.4% of isolates, whereas only three (8.1%) belonged to type C. All isolates, but one, of the epidemic phase were indistinguishable and attributed to type C. In the post-epidemic period, a polymorphic pattern of SE-AFLP types was again recognized but type C accounted for 73.3% of the isolates during the first six months of 2006, whereas during the remaining six months type A regained the first place, including 52.0% of the isolates. Conclusion: The epidemic event was attributed to the emergence and clonal expansion of a strain of S. Enteritidis PT8-SE-AFLP type C. Circulation of the epidemic clone was much more extensive than the surveillance and traditional laboratory data demonstrated.